fix: avoid OpenGL driver loading in unit tests

common/build.gradle.kts

@@ -66,6 +66,8 @@ dependencies {
 
 tasks.withType<Test> {
     useJUnitPlatform()
+    // Static GL mocks must not load a graphics driver on headless CI.
+    systemProperty("org.lwjgl.opengl.explicitInit", "true")
     testLogging {
         setEvents(listOf("failed"))
         setExceptionFormat("full")

common/src/main/java/com/cobblemon/mod/common/mixin/client/RenderStateShardMixin.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.mixin.client;
+
+import com.cobblemon.mod.common.client.render.RenderAlphaScope;
+import net.minecraft.client.renderer.RenderStateShard;
+import net.minecraft.client.renderer.RenderType;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(RenderStateShard.class)
+public abstract class RenderStateShardMixin {
+    @Inject(method = "setupRenderState", at = @At("RETURN"))
+    private void cobblemon$applyScopedAlpha(CallbackInfo ci) {
+        if ((Object) this instanceof RenderType) RenderAlphaScope.afterMaterialSetup();
+    }
+
+    @Inject(method = "clearRenderState", at = @At("RETURN"))
+    private void cobblemon$restoreScopedAlpha(CallbackInfo ci) {
+        if ((Object) this instanceof RenderType) RenderAlphaScope.afterMaterialClear();
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/Cobblemon.kt

@@ -22,6 +22,8 @@ import com.cobblemon.mod.common.api.habitats.HabitatPools
 import com.cobblemon.mod.common.api.interaction.RequestManager
 import com.cobblemon.mod.common.api.molang.MoLangLoadedFilesCache
 import com.cobblemon.mod.common.api.molang.ObjectValue
+import com.cobblemon.mod.common.api.molangui.MolangUiActionRegistry
+import com.cobblemon.mod.common.api.molangui.MolangUiService
 import com.cobblemon.mod.common.api.permission.PermissionValidator
 import com.cobblemon.mod.common.api.pokeball.catching.calculators.CaptureCalculator
 import com.cobblemon.mod.common.api.pokeball.catching.calculators.CaptureCalculators
@@ -69,6 +71,7 @@ import com.cobblemon.mod.common.api.storage.player.adapter.TMMoveNbtBackend
 import com.cobblemon.mod.common.api.storage.player.factory.CachedPlayerDataStoreFactory
 import com.cobblemon.mod.common.api.tags.CobblemonEntityTypeTags
 import com.cobblemon.mod.common.api.tags.CobblemonItemTags
+import com.cobblemon.mod.common.api.vending.VendingMachinePageHandler
 import com.cobblemon.mod.common.battles.BagItems
 import com.cobblemon.mod.common.battles.BattleFormat
 import com.cobblemon.mod.common.battles.BattleRegistry
@@ -259,6 +262,7 @@ object Cobblemon {
             playerDataManager.onPlayerDisconnect(it.player)
             RequestManager.onLogoff(it.player)
             serverPlayerStructs.remove(it.player.uuid)
+            MolangUiService.closeAllForPlayer(it.player.uuid)
         }
         PlatformEvents.PLAYER_DEATH.subscribe {
             PCLinkManager.removeLink(it.player.uuid)
@@ -321,6 +325,11 @@ object Cobblemon {
         // Start up the data provider.
         CobblemonDataProvider.registerDefaults()
 
+        MolangUiActionRegistry.register(
+            VendingMachinePageHandler.ACTION,
+            VendingMachinePageHandler
+        )
+
         SHINY_ASPECT.register()
         GENDER_ASPECT.register()
         COSMETIC_SLOT_ASPECT.register()
@@ -450,6 +459,7 @@ object Cobblemon {
         }
 
         PlatformEvents.SERVER_STOPPED.subscribe {
+            MolangUiService.closeAll()
             storage.unregisterAll(it.server.registryAccess())
             playerDataManager.saveAllStores()
             playerDataManager.saveExecutor.shutdown()

common/src/main/kotlin/com/cobblemon/mod/common/CobblemonBlocks.kt

@@ -808,6 +808,9 @@ object CobblemonBlocks : PlatformRegistry<Registry<Block>, ResourceKey<Registry<
     @JvmField
     val DISPLAY_CASE = create("display_case", DisplayCaseBlock(BlockBehaviour.Properties.of().sound(CobblemonSounds.DISPLAY_CASE_SOUNDS).noOcclusion().pushReaction(PushReaction.BLOCK).mapColor(MapColor.STONE).strength(0.3F).requiresCorrectToolForDrops()))
 
+    @JvmField
+    val VENDING_MACHINE = create("vending_machine", VendingMachineBlock(BlockBehaviour.Properties.of().mapColor(MapColor.METAL).sound(SoundType.METAL).pushReaction(PushReaction.BLOCK).strength(2F).requiresCorrectToolForDrops()))
+
     // TODO after 1.7
 //    @JvmField
 //    val INCENSE_SWEET = create("incense_sweet", SweetIncenseBlock(BlockBehaviour.Properties.of().sound(CobblemonSounds.DISPLAY_CASE_SOUNDS).noOcclusion().pushReaction(PushReaction.BLOCK).mapColor(MapColor.STONE).strength(0.3F)))

common/src/main/kotlin/com/cobblemon/mod/common/CobblemonCommands.kt

@@ -77,5 +77,6 @@ object CobblemonCommands {
         // Possibly lock down registration if and only if under dev environment or running in an environment
         // with a certain system environment variable set
         CobblemonInfoCommand.register(dispatcher)
+        MolangUiCommand.register(dispatcher)
     }
 }

common/src/main/kotlin/com/cobblemon/mod/common/CobblemonItems.kt

@@ -465,6 +465,8 @@ object CobblemonItems : PlatformRegistry<Registry<Item>, ResourceKey<Registry<It
     val PASTURE = blockItem("pasture", CobblemonBlocks.PASTURE)
     @JvmField
     val DISPLAY_CASE = blockItem("display_case", CobblemonBlocks.DISPLAY_CASE)
+    @JvmField
+    val VENDING_MACHINE = blockItem("vending_machine", CobblemonBlocks.VENDING_MACHINE)
 //    @JvmField
 //    val INCENSE_SWEET = blockItem("incense_sweet", CobblemonBlocks.INCENSE_SWEET) // TODO after 1.7
 

common/src/main/kotlin/com/cobblemon/mod/common/CobblemonMenuType.kt

@@ -10,6 +10,7 @@ package com.cobblemon.mod.common
 
 
 import com.cobblemon.mod.common.block.campfirepot.CookingPotMenu
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
 import com.cobblemon.mod.common.block.tmmachine.TMMachineMenu
 import com.cobblemon.mod.common.platform.PlatformRegistry
 import net.minecraft.core.Registry
@@ -22,9 +23,10 @@ object CobblemonMenuType : PlatformRegistry<Registry<MenuType<*>>, ResourceKey<R
 
     val COOKING_POT = MenuType.register("cooking_pot_menu", ::CookingPotMenu)
     val TM_MACHINE = MenuType.register("tm_machine_menu", ::TMMachineMenu)
+    val MOLANG_UI = MenuType.register("molang_ui", ::MolangContainerMenu)
 
     override val registry: Registry<MenuType<*>>
         get() = BuiltInRegistries.MENU
     override val resourceKey: ResourceKey<Registry<MenuType<*>>>
         get() = Registries.MENU
-}
+}

common/src/main/kotlin/com/cobblemon/mod/common/CobblemonNetwork.kt

@@ -32,6 +32,10 @@ import com.cobblemon.mod.common.client.net.effect.SpawnSnowstormParticleHandler
 import com.cobblemon.mod.common.client.net.gui.ExpGainedDataPacketHandler
 import com.cobblemon.mod.common.client.net.gui.InteractPokemonUIPacketHandler
 import com.cobblemon.mod.common.client.net.gui.PokedexUIPacketHandler
+import com.cobblemon.mod.common.client.net.molangui.MolangUiCloseHandler
+import com.cobblemon.mod.common.client.net.molangui.MolangUiCompatibilityRequestHandler
+import com.cobblemon.mod.common.client.net.molangui.MolangUiOpenHandler
+import com.cobblemon.mod.common.client.net.molangui.MolangUiUpdateHandler
 import com.cobblemon.mod.common.client.net.gui.SetActiveTMPacketHandler
 import com.cobblemon.mod.common.client.net.gui.SetTMMachineContainerDataPacketHandler
 import com.cobblemon.mod.common.client.net.gui.SummaryUIPacketHandler
@@ -83,6 +87,10 @@ import com.cobblemon.mod.common.net.messages.client.callback.OpenPartyMoveCallba
 import com.cobblemon.mod.common.net.messages.client.cooking.SeasoningRegistrySyncPacket
 import com.cobblemon.mod.common.net.messages.client.cooking.ToggleCookingPotLidPacket
 import com.cobblemon.mod.common.net.messages.client.data.*
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiClosePacket
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiCompatibilityRequestPacket
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiOpenPacket
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiUpdatePacket
 import com.cobblemon.mod.common.net.messages.client.debug.RequestOpenRidingStatsDebugGUIPacket
 import com.cobblemon.mod.common.net.messages.client.dialogue.DialogueClosedPacket
 import com.cobblemon.mod.common.net.messages.client.dialogue.DialogueOpenedPacket
@@ -180,6 +188,9 @@ import com.cobblemon.mod.common.net.messages.server.dialogue.EscapeDialoguePacke
 import com.cobblemon.mod.common.net.messages.server.dialogue.InputToDialoguePacket
 import com.cobblemon.mod.common.net.messages.server.habitat.SaveHabitatBlockSettingsPacket
 import com.cobblemon.mod.common.net.messages.server.npc.SaveNPCPacket
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiActionPacket
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiClosedPacket
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiCompatibilityResponsePacket
 import com.cobblemon.mod.common.net.messages.server.orientation.ServerboundUpdateOrientationPacket
 import com.cobblemon.mod.common.net.messages.server.pasture.PasturePokemonPacket
 import com.cobblemon.mod.common.net.messages.server.pasture.SetPastureConflictPacket
@@ -239,6 +250,9 @@ import com.cobblemon.mod.common.net.serverhandling.dialogue.InputToDialogueHandl
 import com.cobblemon.mod.common.net.serverhandling.evolution.AcceptEvolutionHandler
 import com.cobblemon.mod.common.net.serverhandling.habitat.SaveHabitatBlockSettingsHandler
 import com.cobblemon.mod.common.net.serverhandling.npc.SaveNPCHandler
+import com.cobblemon.mod.common.net.serverhandling.molangui.MolangUiActionHandler
+import com.cobblemon.mod.common.net.serverhandling.molangui.MolangUiClosedHandler
+import com.cobblemon.mod.common.net.serverhandling.molangui.MolangUiCompatibilityResponseHandler
 import com.cobblemon.mod.common.net.serverhandling.orientation.ServerboundUpdateOrientationHandler
 import com.cobblemon.mod.common.net.serverhandling.pasture.PasturePokemonHandler
 import com.cobblemon.mod.common.net.serverhandling.pasture.SetPastureConflictHandler
@@ -445,6 +459,11 @@ object CobblemonNetwork {
         list.add(PacketRegisterInfo(RideSettingsSyncPacket.ID, RideSettingsSyncPacket::decode, RideSettingsSyncHandler))
         list.add(PacketRegisterInfo(TechnicalMachineRegistrySyncPacket.ID, TechnicalMachineRegistrySyncPacket::decode, DataRegistrySyncPacketHandler()))
 
+        list.add(PacketRegisterInfo(MolangUiOpenPacket.ID, MolangUiOpenPacket::decode, MolangUiOpenHandler))
+        list.add(PacketRegisterInfo(MolangUiClosePacket.ID, MolangUiClosePacket::decode, MolangUiCloseHandler))
+        list.add(PacketRegisterInfo(MolangUiUpdatePacket.ID, MolangUiUpdatePacket::decode, MolangUiUpdateHandler))
+        list.add(PacketRegisterInfo(MolangUiCompatibilityRequestPacket.ID, MolangUiCompatibilityRequestPacket::decode, MolangUiCompatibilityRequestHandler))
+
         // Effects
         list.add(PacketRegisterInfo(SpawnSnowstormParticlePacket.ID, SpawnSnowstormParticlePacket::decode, SpawnSnowstormParticleHandler))
         list.add(PacketRegisterInfo(SpawnSnowstormEntityParticlePacket.ID, SpawnSnowstormEntityParticlePacket::decode, SpawnSnowstormEntityParticleHandler))
@@ -643,6 +662,10 @@ object CobblemonNetwork {
         list.add(PacketRegisterInfo(SetEntityBehaviourPacket.ID, SetEntityBehaviourPacket::decode, SetEntityBehaviourHandler))
         list.add(PacketRegisterInfo(DamageOnCollisionPacket.ID, DamageOnCollisionPacket::decode, DamageOnCollisionPacketHandler))
 
+        list.add(PacketRegisterInfo(MolangUiActionPacket.ID, MolangUiActionPacket::decode, MolangUiActionHandler))
+        list.add(PacketRegisterInfo(MolangUiClosedPacket.ID, MolangUiClosedPacket::decode, MolangUiClosedHandler))
+        list.add(PacketRegisterInfo(MolangUiCompatibilityResponsePacket.ID, MolangUiCompatibilityResponsePacket::decode, MolangUiCompatibilityResponseHandler))
+
         // Habitat blocks
         list.add(PacketRegisterInfo(SaveHabitatBlockSettingsPacket.ID, SaveHabitatBlockSettingsPacket::decode, SaveHabitatBlockSettingsHandler))
 
@@ -656,4 +679,4 @@ object CobblemonNetwork {
     fun sendPacketToPlayer(player: ServerPlayer, packet: NetworkPacket<*>) {
         Cobblemon.implementation.networkManager.sendPacketToPlayer(player, packet)
     }
-}
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molang/function/GeneralMoLangFunctions.kt

@@ -45,6 +45,11 @@ object GeneralMoLangFunctions {
     val functions: MutableMap<String, (MoParams) -> Any> = mutableMapOf()
 
     init {
+        functions["format"] = { params ->
+            val value = params.get<MoValue>(0).asDouble()
+            val decimals = params.get<MoValue>(1).asDouble().toInt().coerceIn(0, 10)
+            StringValue(String.format("%.${decimals}f", value))
+        }
         functions["print"] = { params ->
             val message = params.get<MoValue>(0).asString()
             LOGGER.info(message)

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/GuiMode.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+enum class GuiMode {
+    OVERLAY,
+    SCREEN;
+
+    companion object {
+        fun fromString(value: String): GuiMode? = entries.firstOrNull { it.name.equals(value, ignoreCase = true) }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiActionRegistry.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.api.scripting.CobblemonScripts
+import java.util.concurrent.ConcurrentHashMap
+
+object MolangUiActionRegistry {
+    private val handlers = ConcurrentHashMap<ActionKey, MolangUiActionHandler>()
+
+    fun register(key: ActionKey, handler: MolangUiActionHandler) {
+        check(handlers.putIfAbsent(key, handler) == null) { "Molang UI action '$key' is already registered" }
+    }
+
+    fun unregister(key: ActionKey): Boolean = handlers.remove(key) != null
+
+    internal fun dispatch(
+        session: MolangUiSession,
+        action: ActionKey,
+        parameters: Map<String, ParameterValue>
+    ) {
+        val definition = session.contract.actions[action]
+            ?: throw IllegalArgumentException("undeclared action '${action.value}'")
+        val registered = handlers[action]
+        if (registered != null) {
+            registered.handle(MolangUiActionContext(session, session.player, parameters))
+            return
+        }
+        val scriptId = definition.handler
+            ?: throw IllegalStateException("action '${action.value}' has no registered handler")
+        val script = CobblemonScripts.scripts[scriptId]
+            ?: throw IllegalStateException("Molang UI action script '$scriptId' was not loaded")
+        val runtime = MolangUiServerRuntimeFactory.create(session)
+        parameters.forEach { (name, value) -> runtime.environment.setSimpleVariable(name, value.asMoLangValue()) }
+        script.resolve(runtime, emptyMap())
+    }
+
+    internal fun executeOnOpen(session: MolangUiSession) {
+        val scriptId = session.contract.onOpen ?: return
+        val script = CobblemonScripts.scripts[scriptId]
+            ?: throw IllegalStateException("Molang UI on_open script '$scriptId' was not loaded")
+        script.resolve(MolangUiServerRuntimeFactory.create(session), emptyMap())
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiContractRegistry.kt

@@ -0,0 +1,201 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.google.gson.JsonArray
+import com.cobblemon.mod.common.api.data.DataRegistry
+import com.cobblemon.mod.common.api.molangui.container.CollectionConfig
+import com.cobblemon.mod.common.api.molangui.container.CollectionSource
+import com.cobblemon.mod.common.api.molangui.container.CloseItemsPolicy
+import com.cobblemon.mod.common.api.molangui.container.ItemFilterEntry
+import com.cobblemon.mod.common.api.molangui.container.ShiftClickRule
+import com.cobblemon.mod.common.api.molangui.container.SlotLayout
+import com.cobblemon.mod.common.api.reactive.SimpleObservable
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiCompatibilityRequestPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import com.google.gson.JsonElement
+import com.google.gson.JsonObject
+import com.google.gson.JsonParser
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.server.packs.PackType
+import net.minecraft.server.packs.resources.ResourceManager
+import com.cobblemon.mod.common.Cobblemon
+import java.util.concurrent.atomic.AtomicLong
+
+object MolangUiContractRegistry : DataRegistry {
+    override val id = cobblemonResource("molang_ui_contracts")
+    override val type = PackType.SERVER_DATA
+    override val observable = SimpleObservable<MolangUiContractRegistry>()
+    private val contracts = linkedMapOf<ResourceLocation, MolangUiContract>()
+    private val parseFailures = AtomicLong()
+    private const val DIRECTORY = "molang_ui"
+
+    override fun reload(manager: ResourceManager) {
+        val next = linkedMapOf<ResourceLocation, MolangUiContract>()
+        manager.listResources(DIRECTORY) { it.path.endsWith(".json") }.forEach { (location, resource) ->
+            val id = ResourceLocation.fromNamespaceAndPath(
+                location.namespace,
+                location.path.removePrefix("$DIRECTORY/").removeSuffix(".json")
+            )
+            try {
+                val json = resource.openAsReader().use { JsonParser.parseReader(it).asJsonObject }
+                next[id] = parse(id, json)
+            } catch (error: Exception) {
+                parseFailures.incrementAndGet()
+                Cobblemon.LOGGER.error("Rejected Molang UI contract '$location': ${error.message}")
+            }
+        }
+        require(next.size <= MolangUiPacketLimits.MAX_CONTRACTS) {
+            "Molang UI contract count exceeds ${MolangUiPacketLimits.MAX_CONTRACTS}"
+        }
+        contracts.clear()
+        contracts.putAll(next)
+        Cobblemon.LOGGER.info("Loaded ${contracts.size} validated Molang UI contracts")
+        observable.emit(this)
+    }
+
+    override fun sync(player: ServerPlayer) {
+        MolangUiCompatibilityRequestPacket(versions()).sendToPlayer(player)
+    }
+
+    internal fun parse(
+        id: ResourceLocation,
+        json: JsonObject,
+        itemExists: (ResourceLocation) -> Boolean = BuiltInRegistries.ITEM::containsKey
+    ): MolangUiContract {
+        require(!json.has("blocks_input")) { "blocks_input is unsupported; input handling is determined by mode" }
+        val formatVersion = json.get("format_version").requireUiInt("format_version")
+        require(formatVersion == MOLANG_UI_FORMAT_VERSION) { "unsupported format_version $formatVersion" }
+        val contractVersion = json.get("contract_version").requireUiString("contract_version")
+        MolangUiPacketLimits.requireIdentifier(contractVersion, "contract_version")
+        val mode = GuiMode.fromString(json.get("mode").requireUiString("mode"))
+            ?: throw IllegalArgumentException("unknown mode '${json.get("mode").asString}'")
+        val parameters = parseParameters(json.getAsJsonObject("parameters"))
+        val actions = parseActions(json.getAsJsonObject("actions"))
+        val collections = parseCollections(json.getAsJsonObject("collections"), itemExists)
+        val shiftRules = parseShiftRules(json.getAsJsonArray("shift_click_rules"), collections.keys)
+        val closePolicy = json.get("on_close_items")?.requireUiString("on_close_items")?.let { raw ->
+            CloseItemsPolicy.fromId(raw) ?: throw IllegalArgumentException("unknown on_close_items policy '$raw'")
+        } ?: CloseItemsPolicy.RETURN
+        return MolangUiContract(
+            id, formatVersion, contractVersion, mode, parameters, actions,
+            json.get("on_open")?.requireUiString("on_open")?.let { ResourceLocation.tryParse(it) ?: error("invalid on_open script id") },
+            collections, shiftRules, closePolicy,
+            json.get("exclusive")?.requireUiBoolean("exclusive") ?: false
+        )
+    }
+
+    private fun parseParameters(json: JsonObject?): Map<String, ParameterDefinition> {
+        require((json?.size() ?: 0) <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many parameters" }
+        return json?.entrySet()?.associate { (name, element) ->
+            MolangUiParameters.requireName(name)
+            val obj = element.asJsonObject
+            val type = ParameterType.fromId(obj.get("type").requireUiString("parameters.$name.type"))
+                ?: throw IllegalArgumentException("unknown type for parameter '$name'")
+            val default = obj.get("default")?.let { parseDefault(type, it, "parameters.$name.default") }
+            name to ParameterDefinition(type, obj.get("required")?.requireUiBoolean("parameters.$name.required") ?: (default == null), default)
+        }?.toMap() ?: emptyMap()
+    }
+
+    private fun parseDefault(type: ParameterType, element: JsonElement, path: String): ParameterDefault {
+        if (element.isJsonObject && element.asJsonObject.has("molang")) {
+            require(element.asJsonObject.size() == 1) { "expression '$path' contains unknown fields" }
+            return ParameterDefault.Molang(element.asJsonObject.get("molang").requireUiString("$path.molang"))
+        }
+        val value = when (type) {
+            ParameterType.STRING -> ParameterValue.StringValue(element.requireUiString(path))
+            ParameterType.NUMBER -> ParameterValue.NumberValue(element.requireUiNumber(path))
+            ParameterType.BOOLEAN -> ParameterValue.BooleanValue(element.requireUiBoolean(path))
+            ParameterType.VEC3 -> {
+                require(element.isJsonArray && element.asJsonArray.size() == 3) { "'$path' must be a three-number array" }
+                ParameterValue.Vec3Value(
+                    element.asJsonArray[0].requireUiNumber("$path[0]"),
+                    element.asJsonArray[1].requireUiNumber("$path[1]"),
+                    element.asJsonArray[2].requireUiNumber("$path[2]")
+                )
+            }
+        }
+        MolangUiPacketLimits.requireValue(value.asWireValue(), path)
+        return ParameterDefault.Literal(value)
+    }
+
+    private fun parseActions(json: JsonObject?): Map<ActionKey, ActionDefinition> {
+        val result = json?.entrySet()?.associate { (name, element) ->
+            val key = ActionKey(name)
+            val obj = element.asJsonObject
+            val handler = obj.get("handler")?.requireUiString("actions.$name.handler")?.let { ResourceLocation.tryParse(it) ?: error("invalid handler for action '$name'") }
+            val params = obj.getAsJsonObject("parameters")?.entrySet()?.associate { (paramName, typeElement) ->
+                MolangUiPacketLimits.requireIdentifier(paramName, "action parameter name")
+                paramName to (ParameterType.fromId(typeElement.requireUiString("actions.$name.parameters.$paramName"))
+                    ?: throw IllegalArgumentException("unknown type for action parameter '$name.$paramName'"))
+            }?.toMap() ?: emptyMap()
+            require(params.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "action '$name' has too many parameters" }
+            key to ActionDefinition(handler, params)
+        }?.toMap() ?: emptyMap()
+        require(result.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "contract has too many actions" }
+        return result
+    }
+
+    private fun parseCollections(
+        json: JsonObject?,
+        itemExists: (ResourceLocation) -> Boolean
+    ): Map<String, CollectionConfig> {
+        require((json?.size() ?: 0) <= MolangUiPacketLimits.MAX_CONTRACTS) { "too many container collections" }
+        return json?.entrySet()?.associate { (name, element) ->
+            MolangUiPacketLimits.requireIdentifier(name, "collection name")
+            val obj = element.asJsonObject
+            val source = CollectionSource.fromId(obj.get("source")?.requireUiString("collections.$name.source") ?: "session")
+                ?: throw IllegalArgumentException("unknown source for collection '$name'")
+            val filters = obj.getAsJsonArray("allowed_items")?.map { raw ->
+                val filter = ItemFilterEntry.parse(raw.requireUiString("collections.$name.allowed_items"))
+                    ?: throw IllegalArgumentException("invalid item filter '${raw.asString}' in '$name'")
+                if (filter is ItemFilterEntry.Item) require(itemExists(filter.id)) {
+                    "unknown item '${filter.id}' in collection '$name'"
+                }
+                filter
+            }?.toList() ?: emptyList()
+            val layoutObj = obj.getAsJsonObject("layout") ?: throw IllegalArgumentException("collection '$name' is missing layout")
+            val offset = layoutObj.pair("offset", 0, 0)
+            val grid = layoutObj.pair("grid_dimensions", 1, 1)
+            val slotSize = layoutObj.pair("slot_size", 18, 18)
+            val spacing = layoutObj.pair("slot_spacing", 0, 0)
+            val layout = SlotLayout(offset.first, offset.second, grid.first, grid.second, slotSize.first, slotSize.second, spacing.first, spacing.second)
+            name to CollectionConfig(
+                obj.get("size").requireUiInt("collections.$name.size"), filters,
+                obj.get("take_only")?.requireUiBoolean("collections.$name.take_only") ?: false, source,
+                obj.getAsJsonArray("slots")?.map { it.requireUiInt("collections.$name.slots") }?.toList(), layout
+            )
+        }?.toMap() ?: emptyMap()
+    }
+
+    private fun parseShiftRules(array: JsonArray?, collections: Set<String>): List<ShiftClickRule> =
+        array?.map { element ->
+            val obj = element.asJsonObject
+            val from = obj.stringList("from")
+            val to = obj.stringList("to")
+            val valid = collections + ShiftClickRule.PLAYER_SIDE_NAMES
+            require(from.isNotEmpty() && to.isNotEmpty() && (from + to).all { it in valid }) { "shift-click rule references an unknown collection" }
+            ShiftClickRule(from, to)
+        }?.toList() ?: emptyList()
+
+    fun getContract(id: ResourceLocation): MolangUiContract? = contracts[id]
+    fun getAllContracts(): Map<ResourceLocation, MolangUiContract> = contracts.toMap()
+    fun versions(): Map<ResourceLocation, String> = contracts.mapValues { it.value.contractVersion }
+    fun parseFailureCount(): Long = parseFailures.get()
+}
+
+private fun JsonObject.pair(name: String, first: Int, second: Int): Pair<Int, Int> {
+    val value = getAsJsonArray(name) ?: return first to second
+    require(value.size() == 2) { "'$name' must have two entries" }
+    return value[0].requireUiInt(name) to value[1].requireUiInt(name)
+}
+private fun JsonObject.stringList(name: String): List<String> = getAsJsonArray(name)?.map { it.requireUiString(name) }?.toList()
+    ?: throw IllegalArgumentException("missing array '$name'")

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiDefinition.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetDefinition
+import net.minecraft.resources.ResourceLocation
+
+data class MolangUiDefinition(
+    val guiId: ResourceLocation,
+    val formatVersion: Int,
+    val contractVersion: String,
+    val mode: GuiMode,
+    val parameters: Map<String, ParameterType>,
+    val actions: Set<ActionKey>,
+    val exclusive: Boolean,
+    val root: WidgetDefinition,
+    val variables: Map<String, Double>
+)

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiDiagnostics.kt

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import java.util.concurrent.atomic.AtomicLong
+
+object MolangUiDiagnostics {
+    private val activeSessions = AtomicLong()
+    private val rejectedActions = AtomicLong()
+    private val compatibilityFailures = AtomicLong()
+
+    internal fun sessionOpened() { activeSessions.incrementAndGet() }
+    internal fun sessionClosed() { activeSessions.updateAndGet { value -> (value - 1).coerceAtLeast(0) } }
+    internal fun actionRejected() { rejectedActions.incrementAndGet() }
+    internal fun compatibilityFailures(count: Long) { compatibilityFailures.addAndGet(count) }
+
+    fun snapshot() = Snapshot(activeSessions.get(), rejectedActions.get(), compatibilityFailures.get(), MolangUiContractRegistry.parseFailureCount())
+
+    data class Snapshot(val activeSessions: Long, val rejectedActions: Long, val compatibilityFailures: Long, val parseFailures: Long)
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiJson.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.google.gson.JsonElement
+
+internal fun JsonElement?.requireUiString(path: String): String {
+    require(this != null && isJsonPrimitive && asJsonPrimitive.isString) { "'$path' must be a string" }
+    return asString
+}
+
+internal fun JsonElement?.requireUiBoolean(path: String): Boolean {
+    require(this != null && isJsonPrimitive && asJsonPrimitive.isBoolean) { "'$path' must be a boolean" }
+    return asBoolean
+}
+
+internal fun JsonElement?.requireUiNumber(path: String): Double {
+    require(this != null && isJsonPrimitive && asJsonPrimitive.isNumber) { "'$path' must be a number" }
+    return asDouble.also { require(it.isFinite()) { "'$path' must be finite" } }
+}
+
+internal fun JsonElement?.requireUiInt(path: String): Int {
+    require(this != null && isJsonPrimitive && asJsonPrimitive.isNumber) { "'$path' must be an integer" }
+    return try {
+        asBigDecimal.intValueExact()
+    } catch (error: ArithmeticException) {
+        throw IllegalArgumentException("'$path' must be an integer in the 32-bit range", error)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiModels.kt

@@ -0,0 +1,151 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.api.molangui.container.SlotLayout
+import com.cobblemon.mod.common.api.molangui.container.CollectionConfig
+import com.cobblemon.mod.common.api.molangui.container.CloseItemsPolicy
+import com.cobblemon.mod.common.api.molangui.container.ShiftClickRule
+import net.minecraft.core.BlockPos
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.level.ServerPlayer
+import java.util.UUID
+
+const val MOLANG_UI_FORMAT_VERSION = 1
+
+enum class ParameterType(val id: String) {
+    STRING("string"),
+    NUMBER("number"),
+    BOOLEAN("boolean"),
+    VEC3("vec3");
+
+    companion object {
+        fun fromId(id: String): ParameterType? = entries.firstOrNull { it.id == id }
+    }
+}
+
+sealed interface ParameterValue {
+    val type: ParameterType
+    fun asWireValue(): String
+
+    data class StringValue(val value: String) : ParameterValue {
+        override val type = ParameterType.STRING
+        override fun asWireValue() = value
+    }
+
+    data class NumberValue(val value: Double) : ParameterValue {
+        init { require(value.isFinite()) { "number parameters must be finite" } }
+        override val type = ParameterType.NUMBER
+        override fun asWireValue() = value.toString()
+    }
+
+    data class BooleanValue(val value: Boolean) : ParameterValue {
+        override val type = ParameterType.BOOLEAN
+        override fun asWireValue() = value.toString()
+    }
+
+    data class Vec3Value(val x: Double, val y: Double, val z: Double) : ParameterValue {
+        init { require(x.isFinite() && y.isFinite() && z.isFinite()) { "vec3 parameters must be finite" } }
+        override val type = ParameterType.VEC3
+        override fun asWireValue() = "$x,$y,$z"
+    }
+
+    companion object {
+        fun parse(type: ParameterType, value: String): ParameterValue? = when (type) {
+            ParameterType.STRING -> StringValue(value)
+            ParameterType.NUMBER -> value.toDoubleOrNull()?.takeIf(Double::isFinite)?.let(::NumberValue)
+            ParameterType.BOOLEAN -> value.toBooleanStrictOrNull()?.let(::BooleanValue)
+            ParameterType.VEC3 -> value.split(',').takeIf { it.size == 3 }
+                ?.map { it.trim().toDoubleOrNull() }
+                ?.takeIf { it.all { part -> part != null && part.isFinite() } }
+                ?.let { Vec3Value(it[0]!!, it[1]!!, it[2]!!) }
+        }
+    }
+}
+
+data class ParameterDefinition(
+    val type: ParameterType,
+    val required: Boolean = true,
+    val default: ParameterDefault? = null
+)
+
+sealed interface ParameterDefault {
+    data class Literal(val value: ParameterValue) : ParameterDefault
+    data class Molang(val expression: String) : ParameterDefault
+}
+
+@JvmInline
+value class ActionKey(val value: String) {
+    init { require(value.isNotBlank() && value.length <= MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH) }
+}
+
+data class ActionDefinition(
+    val handler: ResourceLocation? = null,
+    val parameters: Map<String, ParameterType> = emptyMap()
+)
+
+data class MolangUiContract(
+    val id: ResourceLocation,
+    val formatVersion: Int,
+    val contractVersion: String,
+    val mode: GuiMode,
+    val parameters: Map<String, ParameterDefinition>,
+    val actions: Map<ActionKey, ActionDefinition>,
+    val onOpen: ResourceLocation?,
+    val collections: Map<String, CollectionConfig>,
+    val shiftClickRules: List<ShiftClickRule>,
+    val closeItemsPolicy: CloseItemsPolicy,
+    val exclusive: Boolean
+) {
+    init {
+        require(mode == GuiMode.SCREEN || collections.isEmpty()) { "overlay views cannot have container collections" }
+        SlotLayout.validateCollections(collections.mapValues { it.value.layout })
+    }
+}
+
+data class MolangUiOpenRequest(
+    val uiId: ResourceLocation,
+    val parameters: Map<String, ParameterValue> = emptyMap(),
+    val blockPos: BlockPos? = null
+)
+
+sealed interface MolangUiOpenResult {
+    data class Opened(val session: MolangUiSessionHandle) : MolangUiOpenResult
+    data class Rejected(val reason: RejectionReason, val detail: String) : MolangUiOpenResult
+}
+
+enum class RejectionReason {
+    UNKNOWN_UI,
+    INCOMPATIBLE_CLIENT,
+    INVALID_PARAMETERS,
+    SESSION_LIMIT,
+    INVALID_CONTAINER,
+    INTERNAL_ERROR
+}
+
+enum class SessionState { OPENING, ACTIVE, CLOSING, CLOSED }
+
+interface MolangUiSessionHandle : AutoCloseable {
+    val sessionId: UUID
+    val uiId: ResourceLocation
+    val state: SessionState
+    fun updateParameters(parameters: Map<String, ParameterValue>): Boolean
+    fun updateVariables(variables: Map<String, Double>): Boolean
+    override fun close()
+}
+
+data class MolangUiActionContext(
+    val session: MolangUiSessionHandle,
+    val player: ServerPlayer,
+    val parameters: Map<String, ParameterValue>
+)
+
+fun interface MolangUiActionHandler {
+    fun handle(context: MolangUiActionContext)
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiPacketLimits.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+object MolangUiPacketLimits {
+    const val MAX_CONTRACTS = 256
+    const val MAX_ACTION_FIELDS = 32
+    const val MAX_IDENTIFIER_LENGTH = 128
+    const val MAX_VALUE_LENGTH = 4 * 1024
+    const val MAX_PACKET_BYTES = 256 * 1024
+
+    fun requireIdentifier(value: String, label: String): String {
+        require(value.isNotBlank() && value.length <= MAX_IDENTIFIER_LENGTH) {
+            "$label must contain 1..$MAX_IDENTIFIER_LENGTH characters"
+        }
+        return value
+    }
+
+    fun requireValue(value: String, label: String): String {
+        require(value.length <= MAX_VALUE_LENGTH) { "$label exceeds $MAX_VALUE_LENGTH characters" }
+        return value
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiParameters.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.MoValue
+import com.bedrockk.molang.runtime.value.StringValue
+
+internal object MolangUiParameters {
+    val GENERAL_FUNCTIONS = setOf("format", "to_string", "to_number", "to_int", "is_number", "is_int", "is_blank")
+    private val reserved = GENERAL_FUNCTIONS + setOf(
+        "gui_id", "gui_time", "delta", "screen_width", "screen_height", "mouse_x", "mouse_y", "item", "translate", "event"
+    )
+    private val namePattern = Regex("[a-z_][a-z0-9_]*")
+
+    fun requireName(name: String) {
+        MolangUiPacketLimits.requireIdentifier(name, "parameter name")
+        require(namePattern.matches(name) && name !in reserved) { "invalid or reserved parameter name '$name'" }
+    }
+
+    fun decode(types: Map<String, ParameterType>, values: Map<String, String>): Map<String, ParameterValue> {
+        require(values.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many parameters" }
+        return values.mapValues { (name, raw) ->
+            requireName(name)
+            val type = types[name] ?: throw IllegalArgumentException("undeclared parameter '$name'")
+            MolangUiPacketLimits.requireValue(raw, name)
+            ParameterValue.parse(type, raw) ?: throw IllegalArgumentException("invalid ${type.id} parameter '$name'")
+        }
+    }
+
+    fun validate(values: Map<String, ParameterValue>) {
+        require(values.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many parameters" }
+        values.forEach { (name, value) ->
+            requireName(name)
+            MolangUiPacketLimits.requireValue(value.asWireValue(), name)
+        }
+    }
+
+    fun requireVariableName(name: String): String {
+        MolangUiPacketLimits.requireIdentifier(name, "variable name")
+        return name.removePrefix("v.").also {
+            require(namePattern.matches(it)) { "invalid variable '$name'" }
+        }
+    }
+
+    fun validateVariables(values: Map<String, Double>) {
+        val names = mutableSetOf<String>()
+        values.forEach { (name, value) ->
+            val canonical = requireVariableName(name)
+            require(names.add(canonical) && value.isFinite()) {
+                "invalid or duplicate variable '$name'"
+            }
+        }
+    }
+}
+
+internal fun ParameterValue.asMoLangValue(): MoValue = when (this) {
+    is ParameterValue.StringValue -> StringValue(value)
+    is ParameterValue.NumberValue -> DoubleValue(value)
+    is ParameterValue.BooleanValue -> DoubleValue(value)
+    is ParameterValue.Vec3Value -> StringValue(asWireValue())
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiRegistry.kt

@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.molangui.template.TemplateLibraryRegistry
+import com.cobblemon.mod.common.api.molangui.widget.WidgetDefinition
+import com.cobblemon.mod.common.api.molangui.widget.WidgetTree
+import com.google.gson.JsonObject
+import com.google.gson.JsonParser
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.packs.resources.ResourceManager
+import java.util.concurrent.atomic.AtomicLong
+
+/** Client asset registry. Server code must use [MolangUiContractRegistry]. */
+object MolangUiRegistry {
+    private const val DIRECTORY = "molang_ui"
+    private val definitions = linkedMapOf<ResourceLocation, MolangUiDefinition>()
+    private val parseFailures = AtomicLong()
+
+    fun reload(manager: ResourceManager) {
+        TemplateLibraryRegistry.reload(manager)
+        val next = linkedMapOf<ResourceLocation, MolangUiDefinition>()
+        manager.listResources(DIRECTORY) { it.path.endsWith(".json") && !it.path.contains("/templates/") }
+            .forEach { (identifier, resource) ->
+                val guiId = ResourceLocation.fromNamespaceAndPath(
+                    identifier.namespace,
+                    identifier.path.removePrefix("$DIRECTORY/").removeSuffix(".json")
+                )
+                try {
+                    val json = resource.openAsReader().use { JsonParser.parseReader(it).asJsonObject }
+                    next[guiId] = parse(guiId, json)
+                } catch (error: Exception) {
+                    parseFailures.incrementAndGet()
+                    Cobblemon.LOGGER.error("Rejected Molang UI view '$identifier': ${error.message}")
+                }
+            }
+        require(next.size <= MolangUiPacketLimits.MAX_CONTRACTS) { "too many Molang UI views" }
+        definitions.clear()
+        definitions.putAll(next)
+        Cobblemon.LOGGER.info("Loaded ${definitions.size} validated Molang UI views")
+    }
+
+    internal fun parse(guiId: ResourceLocation, json: JsonObject): MolangUiDefinition {
+        require(!json.has("blocks_input")) { "blocks_input is unsupported; input handling is determined by mode" }
+        val formatVersion = json.get("format_version").requireUiInt("format_version")
+        require(formatVersion == MOLANG_UI_FORMAT_VERSION) { "unsupported format_version $formatVersion" }
+        val contractVersion = json.get("contract_version").requireUiString("contract_version")
+        MolangUiPacketLimits.requireIdentifier(contractVersion, "contract_version")
+        val mode = GuiMode.fromString(json.get("mode").requireUiString("mode"))
+            ?: throw IllegalArgumentException("unknown mode '${json.get("mode").asString}'")
+        val parameterJson = json.getAsJsonObject("parameters")
+        require((parameterJson?.size() ?: 0) <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many parameters" }
+        val parameters = parameterJson?.entrySet()?.associate { (name, value) ->
+            MolangUiParameters.requireName(name)
+            val type = ParameterType.fromId(value.requireUiString("parameters.$name"))
+                ?: throw IllegalArgumentException("unknown parameter type for '$name'")
+            name to type
+        }?.toMap() ?: emptyMap()
+        val actions = json.getAsJsonArray("actions")?.map { ActionKey(it.requireUiString("actions")) }?.toSet() ?: emptySet()
+        require(actions.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many declared actions" }
+        val variables = json.getAsJsonObject("variables")?.entrySet()?.associate { (name, value) ->
+            MolangUiPacketLimits.requireIdentifier(name, "variable name")
+            name to value.requireUiNumber("variables.$name")
+        }?.toMap() ?: emptyMap()
+        MolangUiParameters.validateVariables(variables)
+        require(!json.has("imports")) { "template imports are unsupported; use qualified template references" }
+        val root = WidgetTree.parse(json)
+        val referencedActions = linkedSetOf<ActionKey>().also { collectActions(root, it) }
+        val undeclared = referencedActions - actions
+        require(undeclared.isEmpty()) { "widgets reference undeclared actions: ${undeclared.joinToString { it.value }}" }
+        return MolangUiDefinition(
+            guiId, formatVersion, contractVersion, mode, parameters, actions,
+            json.get("exclusive")?.requireUiBoolean("exclusive") ?: false,
+            root, variables
+        )
+    }
+
+    private fun collectActions(node: WidgetDefinition, output: MutableSet<ActionKey>) {
+        node.events.values.forEach { output += it.action }
+        node.children.forEach { collectActions(it, output) }
+        node.itemTemplate?.let { collectActions(it, output) }
+    }
+
+    fun getDefinition(guiId: ResourceLocation): MolangUiDefinition? = definitions[guiId]
+    fun getAllDefinitions(): Map<ResourceLocation, MolangUiDefinition> = definitions.toMap()
+    fun getVersionMap(): Map<ResourceLocation, String> = definitions.mapValues { it.value.contractVersion }
+    fun parseFailureCount(): Long = parseFailures.get()
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiServerRuntimeFactory.kt

@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.MoValue
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.api.molang.function.GeneralMoLangFunctions
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.world.item.Item
+import net.minecraft.world.item.ItemStack
+
+/** Builds a server runtime with only UI-scoped capabilities. It never exposes q.player. */
+internal object MolangUiServerRuntimeFactory {
+    fun create(session: MolangUiSession): MoLangRuntime = MoLangRuntime().also { runtime ->
+        GeneralMoLangFunctions.functions.filterKeys { it in MolangUiParameters.GENERAL_FUNCTIONS }.forEach { (name, function) ->
+            runtime.environment.query.addFunction(name) { function(it) }
+        }
+        runtime.environment.query.addFunction("parameter") { params ->
+            session.getParameter(params.getString(0))?.asMoLangValue() ?: DoubleValue.ZERO
+        }
+        runtime.environment.query.addFunction("update_parameter") { params ->
+            val name = params.getString(0)
+            val raw = params.get<MoValue>(1).asString()
+            val type = session.contract.parameters[name]?.type
+            DoubleValue(type != null && ParameterValue.parse(type, raw)?.let { session.updateParameters(mapOf(name to it)) } == true)
+        }
+        runtime.environment.query.addFunction("update_variable") { params ->
+            val value = params.getDouble(1)
+            DoubleValue(value.isFinite() && session.updateVariables(mapOf(params.getString(0) to value)))
+        }
+        runtime.environment.query.addFunction("close_ui") {
+            session.close()
+            DoubleValue.ONE
+        }
+        runtime.environment.query.addFunction("count_item") { params ->
+            val item = validatedItem(params.getString(0)) ?: return@addFunction DoubleValue.ZERO
+            DoubleValue(session.player.inventory.items.filter { it.`is`(item) }.sumOf(ItemStack::getCount).toDouble())
+        }
+        runtime.environment.query.addFunction("remove_items") { params ->
+            val item = validatedItem(params.getString(0)) ?: return@addFunction DoubleValue.ZERO
+            val count = params.getDouble(1).asNonnegativeInt() ?: return@addFunction DoubleValue.ZERO
+            DoubleValue(count > 0 && removeAtomically(session, item, count))
+        }
+        runtime.environment.query.addFunction("slot") { params ->
+            val index = params.getDouble(1).asNonnegativeInt() ?: return@addFunction StringValue("")
+            val stack = session.containerMenu?.getStack(params.getString(0), index)
+            if (stack == null || stack.isEmpty) StringValue("") else StringValue(BuiltInRegistries.ITEM.getKey(stack.item).toString())
+        }
+        runtime.environment.query.addFunction("replace_slot") { params ->
+            val itemId = params.getString(5)
+            validatedItem(itemId) ?: return@addFunction DoubleValue.ZERO
+            val inputIndex = params.getDouble(1).asNonnegativeInt() ?: return@addFunction DoubleValue.ZERO
+            val consumeCount = params.getDouble(2).asNonnegativeInt() ?: return@addFunction DoubleValue.ZERO
+            val outputIndex = params.getDouble(4).asNonnegativeInt() ?: return@addFunction DoubleValue.ZERO
+            val outputCount = params.getDouble(6).asNonnegativeInt() ?: return@addFunction DoubleValue.ZERO
+            val result = session.containerMenu?.replaceSlotAtomically(
+                params.getString(0), inputIndex, consumeCount,
+                params.getString(3), outputIndex, itemId, outputCount
+            ) ?: false
+            DoubleValue(result)
+        }
+    }
+
+    private fun validatedItem(raw: String) = ResourceLocation.tryParse(raw)
+        ?.takeIf(BuiltInRegistries.ITEM::containsKey)
+        ?.let(BuiltInRegistries.ITEM::get)
+
+    private fun Double.asNonnegativeInt(): Int? =
+        takeIf { it.isFinite() && it in 0.0..Int.MAX_VALUE.toDouble() && it % 1.0 == 0.0 }?.toInt()
+
+    private fun removeAtomically(session: MolangUiSession, item: Item, count: Int): Boolean {
+        val stacks = session.player.inventory.items.filter { it.`is`(item) }
+        if (stacks.sumOf(ItemStack::getCount) < count) return false
+        var remaining = count
+        stacks.forEach { stack ->
+            if (remaining == 0) return@forEach
+            val removed = minOf(stack.count, remaining)
+            stack.shrink(removed)
+            remaining -= removed
+        }
+        session.player.inventory.setChanged()
+        return true
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiSession.kt

@@ -0,0 +1,317 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.core.BlockPos
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.molangui.container.CollectionSource
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
+import com.cobblemon.mod.common.api.molangui.container.ServerCollectionSetup
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiClosePacket
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiOpenPacket
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiUpdatePacket
+import com.cobblemon.mod.common.util.asExpressionLike
+import net.minecraft.network.chat.Component
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.world.Container
+import net.minecraft.world.MenuProvider
+import net.minecraft.world.SimpleContainer
+import net.minecraft.world.entity.player.Inventory
+import net.minecraft.world.entity.player.Player
+import net.minecraft.world.inventory.AbstractContainerMenu
+import java.util.UUID
+
+internal class MolangUiSession(
+    override val sessionId: UUID,
+    override val uiId: ResourceLocation,
+    val player: ServerPlayer,
+    val contract: MolangUiContract,
+    initialParameters: Map<String, ParameterValue>,
+    val blockPos: BlockPos?
+) : MolangUiSessionHandle {
+    override var state: SessionState = SessionState.OPENING
+        private set
+    private val parameters = initialParameters.toMutableMap()
+    internal var containerMenu: MolangContainerMenu? = null
+    private var clientWasNotified = false
+
+    override fun updateParameters(parameters: Map<String, ParameterValue>): Boolean {
+        assertServerThread()
+        if (state != SessionState.ACTIVE || !validateParameterUpdate(parameters)) return false
+        MolangUiUpdatePacket(sessionId, parameters.mapValues { it.value.asWireValue() }).sendToPlayer(player)
+        this.parameters.putAll(parameters)
+        return true
+    }
+
+    override fun updateVariables(variables: Map<String, Double>): Boolean {
+        assertServerThread()
+        if (state != SessionState.ACTIVE || variables.size > MolangUiPacketLimits.MAX_ACTION_FIELDS) return false
+        try {
+            MolangUiParameters.validateVariables(variables)
+        } catch (error: IllegalArgumentException) {
+            return false
+        }
+        MolangUiUpdatePacket(sessionId, emptyMap(), variables).sendToPlayer(player)
+        return true
+    }
+
+    override fun close() = closeInternal(notifyClient = true, closeMenu = true)
+
+    internal fun closeFromClient() = closeInternal(notifyClient = false, closeMenu = true)
+    internal fun closeFromMenu() = closeInternal(notifyClient = true, closeMenu = false)
+
+    internal fun activate() {
+        assertServerThread()
+        check(state == SessionState.OPENING)
+        state = SessionState.ACTIVE
+    }
+
+    internal fun notifyOpened(hasContainer: Boolean) {
+        check(state == SessionState.OPENING)
+        clientWasNotified = true
+        MolangUiOpenPacket(
+            sessionId, uiId, contract.contractVersion, contract.mode,
+            parameters.mapValues { it.value.asWireValue() }, contract.exclusive,
+            hasContainer, contract.collections.mapValues { it.value.layout }
+        ).sendToPlayer(player)
+    }
+
+    internal fun getParameter(name: String): ParameterValue? = parameters[name]
+
+    private fun validateParameterUpdate(values: Map<String, ParameterValue>): Boolean =
+        values.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS && values.all { (name, value) ->
+            contract.parameters[name]?.type == value.type && value.asWireValue().length <= MolangUiPacketLimits.MAX_VALUE_LENGTH
+        }
+
+    private fun closeInternal(notifyClient: Boolean, closeMenu: Boolean) {
+        assertServerThread()
+        if (state == SessionState.CLOSING || state == SessionState.CLOSED) return
+        state = SessionState.CLOSING
+        val menu = containerMenu
+        containerMenu = null
+        try {
+            cleanup("notify client") {
+                if (notifyClient && clientWasNotified) MolangUiClosePacket(sessionId).sendToPlayer(player)
+            }
+            cleanup("close container") {
+                if (closeMenu && menu != null && player.containerMenu === menu) player.closeContainer()
+            }
+            cleanup("release items") { menu?.releaseOwnedItemsOnce(player) }
+        } finally {
+            state = SessionState.CLOSED
+            MolangUiService.remove(sessionId)
+        }
+    }
+
+    private inline fun cleanup(operation: String, action: () -> Unit) {
+        try {
+            action()
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.error("Failed to $operation while closing Molang UI session $sessionId", error)
+        }
+    }
+
+    private fun assertServerThread() {
+        check(player.server.isSameThread) { "Molang UI sessions are confined to the server thread" }
+    }
+}
+
+object MolangUiService {
+    private const val MAX_OVERLAYS_PER_PLAYER = 8
+    private val sessions = linkedMapOf<UUID, MolangUiSession>()
+    private val disabled = mutableMapOf<UUID, Set<ResourceLocation>>()
+    private val rateLimiters = mutableMapOf<UUID, MolangUiRateLimiter>()
+
+    fun open(player: ServerPlayer, request: MolangUiOpenRequest): MolangUiOpenResult {
+        check(player.server.isSameThread) { "MolangUiService.open must run on the server thread" }
+        val contract = MolangUiContractRegistry.getContract(request.uiId)
+            ?: return MolangUiOpenResult.Rejected(RejectionReason.UNKNOWN_UI, "Unknown Molang UI '${request.uiId}'")
+        // Intentionally does not wait for the compatibility round-trip; the client re-validates on open.
+        if (disabled[player.uuid]?.contains(request.uiId) == true) {
+            return MolangUiOpenResult.Rejected(RejectionReason.INCOMPATIBLE_CLIENT, "Client view is missing or incompatible")
+        }
+        val parameters = resolveParameters(contract, request.parameters)
+            ?: return MolangUiOpenResult.Rejected(RejectionReason.INVALID_PARAMETERS, "Parameters do not match the contract")
+        val playerSessions = sessions.values.filter { it.player.uuid == player.uuid && it.state != SessionState.CLOSED }
+        if (contract.mode == GuiMode.OVERLAY && playerSessions.count { it.contract.mode == GuiMode.OVERLAY } >= MAX_OVERLAYS_PER_PLAYER) {
+            return MolangUiOpenResult.Rejected(RejectionReason.SESSION_LIMIT, "At most $MAX_OVERLAYS_PER_PLAYER overlays may be active")
+        }
+        val collectionSetup = try {
+            createCollections(player, contract, request.blockPos)
+                ?: return MolangUiOpenResult.Rejected(RejectionReason.INVALID_CONTAINER, "The bound block container is unavailable or invalid")
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.error("Failed to prepare Molang UI '${request.uiId}'", error)
+            return MolangUiOpenResult.Rejected(RejectionReason.INTERNAL_ERROR, error.message ?: "Unable to prepare UI")
+        }
+        val session = MolangUiSession(UUID.randomUUID(), request.uiId, player, contract, parameters, request.blockPos)
+        sessions[session.sessionId] = session
+        MolangUiDiagnostics.sessionOpened()
+        return try {
+            if (contract.mode == GuiMode.SCREEN) {
+                playerSessions.filter { it.contract.mode == GuiMode.SCREEN }.forEach(MolangUiSession::close)
+            }
+            check(session.state == SessionState.OPENING) { "Molang UI closed while replacing its previous screen" }
+            MolangUiActionRegistry.executeOnOpen(session)
+            check(session.state == SessionState.OPENING) { "Molang UI closed during on_open" }
+            val hasContainer = contract.collections.isNotEmpty()
+            session.notifyOpened(hasContainer)
+            check(session.state == SessionState.OPENING) { "Molang UI closed while notifying the client" }
+            if (hasContainer) openContainer(session, collectionSetup)
+            session.activate()
+            MolangUiOpenResult.Opened(session)
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.error("Failed to open Molang UI '${request.uiId}'", error)
+            session.close()
+            MolangUiOpenResult.Rejected(RejectionReason.INTERNAL_ERROR, error.message ?: "Unable to open UI")
+        }
+    }
+
+    fun handleAction(sessionId: UUID, player: ServerPlayer, actionRaw: String, raw: Map<String, String>) {
+        check(player.server.isSameThread)
+        val session = sessions[sessionId]
+        if (session == null || session.player.uuid != player.uuid || session.state != SessionState.ACTIVE) {
+            MolangUiDiagnostics.actionRejected()
+            return
+        }
+        if (!rateLimiters.getOrPut(sessionId) { MolangUiRateLimiter() }.tryAcquire()) {
+            MolangUiDiagnostics.actionRejected()
+            return
+        }
+        val action = runCatching { ActionKey(actionRaw) }.getOrNull()
+        val definition = action?.let(session.contract.actions::get)
+        if (action == null || definition == null || raw.keys != definition.parameters.keys || raw.size > MolangUiPacketLimits.MAX_ACTION_FIELDS) {
+            MolangUiDiagnostics.actionRejected()
+            return
+        }
+        val typed = linkedMapOf<String, ParameterValue>()
+        for ((name, type) in definition.parameters) {
+            val value = raw[name]
+            if (value == null || value.length > MolangUiPacketLimits.MAX_VALUE_LENGTH) {
+                MolangUiDiagnostics.actionRejected(); return
+            }
+            typed[name] = ParameterValue.parse(type, value) ?: run { MolangUiDiagnostics.actionRejected(); return }
+        }
+        try {
+            MolangUiActionRegistry.dispatch(session, action, typed)
+        } catch (error: Exception) {
+            MolangUiDiagnostics.actionRejected()
+            Cobblemon.LOGGER.error("Molang UI action '${action.value}' failed in session $sessionId", error)
+        }
+    }
+
+    fun handleClosed(sessionId: UUID, player: ServerPlayer) {
+        val session = sessions[sessionId] ?: return
+        if (session.player.uuid == player.uuid) session.closeFromClient() else MolangUiDiagnostics.actionRejected()
+    }
+
+    fun applyCompatibility(player: ServerPlayer, clientVersions: Map<ResourceLocation, String>) {
+        val incompatible = MolangUiContractRegistry.versions().filter { (id, version) -> clientVersions[id] != version }.keys
+        if (incompatible.isEmpty()) disabled.remove(player.uuid) else disabled[player.uuid] = incompatible
+        MolangUiDiagnostics.compatibilityFailures(incompatible.size.toLong())
+        sessions.values.filter { it.player.uuid == player.uuid && it.uiId in incompatible }.toList().forEach(MolangUiSession::close)
+    }
+
+    fun requestCompatibility(player: ServerPlayer) = MolangUiContractRegistry.sync(player)
+
+    fun closeAllForPlayer(playerId: UUID) {
+        sessions.values.filter { it.player.uuid == playerId }.toList().forEach(MolangUiSession::close)
+        disabled.remove(playerId)
+    }
+
+    fun closeAll() {
+        sessions.values.toList().forEach(MolangUiSession::close)
+        disabled.clear()
+    }
+
+    fun getSession(sessionId: UUID): MolangUiSessionHandle? = sessions[sessionId]
+    fun getSessionForPlayer(player: ServerPlayer, id: ResourceLocation): MolangUiSessionHandle? =
+        sessions.values.firstOrNull { it.player.uuid == player.uuid && it.uiId == id && it.state == SessionState.ACTIVE }
+    fun activeSessionCount(): Int = sessions.size
+
+    internal fun remove(sessionId: UUID) {
+        if (sessions.remove(sessionId) != null) MolangUiDiagnostics.sessionClosed()
+        rateLimiters.remove(sessionId)
+    }
+
+    private fun resolveParameters(contract: MolangUiContract, supplied: Map<String, ParameterValue>): Map<String, ParameterValue>? {
+        if (supplied.keys.any { it !in contract.parameters }) return null
+        val values = linkedMapOf<String, ParameterValue>()
+        for ((name, definition) in contract.parameters) {
+            val suppliedValue = supplied[name]
+            val value = suppliedValue ?: when (val default = definition.default) {
+                is ParameterDefault.Literal -> default.value
+                is ParameterDefault.Molang -> runCatching {
+                    val result = default.expression.asExpressionLike().resolveString(MoLangRuntime())
+                    ParameterValue.parse(definition.type, result)
+                }.getOrNull()
+                null -> null
+            }
+            if (value == null) {
+                if (definition.required) return null else continue
+            }
+            if (value.type != definition.type || value.asWireValue().length > MolangUiPacketLimits.MAX_VALUE_LENGTH) return null
+            values[name] = value
+        }
+        return values
+    }
+
+    private fun createCollections(
+        player: ServerPlayer,
+        contract: MolangUiContract,
+        blockPos: BlockPos?
+    ): List<ServerCollectionSetup>? {
+        val setups = mutableListOf<ServerCollectionSetup>()
+        for ((name, config) in contract.collections) {
+            val container: Container = when (config.source) {
+                CollectionSource.SESSION -> SimpleContainer(config.size)
+                CollectionSource.BLOCK_ENTITY -> {
+                    val pos = blockPos ?: return null
+                    if (player.distanceToSqr(pos.x + .5, pos.y + .5, pos.z + .5) > 64.0 || !player.level().hasChunk(pos.x shr 4, pos.z shr 4)) return null
+                    val blockEntity = player.level().getBlockEntity(pos)
+                    if (blockEntity !is Container) return null
+                    if ((config.slots ?: (0 until config.size).toList()).any { it >= blockEntity.containerSize }) return null
+                    blockEntity
+                }
+            }
+            setups += ServerCollectionSetup(name, config, container)
+        }
+        return setups
+    }
+
+    private fun openContainer(session: MolangUiSession, setup: List<ServerCollectionSetup>) {
+        val owned = setup.filter { it.config.source == CollectionSource.SESSION }.mapTo(mutableSetOf()) { it.name }
+        val opened = session.player.openMenu(object : MenuProvider {
+            override fun getDisplayName() = Component.empty()
+            override fun createMenu(containerId: Int, inventory: Inventory, player: Player): AbstractContainerMenu =
+                MolangContainerMenu(containerId, inventory).also { menu ->
+                    check(session.state == SessionState.OPENING) { "Molang UI closed before creating its container" }
+                    session.containerMenu = menu
+                    menu.initializeServer(
+                        setup, session.contract.shiftClickRules, owned, session.contract.closeItemsPolicy,
+                        owner = session.player, session = session, blockPos = session.blockPos
+                    )
+                }
+        })
+        check(opened.isPresent) { "player rejected the Molang UI container menu" }
+    }
+}
+
+internal class MolangUiRateLimiter(private var lastNanos: Long = System.nanoTime()) {
+    private var tokens = 40.0
+    fun tryAcquire(now: Long = System.nanoTime()): Boolean {
+        val elapsed = (now - lastNanos).coerceAtLeast(0) / 1_000_000_000.0
+        tokens = minOf(40.0, tokens + elapsed * 20.0)
+        lastNanos = now
+        if (tokens < 1.0) return false
+        tokens -= 1.0
+        return true
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/container/CollectionConfig.kt

@@ -0,0 +1,94 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.container
+
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.core.registries.Registries
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.tags.TagKey
+import net.minecraft.world.item.ItemStack
+
+data class CollectionConfig(
+    val size: Int,
+    val allowedItems: List<ItemFilterEntry> = emptyList(),
+    val takeOnly: Boolean = false,
+    val source: CollectionSource = CollectionSource.SESSION,
+    val slots: List<Int>? = null,
+    val layout: SlotLayout
+) {
+    init {
+        require(size in 1..MAX_COLLECTION_SIZE) { "collection size must be in 1..$MAX_COLLECTION_SIZE" }
+        require(layout.slotCount == size) { "collection size must match its grid dimensions" }
+        require(slots == null || slots.size == size) { "block collection slot mapping must match collection size" }
+        require(slots == null || slots.distinct().size == slots.size) { "block collection slot mapping contains duplicates" }
+        require(slots == null || slots.all { it >= 0 }) { "block collection slot indices must be nonnegative" }
+        require(source == CollectionSource.BLOCK_ENTITY || slots == null) { "slot mapping is only valid for block_entity sources" }
+    }
+
+    fun isItemAllowed(stack: ItemStack): Boolean {
+        return !takeOnly && matchesFilter(stack)
+    }
+
+    fun matchesFilter(stack: ItemStack): Boolean {
+        if (allowedItems.isEmpty()) return true
+        return allowedItems.any { it.matches(stack) }
+    }
+
+    companion object { const val MAX_COLLECTION_SIZE = 256 }
+}
+
+enum class CollectionSource(val id: String) {
+    SESSION("session"), BLOCK_ENTITY("block_entity");
+    companion object { fun fromId(id: String) = entries.firstOrNull { it.id == id } }
+}
+
+data class ShiftClickRule(
+    val from: List<String>,
+    val to: List<String>
+) {
+    companion object {
+        const val PLAYER_INVENTORY = "player_inventory"
+        const val PLAYER_HOTBAR = "player_hotbar"
+        val PLAYER_SIDE_NAMES = listOf(PLAYER_INVENTORY, PLAYER_HOTBAR)
+    }
+}
+
+sealed interface ItemFilterEntry {
+    fun matches(stack: ItemStack): Boolean
+
+    data class Item(val id: ResourceLocation) : ItemFilterEntry {
+        override fun matches(stack: ItemStack): Boolean {
+            if (!BuiltInRegistries.ITEM.containsKey(id)) return false
+            return stack.`is`(BuiltInRegistries.ITEM.get(id))
+        }
+    }
+
+    data class Tag(val id: ResourceLocation) : ItemFilterEntry {
+        override fun matches(stack: ItemStack): Boolean = stack.`is`(TagKey.create(Registries.ITEM, id))
+    }
+
+    companion object {
+        fun parse(value: String): ItemFilterEntry? {
+            val trimmed = value.trim()
+            val isTag = trimmed.startsWith('#')
+            val id = ResourceLocation.tryParse(if (isTag) trimmed.substring(1) else trimmed) ?: return null
+            return if (isTag) Tag(id) else Item(id)
+        }
+    }
+}
+
+enum class CloseItemsPolicy(val id: String) {
+    DROP("drop"),
+    RETURN("return"),
+    DESTROY("destroy");
+
+    companion object {
+        fun fromId(id: String): CloseItemsPolicy? = entries.firstOrNull { it.id == id }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/container/CollectionSetup.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.container
+
+import net.minecraft.world.Container
+
+data class ServerCollectionSetup(
+    val name: String,
+    val config: CollectionConfig,
+    val container: Container
+)

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/container/MolangContainerMenu.kt

@@ -0,0 +1,300 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.container
+
+import com.cobblemon.mod.common.CobblemonMenuType
+import com.cobblemon.mod.common.api.molangui.MolangUiSession
+import com.cobblemon.mod.common.api.molangui.SessionState
+import net.minecraft.core.BlockPos
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.world.Container
+import net.minecraft.world.SimpleContainer
+import net.minecraft.world.entity.player.Inventory
+import net.minecraft.world.entity.player.Player
+import net.minecraft.world.inventory.AbstractContainerMenu
+import net.minecraft.world.inventory.Slot
+import net.minecraft.world.item.ItemStack
+
+private data class CollectionSlotRange(
+    val name: String,
+    val startIndex: Int,
+    val endIndex: Int
+)
+
+class MolangContainerMenu(
+    containerId: Int,
+    private val playerInventory: Inventory
+) : AbstractContainerMenu(CobblemonMenuType.MOLANG_UI, containerId) {
+
+    private val collections: MutableMap<String, Container> = mutableMapOf()
+
+    private val collectionRanges: MutableList<CollectionSlotRange> = mutableListOf()
+    private var playerInventoryStart: Int = 0
+    private var playerHotbarStart: Int = 0
+    private var playerHotbarEnd: Int = 0
+
+    private var shiftClickRules: List<ShiftClickRule> = emptyList()
+
+    private val collectionConfigs: MutableMap<String, CollectionConfig> = mutableMapOf()
+    private val sessionOwnedCollections: MutableSet<String> = mutableSetOf()
+    private var closeItemsPolicy: CloseItemsPolicy = CloseItemsPolicy.RETURN
+    private var owner: ServerPlayer? = null
+    private var owningSession: MolangUiSession? = null
+    private var blockPos: BlockPos? = null
+    private var originalBlockContainer: Container? = null
+    private var ownedItemsReleased = false
+
+    internal fun initializeServer(
+        collectionSetup: List<ServerCollectionSetup>,
+        shiftClickRules: List<ShiftClickRule> = emptyList(),
+        sessionOwned: Set<String> = emptySet(),
+        closeItemsPolicy: CloseItemsPolicy = CloseItemsPolicy.RETURN,
+        owner: ServerPlayer,
+        session: MolangUiSession,
+        blockPos: BlockPos? = null
+    ) {
+        this.shiftClickRules = shiftClickRules
+        this.sessionOwnedCollections.addAll(sessionOwned)
+        this.closeItemsPolicy = closeItemsPolicy
+        this.owner = owner
+        this.owningSession = session
+        this.blockPos = blockPos
+
+        for (setup in collectionSetup) {
+            val (name, config, container) = setup
+            collections[name] = container
+            if (config.source == CollectionSource.BLOCK_ENTITY) originalBlockContainer = container
+            collectionConfigs[name] = config
+
+            val startIndex = slots.size
+            addCollectionSlots(name, config, container)
+            val endIndex = slots.size
+            collectionRanges.add(CollectionSlotRange(name, startIndex, endIndex))
+        }
+
+        addPlayerSlots()
+    }
+
+    fun initializeClient(
+        collectionLayouts: Map<String, SlotLayout>
+    ) {
+        SlotLayout.validateCollections(collectionLayouts)
+        for ((name, layout) in collectionLayouts) {
+            val config = CollectionConfig(size = layout.slotCount, layout = layout)
+            val container = SimpleContainer(layout.slotCount)
+            collections[name] = container
+            collectionConfigs[name] = config
+
+            val startIndex = slots.size
+            addCollectionSlots(name, config, container)
+            val endIndex = slots.size
+            collectionRanges.add(CollectionSlotRange(name, startIndex, endIndex))
+        }
+
+        addPlayerSlots()
+    }
+
+    private fun addCollectionSlots(
+        name: String,
+        config: CollectionConfig,
+        container: Container
+    ) {
+        val layout = config.layout
+        var containerIndex = 0
+        for (row in 0 until layout.rows) {
+            for (col in 0 until layout.columns) {
+                val x = layout.offsetX + col * (layout.slotWidth + layout.spacingX)
+                val y = layout.offsetY + row * (layout.slotHeight + layout.spacingY)
+                val mappedIndex = config.slots?.getOrNull(containerIndex) ?: containerIndex
+                require(mappedIndex in 0 until container.containerSize) { "collection '$name' maps outside its container" }
+                addSlot(MolangSlot(
+                    container = container,
+                    index = mappedIndex,
+                    x = x,
+                    y = y,
+                    collectionConfig = config
+                ))
+                containerIndex++
+            }
+        }
+    }
+
+    private fun addPlayerSlots() {
+        playerInventoryStart = slots.size
+
+        for (row in 0 until 3) {
+            for (col in 0 until 9) {
+                val inventoryIndex = 9 + row * 9 + col
+                val x = 8 + col * 18
+                val y = 84 + row * 18
+                addSlot(Slot(playerInventory, inventoryIndex, x, y))
+            }
+        }
+
+        playerHotbarStart = slots.size
+
+        for (col in 0 until 9) {
+            addSlot(Slot(playerInventory, col, 8 + col * 18, 142))
+        }
+
+        playerHotbarEnd = slots.size
+    }
+
+    override fun quickMoveStack(player: Player, index: Int): ItemStack {
+        val slot = slots.getOrNull(index) ?: return ItemStack.EMPTY
+        if (!slot.hasItem() || !slot.mayPickup(player)) return ItemStack.EMPTY
+
+        val slotItem = slot.item
+        val original = slotItem.copy()
+
+        val clickedCollection = collectionRanges.firstOrNull { index >= it.startIndex && index < it.endIndex }
+        val isPlayerInventory = index >= playerInventoryStart && index < playerInventoryEnd()
+        val isHotbar = index >= playerHotbarStart && index < playerHotbarEnd
+
+        val sourceName = clickedCollection?.name ?: when {
+            isPlayerInventory -> ShiftClickRule.PLAYER_INVENTORY
+            isHotbar -> ShiftClickRule.PLAYER_HOTBAR
+            else -> return ItemStack.EMPTY
+        }
+        val destinations = findShiftClickRule(sourceName)?.to ?: if (clickedCollection != null) {
+            ShiftClickRule.PLAYER_SIDE_NAMES
+        } else {
+            collectionRanges.map { it.name }
+        }
+        if (!tryMoveToCollections(slotItem, slot, destinations)) return ItemStack.EMPTY
+
+        if (slotItem.isEmpty) {
+            slot.setByPlayer(ItemStack.EMPTY)
+        } else {
+            slot.setChanged()
+        }
+
+        if (slotItem.count == original.count) return ItemStack.EMPTY
+
+        slot.onTake(player, slotItem)
+        return original
+    }
+
+    override fun stillValid(player: Player): Boolean {
+        val expectedOwner = owner ?: return true // client menu
+        if (player.uuid != expectedOwner.uuid || owningSession?.state != SessionState.ACTIVE) return false
+        val pos = blockPos ?: return true
+        if (player.distanceToSqr(pos.x + .5, pos.y + .5, pos.z + .5) > 64.0) return false
+        return player.level().getBlockEntity(pos) === originalBlockContainer
+    }
+
+    override fun removed(player: Player) {
+        try {
+            super.removed(player)
+        } finally {
+            try {
+                releaseOwnedItemsOnce(player)
+            } finally {
+                owningSession?.closeFromMenu()
+            }
+        }
+    }
+
+    fun releaseOwnedItemsOnce(player: Player) {
+        if (ownedItemsReleased) return
+        ownedItemsReleased = true
+        for (name in sessionOwnedCollections) {
+            val container = collections[name] ?: continue
+            when (closeItemsPolicy) {
+                CloseItemsPolicy.DROP -> clearContainer(player, container)
+                CloseItemsPolicy.RETURN -> returnItemsToPlayer(player, container)
+                CloseItemsPolicy.DESTROY -> container.clearContent()
+            }
+        }
+    }
+
+    fun getStack(collection: String, index: Int): ItemStack? {
+        return collectionSlot(collection, index)?.item
+    }
+
+    fun replaceSlotAtomically(
+        inputCollection: String,
+        inputIndex: Int,
+        consumeCount: Int,
+        outputCollection: String,
+        outputIndex: Int,
+        outputItemId: String,
+        outputCount: Int
+    ): Boolean {
+        if (consumeCount <= 0 || outputCount <= 0) return false
+        val input = collectionSlot(inputCollection, inputIndex) ?: return false
+        val output = collectionSlot(outputCollection, outputIndex) ?: return false
+        val id = ResourceLocation.tryParse(outputItemId)?.takeIf(BuiltInRegistries.ITEM::containsKey) ?: return false
+        val produced = ItemStack(BuiltInRegistries.ITEM.get(id), outputCount)
+        val outputConfig = collectionConfigs[outputCollection] ?: return false
+        if (!outputConfig.matchesFilter(produced)) return false
+        val source = input.item
+        if (source.count < consumeCount) return false
+        val remaining = source.copyWithCount(source.count - consumeCount)
+        val sameSlot = input.container === output.container && input.containerSlot == output.containerSlot
+        val existing = if (sameSlot) remaining else output.item
+        if (!existing.isEmpty && !ItemStack.isSameItemSameComponents(existing, produced)) return false
+        val limit = output.getMaxStackSize(produced)
+        if (existing.count > limit || outputCount > limit - existing.count) return false
+        val result = if (existing.isEmpty) produced else existing.copyWithCount(existing.count + outputCount)
+        if (!sameSlot) input.set(remaining)
+        output.set(result)
+        return true
+    }
+
+    private fun playerInventoryEnd() = playerHotbarStart
+
+    private fun findShiftClickRule(fromCollection: String): ShiftClickRule? {
+        return shiftClickRules.firstOrNull { rule ->
+            rule.from.contains(fromCollection)
+        }
+    }
+
+    private fun collectionSlot(name: String, index: Int): Slot? {
+        val range = collectionRanges.firstOrNull { it.name == name } ?: return null
+        if (index !in 0 until range.endIndex - range.startIndex) return null
+        return slots[range.startIndex + index]
+    }
+
+    private fun tryMoveToCollections(stack: ItemStack, source: Slot, toNames: List<String>): Boolean {
+        val originalCount = stack.count
+        for (name in toNames) {
+            val indices = when (name) {
+                ShiftClickRule.PLAYER_INVENTORY -> playerInventoryStart until playerHotbarStart
+                ShiftClickRule.PLAYER_HOTBAR -> playerHotbarStart until playerHotbarEnd
+                else -> collectionRanges.firstOrNull { it.name == name }?.let { it.startIndex until it.endIndex } ?: continue
+            }
+            for (merge in listOf(true, false)) {
+                for (index in indices) {
+                    if (stack.isEmpty) return true
+                    val destination = slots[index]
+                    if (destination.container === source.container && destination.containerSlot == source.containerSlot) continue
+                    if (destination.hasItem() != merge || destination.item.count >= destination.getMaxStackSize(stack)) continue
+                    destination.safeInsert(stack)
+                }
+            }
+        }
+        return stack.count < originalCount
+    }
+
+    private fun returnItemsToPlayer(player: Player, container: Container) {
+        val inventory = player.inventory
+        for (i in 0 until container.containerSize) {
+            val stack = container.getItem(i)
+            if (!stack.isEmpty) {
+                if (!inventory.add(stack)) {
+                    player.drop(stack, false)
+                }
+                container.setItem(i, ItemStack.EMPTY)
+            }
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/container/MolangSlot.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.container
+
+import net.minecraft.world.Container
+import net.minecraft.world.inventory.Slot
+import net.minecraft.world.item.ItemStack
+
+class MolangSlot(
+    container: Container,
+    index: Int,
+    x: Int,
+    y: Int,
+    private val collectionConfig: CollectionConfig
+) : Slot(container, index, x, y) {
+
+    override fun mayPlace(stack: ItemStack): Boolean {
+        return collectionConfig.isItemAllowed(stack) && container.canPlaceItem(containerSlot, stack)
+    }
+
+    override fun getMaxStackSize(stack: ItemStack): Int = container.getMaxStackSize(stack)
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/container/SlotLayout.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.container
+
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import net.minecraft.world.entity.player.Inventory
+
+data class SlotLayout(
+    val offsetX: Int,
+    val offsetY: Int,
+    val columns: Int,
+    val rows: Int,
+    val slotWidth: Int = 18,
+    val slotHeight: Int = 18,
+    val spacingX: Int = 0,
+    val spacingY: Int = 0
+) {
+    init {
+        require(columns in 1..MAX_GRID_DIMENSION && rows in 1..MAX_GRID_DIMENSION) { "slot grid dimensions must be in 1..$MAX_GRID_DIMENSION" }
+        validateAxis(offsetX, columns, slotWidth, spacingX)
+        validateAxis(offsetY, rows, slotHeight, spacingY)
+    }
+
+    val slotCount: Int get() = columns * rows
+
+    private fun validateAxis(offset: Int, count: Int, size: Int, spacing: Int) {
+        require(size > 0 && spacing >= 0) { "slot size must be positive and spacing nonnegative" }
+        val stride = size.toLong() + spacing
+        val lastSlotOffset = (count - 1) * stride
+        require(stride <= Int.MAX_VALUE && lastSlotOffset <= Int.MAX_VALUE && offset + lastSlotOffset + size <= Int.MAX_VALUE) {
+            "slot grid coordinates exceed the integer range"
+        }
+    }
+
+    companion object {
+        const val MAX_GRID_DIMENSION = 16
+
+        fun validateCollections(layouts: Map<String, SlotLayout>) {
+            require(layouts.size <= MolangUiPacketLimits.MAX_CONTRACTS) { "too many container collections" }
+            layouts.keys.forEach { name ->
+                MolangUiPacketLimits.requireIdentifier(name, "collection name")
+                require(name !in ShiftClickRule.PLAYER_SIDE_NAMES) { "collection name '$name' is reserved for player slots" }
+            }
+            // Native container packets encode slot indices as signed shorts.
+            require(layouts.values.sumOf { it.slotCount.toLong() } + Inventory.INVENTORY_SIZE <= Short.MAX_VALUE + 1L) {
+                "container slot indices exceed the native packet range"
+            }
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/event/ClientEventProcessor.kt

@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.event
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.struct.QueryStruct
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.molangui.widget.WidgetDefinition
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.cobblemon.mod.common.api.molangui.widget.WidgetType
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiActionPacket
+import com.cobblemon.mod.common.util.asExpressionLike
+import java.util.UUID
+
+object ClientEventProcessor {
+
+    fun processEvent(
+        event: WidgetEvent,
+        widget: WidgetDefinition,
+        runtime: MoLangRuntime,
+        sessionId: UUID
+    ) {
+        val context = QueryStruct(hashMapOf())
+        event.data.forEach { (key, value) -> context.addFunction(key) { DoubleValue(value) } }
+        context.addFunction("type") { StringValue(event.type.id) }
+        context.addFunction("widget_id") { StringValue(event.widgetId) }
+        val functions = runtime.environment.query.functions
+        val previous = functions.remove("event")
+        runtime.environment.query.addFunction("event") { context }
+        try {
+            widget.clientEvents[event.type.clientId]?.asExpressionLike()?.resolve(runtime)
+            widget.events[event.type.id]?.let { serverHandler ->
+                // Action arguments may depend on variables changed by the client handler.
+                val parameters = serverHandler.parameters.mapValues { (_, property) -> property.resolveString(runtime) }
+                require(parameters.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many action parameters" }
+                parameters.forEach { (name, value) ->
+                    MolangUiPacketLimits.requireIdentifier(name, "action parameter")
+                    MolangUiPacketLimits.requireValue(value, name)
+                }
+                MolangUiActionPacket(sessionId, serverHandler.action.value, parameters).sendToServer()
+            }
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.warn("Failed UI event {} on widget {}: {}", event.type.id, event.widgetId, error.message)
+        } finally {
+            functions.remove("event")
+            if (previous != null) functions["event"] = previous
+        }
+    }
+
+    fun hitTest(root: WidgetInstance, x: Float, y: Float): WidgetInstance? {
+        if (!root.visible || !root.enabled) return null
+        val isScrollPanel = root.definition.type == WidgetType.SCROLL_PANEL
+        if (isScrollPanel && !root.containsPoint(x, y)) return null
+
+        for (child in root.children.asReversed()) {
+            if (!child.visible) continue
+            val testY = if (isScrollPanel) y - root.scrollOffset else y
+            val hit = hitTest(child, x, testY)
+            if (hit != null) return hit
+        }
+        return if (root.containsPoint(x, y)) root else null
+    }
+}
+
+enum class WidgetEventType(val id: String) {
+    CLICK("on_click"),
+    CHANGE("on_change"),
+    HOVER_ENTER("on_hover_enter"),
+    HOVER_EXIT("on_hover_exit"),
+    SCROLL("on_scroll"),
+    KEY("on_key");
+
+    val clientId: String get() = "${id}_client"
+}
+
+data class WidgetEvent(
+    val type: WidgetEventType,
+    val widgetId: String,
+    val data: Map<String, Double> = emptyMap()
+)

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/layout/LayoutEngine.kt

@@ -0,0 +1,151 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.layout
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+
+object LayoutEngine {
+
+    fun layout(root: WidgetInstance, screenWidth: Float, screenHeight: Float, runtime: MoLangRuntime): Unit = root.withItemContext(runtime) {
+        require(screenWidth.isFinite() && screenHeight.isFinite() && screenWidth >= 0f && screenHeight >= 0f) {
+            "invalid screen dimensions"
+        }
+        if (!root.visible) return@withItemContext
+        root.computedWidth = root.widthExpr.resolve(screenWidth, runtime)
+        root.computedHeight = root.heightExpr.resolve(screenHeight, runtime)
+        val anchor = root.definition.getAnchor()
+        val offsetX = root.offsetXExpr.resolve(screenWidth, runtime)
+        val offsetY = root.offsetYExpr.resolve(screenHeight, runtime)
+        root.absoluteX = (screenWidth - root.computedWidth) * anchor.xFactor + offsetX
+        root.absoluteY = (screenHeight - root.computedHeight) * anchor.yFactor + offsetY
+        validateBounds(root)
+        layoutChildren(root, runtime)
+    }
+
+    private fun layoutChildren(parent: WidgetInstance, runtime: MoLangRuntime): Unit = parent.withItemContext(runtime) {
+        val layout = parent.definition.getStringProperty("layout")
+        val spacing = parent.definition.getProperty("spacing")?.resolveFloat(runtime) ?: 0f
+        require(spacing.isFinite()) { "non-finite spacing at '${parent.definition.id}'" }
+        val padding = parent.paddingProperties.map { it.resolveFloat(runtime) }
+        require(padding.size == 4 && padding.all { it.isFinite() && it >= 0f }) {
+            "invalid padding at '${parent.definition.id}'"
+        }
+        val innerWidth = (parent.computedWidth - padding[3] - padding[1]).coerceAtLeast(0f)
+        val innerHeight = (parent.computedHeight - padding[0] - padding[2]).coerceAtLeast(0f)
+        val innerX = parent.absoluteX + padding[3]
+        val innerY = parent.absoluteY + padding[0]
+
+        for (child in parent.children) {
+            if (!child.visible) continue
+            child.computedWidth = child.withItemContext(runtime) { child.widthExpr.resolve(innerWidth, runtime) }
+            child.computedHeight = child.withItemContext(runtime) { child.heightExpr.resolve(innerHeight, runtime) }
+        }
+
+        when (layout) {
+            "stack_vertical" -> layoutStackVertical(parent, innerX, innerY, innerWidth, innerHeight, spacing, runtime)
+            "stack_horizontal" -> layoutStackHorizontal(parent, innerX, innerY, innerWidth, innerHeight, spacing, runtime)
+            "grid" -> layoutGrid(parent, innerX, innerY, innerWidth, spacing, runtime)
+            else -> layoutAnchored(parent, innerX, innerY, innerWidth, innerHeight, runtime)
+        }
+
+        for (child in parent.children) {
+            if (!child.visible) continue
+            validateBounds(child)
+            layoutChildren(child, runtime)
+        }
+    }
+
+    private fun layoutAnchored(parent: WidgetInstance, innerX: Float, innerY: Float, innerWidth: Float, innerHeight: Float, runtime: MoLangRuntime) {
+        for (child in parent.children) {
+            if (!child.visible) continue
+            val anchor = child.definition.getAnchor()
+            val offsetX = child.withItemContext(runtime) { child.offsetXExpr.resolve(innerWidth, runtime) }
+            val offsetY = child.withItemContext(runtime) { child.offsetYExpr.resolve(innerHeight, runtime) }
+            child.absoluteX = innerX + (innerWidth - child.computedWidth) * anchor.xFactor + offsetX
+            child.absoluteY = innerY + (innerHeight - child.computedHeight) * anchor.yFactor + offsetY
+        }
+    }
+
+    private fun layoutStackVertical(parent: WidgetInstance, innerX: Float, innerY: Float, innerWidth: Float, innerHeight: Float, spacing: Float, runtime: MoLangRuntime) {
+        val visibleChildren = parent.children.filter { it.visible }
+        resolveFillSizes(visibleChildren, innerHeight, spacing, isVertical = true)
+        var currentY = innerY
+        for (child in visibleChildren) {
+            child.absoluteX = innerX + child.withItemContext(runtime) { child.offsetXExpr.resolve(innerWidth, runtime) }
+            child.absoluteY = currentY + child.withItemContext(runtime) { child.offsetYExpr.resolve(innerHeight, runtime) }
+            currentY += child.computedHeight + spacing
+        }
+    }
+
+    private fun layoutStackHorizontal(parent: WidgetInstance, innerX: Float, innerY: Float, innerWidth: Float, innerHeight: Float, spacing: Float, runtime: MoLangRuntime) {
+        val visibleChildren = parent.children.filter { it.visible }
+        resolveFillSizes(visibleChildren, innerWidth, spacing, isVertical = false)
+        var currentX = innerX
+        for (child in visibleChildren) {
+            child.absoluteX = currentX + child.withItemContext(runtime) { child.offsetXExpr.resolve(innerWidth, runtime) }
+            child.absoluteY = innerY + child.withItemContext(runtime) { child.offsetYExpr.resolve(innerHeight, runtime) }
+            currentX += child.computedWidth + spacing
+        }
+    }
+
+    private fun layoutGrid(parent: WidgetInstance, innerX: Float, innerY: Float, innerWidth: Float, spacing: Float, runtime: MoLangRuntime) {
+        val columns = (parent.definition.getProperty("columns")?.resolveInt(runtime) ?: 1).coerceIn(1, 256)
+        val visibleChildren = parent.children.filter { it.visible }
+        val cellWidth = ((innerWidth - spacing * (columns - 1)) / columns).coerceAtLeast(0f)
+        var col = 0
+        var currentY = innerY
+        var rowHeight = 0f
+        for (child in visibleChildren) {
+            if (child.widthExpr is SizeExpression.Fill) child.computedWidth = cellWidth
+            child.absoluteX = innerX + col * (cellWidth + spacing)
+            child.absoluteY = currentY
+            rowHeight = maxOf(rowHeight, child.computedHeight)
+            col++
+            if (col >= columns) {
+                col = 0
+                currentY += rowHeight + spacing
+                rowHeight = 0f
+            }
+        }
+    }
+
+    private fun resolveFillSizes(children: List<WidgetInstance>, totalSpace: Float, spacing: Float, isVertical: Boolean) {
+        val totalSpacing = spacing * (children.size - 1).coerceAtLeast(0)
+        var usedSpace = totalSpacing
+        var fillCount = 0
+        for (child in children) {
+            val expr = if (isVertical) child.heightExpr else child.widthExpr
+            if (expr is SizeExpression.Fill) {
+                fillCount++
+            } else {
+                usedSpace += if (isVertical) child.computedHeight else child.computedWidth
+            }
+        }
+        if (fillCount > 0) {
+            val fillSize = ((totalSpace - usedSpace) / fillCount).coerceAtLeast(0f)
+            for (child in children) {
+                val expr = if (isVertical) child.heightExpr else child.widthExpr
+                if (expr is SizeExpression.Fill) {
+                    if (isVertical) child.computedHeight = fillSize else child.computedWidth = fillSize
+                }
+            }
+        }
+    }
+
+    private fun validateBounds(widget: WidgetInstance) {
+        fun validAxis(position: Float, size: Float): Boolean =
+            position.isFinite() && size.isFinite() && size >= 0f && size.toDouble() <= Int.MAX_VALUE &&
+                position.toDouble() >= Int.MIN_VALUE && position.toDouble() + size.toDouble() <= Int.MAX_VALUE
+
+        require(validAxis(widget.absoluteX, widget.computedWidth) && validAxis(widget.absoluteY, widget.computedHeight)) {
+            "widget '${widget.definition.id}' has invalid or overflowing bounds"
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/layout/SizeExpression.kt

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.layout
+
+import com.cobblemon.mod.common.api.molang.ExpressionLike
+import com.cobblemon.mod.common.util.asExpressionLike
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.cobblemon.mod.common.api.molangui.widget.WidgetExpression
+
+sealed class SizeExpression {
+    abstract fun resolve(parentSize: Float, runtime: MoLangRuntime): Float
+
+    data class Pixels(val value: Float) : SizeExpression() {
+        init { require(value.isFinite()) { "pixel size must be finite" } }
+        override fun resolve(parentSize: Float, runtime: MoLangRuntime) = value
+    }
+
+    data class Percent(val fraction: Float) : SizeExpression() {
+        init { require(fraction.isFinite()) { "percentage must be finite" } }
+        override fun resolve(parentSize: Float, runtime: MoLangRuntime) = parentSize * fraction
+    }
+
+    data class PercentWithOffset(val fraction: Float, val offset: Float) : SizeExpression() {
+        init { require(fraction.isFinite() && offset.isFinite()) { "percentage and offset must be finite" } }
+        override fun resolve(parentSize: Float, runtime: MoLangRuntime) = parentSize * fraction + offset
+    }
+
+    data object Fill : SizeExpression() {
+        override fun resolve(parentSize: Float, runtime: MoLangRuntime) = parentSize
+    }
+
+    data class MolangSize(val expression: ExpressionLike) : SizeExpression() {
+        override fun resolve(parentSize: Float, runtime: MoLangRuntime) = expression.resolveFloat(runtime)
+    }
+
+    companion object {
+        private val PERCENT_PATTERN = Regex("""^(\d+(?:\.\d+)?)%$""")
+        private val PERCENT_OFFSET_PATTERN = Regex("""^(\d+(?:\.\d+)?)%\s*([+-])\s*(\d+(?:\.\d+)?)$""")
+
+        fun parse(value: Any): SizeExpression {
+            return when (value) {
+                is Number -> Pixels(value.toFloat())
+                is String -> parseString(value)
+                is WidgetExpression -> MolangSize(value.source.asExpressionLike())
+                else -> throw IllegalArgumentException("size must be a number, percentage, fill, or explicit Molang expression")
+            }
+        }
+
+        private fun parseString(value: String): SizeExpression {
+            val trimmed = value.trim()
+            return when {
+                trimmed == "fill" -> Fill
+                trimmed.toFloatOrNull() != null -> Pixels(trimmed.toFloat())
+                PERCENT_PATTERN.matches(trimmed) -> {
+                    val pct = PERCENT_PATTERN.matchEntire(trimmed)!!.groupValues[1].toFloat()
+                    Percent(pct / 100f)
+                }
+                PERCENT_OFFSET_PATTERN.matches(trimmed) -> {
+                    val match = PERCENT_OFFSET_PATTERN.matchEntire(trimmed)!!
+                    val pct = match.groupValues[1].toFloat() / 100f
+                    val sign = if (match.groupValues[2] == "+") 1f else -1f
+                    val offset = match.groupValues[3].toFloat() * sign
+                    PercentWithOffset(pct, offset)
+                }
+                else -> throw IllegalArgumentException("invalid size '$value'; Molang requires an explicit expression object")
+            }
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/template/TemplateLibraryRegistry.kt

@@ -0,0 +1,114 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.template
+
+import com.google.gson.Gson
+import com.google.gson.JsonObject
+import com.google.gson.JsonElement
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.packs.resources.ResourceManager
+import com.cobblemon.mod.common.Cobblemon
+
+
+object TemplateLibraryRegistry {
+    private val gson = Gson()
+    private val libraries: MutableMap<ResourceLocation, Map<String, TemplateDefinition>> = mutableMapOf()
+
+    private const val DIRECTORY = "molang_ui/templates"
+
+    fun reload(manager: ResourceManager) {
+        val next = mutableMapOf<ResourceLocation, Map<String, TemplateDefinition>>()
+        val resources = manager.listResources(DIRECTORY) { it.path.endsWith(".json") }
+        for ((location, resource) in resources) {
+            try {
+                val json = resource.openAsReader().use { gson.fromJson(it, JsonObject::class.java) }
+                val namespace = location.namespace
+                val name = location.path.removePrefix("$DIRECTORY/").removeSuffix(".json")
+                val libraryId = ResourceLocation.fromNamespaceAndPath(namespace, name)
+                val templates = parseTemplates(json, namespace)
+                next[libraryId] = templates
+            } catch (e: Exception) {
+                Cobblemon.LOGGER.error("Rejected Molang UI template library '$location': ${e.message}")
+            }
+        }
+        libraries.clear()
+        libraries.putAll(next)
+    }
+
+    fun getTemplate(qualifiedName: String): TemplateDefinition? {
+        val dotIndex = qualifiedName.lastIndexOf('.')
+        if (dotIndex == -1) return null
+        val libraryName = qualifiedName.substring(0, dotIndex)
+        val templateName = qualifiedName.substring(dotIndex + 1)
+        val library = if (':' in libraryName) {
+            ResourceLocation.tryParse(libraryName)?.let(libraries::get)
+        } else {
+            libraries.entries.singleOrNull { it.key.path == libraryName }?.value
+        }
+        return library?.get(templateName)
+    }
+
+    private fun parseTemplates(json: JsonObject, namespace: String): Map<String, TemplateDefinition> {
+        val result = mutableMapOf<String, TemplateDefinition>()
+        val templatesObj = json.getAsJsonObject("templates") ?: return result
+        for ((name, element) in templatesObj.entrySet()) {
+            val templateJson = element.asJsonObject
+            val inputs = mutableMapOf<String, TemplateInput>()
+            templateJson.getAsJsonObject("inputs")?.let { inputsObj ->
+                for ((inputName, inputEl) in inputsObj.entrySet()) {
+                    val inputObj = inputEl.asJsonObject
+                    val type = TemplateInputType.fromId(inputObj.get("type")?.asString ?: "string")
+                        ?: throw IllegalArgumentException("unknown template input type for '$inputName'")
+                    val default = inputObj.get("default")
+                    require(default == null || type.accepts(default)) { "invalid template default at '$name.$inputName'" }
+                    inputs[inputName] = TemplateInput(type, default)
+                }
+            }
+            val extends = templateJson.get("extends")?.asString
+            result[name] = TemplateDefinition(
+                name = name,
+                namespace = namespace,
+                inputs = inputs,
+                extends = extends,
+                body = templateJson
+            )
+        }
+        return result
+    }
+}
+
+data class TemplateDefinition(
+    val name: String,
+    val namespace: String,
+    val inputs: Map<String, TemplateInput> = emptyMap(),
+    val extends: String? = null,
+    val body: JsonObject
+)
+
+data class TemplateInput(
+    val type: TemplateInputType = TemplateInputType.STRING,
+    val default: JsonElement? = null
+)
+
+enum class TemplateInputType(val id: String) {
+    STRING("string"), NUMBER("number"), BOOLEAN("boolean"), EXPRESSION("expression"), OBJECT("object"), ARRAY("array");
+
+    fun accepts(value: JsonElement): Boolean = when (this) {
+        STRING -> value.isJsonPrimitive && value.asJsonPrimitive.isString
+        NUMBER -> value.isJsonPrimitive && value.asJsonPrimitive.isNumber && value.asDouble.isFinite()
+        BOOLEAN -> value.isJsonPrimitive && value.asJsonPrimitive.isBoolean
+        EXPRESSION -> value.isJsonObject && value.asJsonObject.size() == 1 && value.asJsonObject.get("molang")?.let {
+            it.isJsonPrimitive && it.asJsonPrimitive.isString
+        } == true
+        OBJECT -> value.isJsonObject
+        ARRAY -> value.isJsonArray
+    }
+
+    companion object { fun fromId(id: String) = entries.firstOrNull { it.id == id } }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/template/TemplateResolver.kt

@@ -0,0 +1,135 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.template
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetTree
+import com.google.gson.JsonArray
+import com.google.gson.JsonElement
+import com.google.gson.JsonObject
+import com.google.gson.JsonPrimitive
+
+object TemplateResolver {
+    fun resolve(root: JsonObject, localTemplates: Map<String, TemplateDefinition>): JsonObject =
+        Resolution(localTemplates).resolveNode(root, emptyList(), 1)
+
+    private class Resolution(private val localTemplates: Map<String, TemplateDefinition>) {
+        private var nodes = 0
+        private var values = 0
+
+        fun resolveNode(node: JsonObject, chain: List<String>, depth: Int): JsonObject {
+            require(depth <= WidgetTree.MAX_DEPTH) { "template expansion exceeds maximum widget depth ${WidgetTree.MAX_DEPTH}" }
+            require(++nodes <= WidgetTree.MAX_NODES) { "template expansion exceeds maximum widget count ${WidgetTree.MAX_NODES}" }
+            val result = JsonObject()
+            node.entrySet().filter { it.key != "controls" && it.key != "item_template" }.forEach { (key, value) ->
+                result.add(key, copy(value))
+            }
+            node.getAsJsonArray("controls")?.let { controls ->
+                val resolvedControls = JsonArray()
+                controls.forEach { element ->
+                    val wrapper = element.takeIf(JsonElement::isJsonObject)?.asJsonObject
+                        ?: throw IllegalArgumentException("control entries must be objects")
+                    require(wrapper.size() == 1) { "control entries must contain exactly one widget" }
+                    val (key, value) = wrapper.entrySet().single()
+                    val output = JsonObject()
+                    require(value.isJsonObject) { "widget '$key' must be an object" }
+                    output.add(key.substringBefore('@'), resolveWidgetEntry(key, value.asJsonObject, chain, depth + 1))
+                    resolvedControls.add(output)
+                }
+                result.add("controls", resolvedControls)
+            }
+            node.getAsJsonObject("item_template")?.let {
+                result.add("item_template", resolveNode(it, chain, depth + 1))
+            }
+            return result
+        }
+
+        private fun resolveWidgetEntry(
+            key: String,
+            overrides: JsonObject,
+            chain: List<String>,
+            depth: Int
+        ): JsonObject {
+            if ('@' !in key) return resolveNode(overrides, chain, depth)
+            val templateRef = key.substringAfter('@')
+            require(templateRef !in chain) { "template expansion cycle: ${(chain + templateRef).joinToString(" -> ")}" }
+            require(depth <= WidgetTree.MAX_DEPTH) { "template expansion exceeds maximum widget depth ${WidgetTree.MAX_DEPTH}" }
+            val template = findTemplate(templateRef)
+                ?: throw IllegalArgumentException("missing template '$templateRef'")
+            val inherited = resolveTemplateChain(templateRef, template, emptyList())
+            val inputs = inherited.inputs.mapValues { (name, input) ->
+                val supplied = overrides.get(name)
+                val value = supplied ?: input.default
+                    ?: throw IllegalArgumentException("missing required template input '$name' for '$templateRef'")
+                require(input.type.accepts(value)) {
+                    "template input '$name' for '$templateRef' must be ${input.type.id}"
+                }
+                value
+            }
+            val substituted = copy(inherited.body, inputs).asJsonObject
+            val nonInputs = JsonObject().also { obj ->
+                overrides.entrySet().filter { it.key !in inherited.inputs }.forEach { (name, value) -> obj.add(name, value) }
+            }
+            return resolveNode(mergeJson(substituted, nonInputs), chain + templateRef, depth)
+        }
+
+        private fun resolveTemplateChain(
+            ref: String,
+            template: TemplateDefinition,
+            chain: List<String>
+        ): TemplateDefinition {
+            require(ref !in chain) { "template inheritance cycle: ${(chain + ref).joinToString(" -> ")}" }
+            require(chain.size < WidgetTree.MAX_DEPTH) { "template inheritance exceeds maximum depth ${WidgetTree.MAX_DEPTH}" }
+            val nextChain = chain + ref
+            val body = JsonObject().also { obj ->
+                template.body.entrySet().filter { it.key != "inputs" && it.key != "extends" }
+                    .forEach { (name, value) -> obj.add(name, value) }
+            }
+            val parentRef = template.extends ?: return template.copy(body = body)
+            val parent = findTemplate(parentRef)
+                ?: throw IllegalArgumentException("missing parent template '$parentRef' referenced by '$ref'")
+            val inherited = resolveTemplateChain(parentRef, parent, nextChain)
+            return template.copy(inputs = inherited.inputs + template.inputs, body = mergeJson(inherited.body, body))
+        }
+
+        private fun findTemplate(ref: String) =
+            localTemplates[ref] ?: TemplateLibraryRegistry.getTemplate(ref)
+
+        private fun copy(element: JsonElement, inputs: Map<String, JsonElement>? = null, depth: Int = 0): JsonElement {
+            require(depth <= 128) { "template JSON exceeds maximum depth 128" }
+            require(++values <= 1_048_576) { "template expansion exceeds maximum JSON work" }
+            if (inputs != null && element.isJsonPrimitive && element.asJsonPrimitive.isString) {
+                val text = element.asString
+                if (text.startsWith('$') && text.length > 1 && text.indexOf('$', 1) == -1) {
+                    val supplied = inputs[text.substring(1)]
+                        ?: throw IllegalArgumentException("unknown template input reference '$text'")
+                    return copy(supplied, depth = depth + 1)
+                }
+                return JsonPrimitive(text)
+            }
+            if (element.isJsonArray) return JsonArray().also { out -> element.asJsonArray.forEach { out.add(copy(it, inputs, depth + 1)) } }
+            if (element.isJsonObject) return JsonObject().also { out ->
+                element.asJsonObject.entrySet().forEach { (key, value) ->
+                    out.add(key, copy(value, inputs, depth + 1))
+                }
+            }
+            return element.deepCopy()
+        }
+
+        private fun mergeJson(base: JsonObject, overlay: JsonObject): JsonObject {
+            val result = copy(base).asJsonObject
+            overlay.entrySet().forEach { (key, value) ->
+                val existing = result.get(key)
+                result.add(key, if (value.isJsonObject && existing?.isJsonObject == true) {
+                    mergeJson(existing.asJsonObject, value.asJsonObject)
+                } else copy(value))
+            }
+            return result
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/widget/WidgetDefinition.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.widget
+
+import com.cobblemon.mod.common.api.molangui.ActionKey
+
+data class WidgetDefinition(
+    val id: String,
+    val type: WidgetType,
+    val properties: Map<String, WidgetProperty> = emptyMap(),
+    val children: List<WidgetDefinition> = emptyList(),
+    val events: Map<String, ServerWidgetEvent> = emptyMap(),
+    val clientEvents: Map<String, String> = emptyMap(),
+    val hoverOverrides: Map<String, WidgetProperty> = emptyMap(),
+    val itemTemplate: WidgetDefinition? = null
+) {
+    fun getProperty(name: String): WidgetProperty? = properties[name]
+    fun getStringProperty(name: String): String? = (properties[name]?.raw as? String)
+    fun getAnchor(): Anchor = Anchor.fromId(getStringProperty("anchor") ?: "top_left")
+}
+
+data class ServerWidgetEvent(
+    val action: ActionKey,
+    val parameters: Map<String, WidgetProperty> = emptyMap()
+)
+
+enum class WidgetType(val id: String) {
+    PANEL("panel"),
+    LABEL("label"),
+    IMAGE("image"),
+    FILL("fill"),
+    BUTTON("button"),
+    LIST("list"),
+    PROGRESS_BAR("progress_bar"),
+    SCROLL_PANEL("scroll_panel"),
+    POKEMON_RENDER("pokemon_render"),
+    NUMBER_INPUT("number_input"),
+    ITEM_SLOT("item_slot"),
+    SLOT_GRID("slot_grid");
+
+    companion object {
+        private val BY_ID = entries.associateBy { it.id }
+        fun fromId(id: String): WidgetType? = BY_ID[id]
+    }
+}
+
+enum class Anchor(val id: String, val xFactor: Float, val yFactor: Float) {
+    TOP_LEFT("top_left", 0f, 0f),
+    TOP_CENTER("top_center", 0.5f, 0f),
+    TOP_RIGHT("top_right", 1f, 0f),
+    CENTER_LEFT("center_left", 0f, 0.5f),
+    CENTER("center", 0.5f, 0.5f),
+    CENTER_RIGHT("center_right", 1f, 0.5f),
+    BOTTOM_LEFT("bottom_left", 0f, 1f),
+    BOTTOM_CENTER("bottom_center", 0.5f, 1f),
+    BOTTOM_RIGHT("bottom_right", 1f, 1f);
+
+    companion object {
+        private val BY_ID = entries.associateBy { it.id }
+        fun fromId(id: String): Anchor = BY_ID[id] ?: TOP_LEFT
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/widget/WidgetExpansion.kt

@@ -0,0 +1,92 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.widget
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.struct.QueryStruct
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.MoValue
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.molangui.layout.SizeExpression
+import com.google.gson.JsonElement
+import com.google.gson.JsonParser
+
+internal object WidgetExpansion {
+    private data class Node(val definition: WidgetDefinition, val context: QueryStruct?, val children: List<Node>)
+
+    fun refresh(definition: WidgetDefinition, runtime: MoLangRuntime, previous: WidgetInstance? = null): WidgetInstance {
+        var count = 0
+        fun plan(def: WidgetDefinition, context: QueryStruct?, depth: Int): Node = withItemContext(runtime, context) {
+            require(depth <= WidgetTree.MAX_DEPTH && ++count <= WidgetTree.MAX_NODES) {
+                "expanded widget tree exceeds ${WidgetTree.MAX_NODES} widgets or depth ${WidgetTree.MAX_DEPTH}"
+            }
+            val children = if (def.type == WidgetType.LIST) {
+                val template = requireNotNull(def.itemTemplate) { "list '${def.id}' requires item_template" }
+                items(def, runtime).mapIndexed { index, values ->
+                    val item = QueryStruct(hashMapOf())
+                    values.forEach { (key, value) -> item.addFunction(key) { value } }
+                    item.addFunction("index") { DoubleValue(index.toDouble()) }
+                    plan(template.copy(id = "${template.id}[$index]"), item, depth + 1)
+                }
+            } else def.children.map { plan(it, context, depth + 1) }
+            Node(def, context, children)
+        }
+        val planned = plan(definition, null, 1)
+        fun apply(node: Node, old: WidgetInstance?, parent: WidgetInstance?): WidgetInstance {
+            val widget = old?.takeIf { it.definition == node.definition } ?: WidgetInstance(node.definition, parent).apply {
+                val size = node.definition.getProperty("size")?.raw as? List<*>
+                if (size?.size == 2) {
+                    widthExpr = SizeExpression.parse(size[0] ?: 0)
+                    heightExpr = SizeExpression.parse(size[1] ?: 0)
+                }
+                val offset = node.definition.getProperty("offset")?.raw as? List<*>
+                if (offset?.size == 2) {
+                    offsetXExpr = SizeExpression.parse(offset[0] ?: 0)
+                    offsetYExpr = SizeExpression.parse(offset[1] ?: 0)
+                }
+            }
+            widget.itemContext = node.context
+            val children = node.children.mapIndexed { index, child -> apply(child, widget.children.getOrNull(index), widget) }
+            widget.children.clear()
+            widget.children.addAll(children)
+            return widget
+        }
+        return apply(planned, previous, null)
+    }
+
+    private fun items(definition: WidgetDefinition, runtime: MoLangRuntime): List<Map<String, MoValue>> =
+        when (definition.getStringProperty("source_type") ?: "data") {
+            "count" -> {
+                val count = definition.getProperty("source")?.resolveDouble(runtime) ?: 0.0
+                require(count.isFinite() && count in 0.0..WidgetTree.MAX_NODES.toDouble() && count % 1.0 == 0.0) {
+                    "list '${definition.id}' requires a whole count between 0 and ${WidgetTree.MAX_NODES}"
+                }
+                List(count.toInt()) { emptyMap() }
+            }
+            "data" -> {
+                val source = definition.getProperty("source")?.resolveString(runtime) ?: "[]"
+                require(source.length <= MolangUiPacketLimits.MAX_VALUE_LENGTH) { "list '${definition.id}' source is too large" }
+                val array = JsonParser.parseString(source).asJsonArray
+                require(array.size() <= WidgetTree.MAX_NODES) { "list '${definition.id}' has too many items" }
+                array.map { element -> element.asJsonObject.entrySet().associate { (key, item) -> key to value(item) } }
+            }
+            else -> throw IllegalArgumentException("unsupported list source_type at '${definition.id}'")
+        }
+
+    private fun value(element: JsonElement): MoValue {
+        if (!element.isJsonPrimitive) return StringValue(element.toString())
+        val primitive = element.asJsonPrimitive
+        return when {
+            primitive.isBoolean -> DoubleValue(primitive.asBoolean)
+            primitive.isNumber -> DoubleValue(primitive.asDouble.also { require(it.isFinite()) { "non-finite list item value" } })
+            else -> StringValue(primitive.asString)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/widget/WidgetInstance.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.widget
+
+import com.cobblemon.mod.common.api.molangui.layout.SizeExpression
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.struct.QueryStruct
+
+class WidgetInstance(
+    val definition: WidgetDefinition,
+    val parent: WidgetInstance? = null
+) {
+    var absoluteX: Float = 0f
+    var absoluteY: Float = 0f
+    var computedWidth: Float = 0f
+    var computedHeight: Float = 0f
+
+    var widthExpr: SizeExpression = SizeExpression.Pixels(0f)
+    var heightExpr: SizeExpression = SizeExpression.Pixels(0f)
+    var offsetXExpr: SizeExpression = SizeExpression.Pixels(0f)
+    var offsetYExpr: SizeExpression = SizeExpression.Pixels(0f)
+
+    internal val paddingProperties: List<WidgetProperty> =
+        (definition.getProperty("padding")?.raw as? List<*>)?.map {
+            when (it) {
+                is Number -> WidgetProperty.literal(it)
+                is WidgetExpression -> WidgetProperty.molang(it.source)
+                else -> throw IllegalArgumentException("invalid padding at '${definition.id}'")
+            }
+        } ?: List(4) { WidgetProperty.literal(0) }
+
+    val children: MutableList<WidgetInstance> = mutableListOf()
+
+    var visible: Boolean = true
+    var enabled: Boolean = true
+    var hovered: Boolean = false
+
+    var scrollOffset: Float = 0f
+    var rendererState: Any? = null
+    var itemContext: QueryStruct? = null
+
+    inline fun <T> withItemContext(runtime: MoLangRuntime, action: () -> T): T =
+        withItemContext(runtime, itemContext, action)
+
+    fun containsPoint(x: Float, y: Float): Boolean {
+        return x >= absoluteX && x <= absoluteX + computedWidth &&
+               y >= absoluteY && y <= absoluteY + computedHeight
+    }
+}
+
+inline fun <T> withItemContext(runtime: MoLangRuntime, context: QueryStruct?, action: () -> T): T {
+    val functions = runtime.environment.query.functions
+    val previous = functions.remove("item")
+    if (context != null) runtime.environment.query.addFunction("item") { context }
+    try {
+        return action()
+    } finally {
+        functions.remove("item")
+        if (previous != null) functions["item"] = previous
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/widget/WidgetProperty.kt

@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.widget
+
+import com.cobblemon.mod.common.api.molang.ExpressionLike
+import com.cobblemon.mod.common.util.asExpressionLike
+import com.bedrockk.molang.runtime.MoLangRuntime
+
+class WidgetProperty private constructor(val raw: Any, expressionSource: String?) {
+    val expression: ExpressionLike? = expressionSource?.asExpressionLike()
+
+    fun resolveDouble(runtime: MoLangRuntime): Double {
+        return expression?.resolveDouble(runtime) ?: when (raw) {
+            is Number -> raw.toDouble()
+            is String -> parseNumericString(raw)
+            is Boolean -> if (raw) 1.0 else 0.0
+            else -> 0.0
+        }
+    }
+
+    fun resolveString(runtime: MoLangRuntime): String {
+        return expression?.resolveString(runtime) ?: raw.toString()
+    }
+
+    fun resolveBoolean(runtime: MoLangRuntime): Boolean {
+        return expression?.resolveBoolean(runtime) ?: when (raw) {
+            is Boolean -> raw
+            is Number -> raw.toDouble() != 0.0
+            is String -> raw.toBooleanStrictOrNull() ?: (raw.isNotEmpty())
+            else -> false
+        }
+    }
+
+    fun resolveInt(runtime: MoLangRuntime): Int {
+        if (expression == null && raw is String && raw.startsWith("0x", ignoreCase = true)) {
+            return raw.substring(2).toLongOrNull(16)?.toInt() ?: 0
+        }
+        return resolveDouble(runtime).toInt()
+    }
+
+    fun resolveFloat(runtime: MoLangRuntime): Float = resolveDouble(runtime).toFloat()
+
+    companion object {
+        fun literal(value: Any): WidgetProperty = WidgetProperty(value, null)
+        fun molang(source: String): WidgetProperty = WidgetProperty(source, source)
+
+        private fun parseNumericString(value: String): Double {
+            value.toDoubleOrNull()?.let { return it }
+            if (value.startsWith("0x", ignoreCase = true)) {
+                value.substring(2).toLongOrNull(16)?.let { return it.toDouble() }
+            }
+            return 0.0
+        }
+    }
+}
+
+data class WidgetExpression(val source: String)

common/src/main/kotlin/com/cobblemon/mod/common/api/molangui/widget/WidgetTree.kt

@@ -0,0 +1,195 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui.widget
+
+import com.cobblemon.mod.common.api.molangui.layout.SizeExpression
+import com.cobblemon.mod.common.api.molangui.ActionKey
+import com.cobblemon.mod.common.api.molangui.requireUiString
+import com.cobblemon.mod.common.api.molangui.template.TemplateDefinition
+import com.cobblemon.mod.common.api.molangui.template.TemplateInput
+import com.cobblemon.mod.common.api.molangui.template.TemplateInputType
+import com.cobblemon.mod.common.api.molangui.template.TemplateResolver
+import com.google.gson.JsonElement
+import com.google.gson.JsonObject
+
+object WidgetTree {
+    const val MAX_DEPTH = 32
+    const val MAX_NODES = 2048
+    private val UNSUPPORTED_PROPERTIES = setOf(
+        "transition", "enter", "exit", "static", "on_submit", "on_submit_client", "on_slot_change", "on_slot_change_client"
+    )
+    private val EVENT_NAMES = setOf(
+        "on_click", "on_change", "on_hover_enter", "on_hover_exit", "on_scroll", "on_key"
+    )
+
+    fun parse(guiJson: JsonObject): WidgetDefinition {
+        val localTemplates = parseLocalTemplates(guiJson)
+        val rootJson = guiJson.getAsJsonObject("root")
+            ?: throw IllegalArgumentException("UI definition is missing 'root'")
+        val resolved = TemplateResolver.resolve(rootJson, localTemplates)
+        val ids = mutableSetOf<String>()
+        val count = intArrayOf(0)
+        return parseWidget("root", resolved, ids, 1, count)
+    }
+
+    fun parseWidget(id: String, json: JsonObject): WidgetDefinition =
+        parseWidget(id, json, mutableSetOf(), 1, intArrayOf(0))
+
+    private fun parseWidget(
+        id: String,
+        json: JsonObject,
+        ids: MutableSet<String>,
+        depth: Int,
+        count: IntArray
+    ): WidgetDefinition {
+        require(id.isNotBlank() && id.length <= 128) { "invalid widget id '$id'" }
+        require(ids.add(id)) { "duplicate widget id '$id'" }
+        require(depth <= MAX_DEPTH) { "widget tree exceeds maximum depth $MAX_DEPTH at '$id'" }
+        count[0]++
+        require(count[0] <= MAX_NODES) { "widget tree exceeds maximum node count $MAX_NODES" }
+
+        val typeName = json.get("type")?.asString ?: "panel"
+        val type = WidgetType.fromId(typeName)
+            ?: throw IllegalArgumentException("unsupported widget renderer '$typeName' at '$id'")
+        val properties = linkedMapOf<String, WidgetProperty>()
+        val events = linkedMapOf<String, ServerWidgetEvent>()
+        val clientEvents = linkedMapOf<String, String>()
+        var hoverOverrides = emptyMap<String, WidgetProperty>()
+        val reserved = setOf("type", "controls", "item_template", "hover", "inputs", "extends")
+        json.entrySet().forEach { (key, value) ->
+            require(key !in UNSUPPORTED_PROPERTIES) { "unsupported widget property '$id.$key'" }
+            when {
+                key.endsWith("_client") && key.removeSuffix("_client") in EVENT_NAMES ->
+                    clientEvents[key] = readExpression(value, "$id.$key")
+                key in EVENT_NAMES -> {
+                    events[key] = parseServerEvent(value, "$id.$key")
+                }
+                key == "hover" && value.isJsonObject -> hoverOverrides = parsePropertyMap(value.asJsonObject, "$id.hover")
+                key !in reserved -> properties[key] = parseProperty(value, "$id.$key")
+            }
+        }
+
+        val children = mutableListOf<WidgetDefinition>()
+        json.getAsJsonArray("controls")?.forEach { element ->
+            require(element.isJsonObject && element.asJsonObject.size() == 1) { "controls under '$id' must be single-entry objects" }
+            val (childId, childValue) = element.asJsonObject.entrySet().single()
+            require(childValue.isJsonObject) { "widget '$childId' must be an object" }
+            children += parseWidget(childId, childValue.asJsonObject, ids, depth + 1, count)
+        }
+
+        val itemTemplate = json.getAsJsonObject("item_template")?.let { template ->
+            parseWidget("$id#item_template", template, ids, depth + 1, count)
+        }
+        validateGeometry(id, type, properties)
+        return WidgetDefinition(
+            id, type, properties.toMap(), children.toList(), events.toMap(), clientEvents.toMap(),
+            hoverOverrides.toMap(), itemTemplate
+        )
+    }
+
+    private fun parseLocalTemplates(guiJson: JsonObject): Map<String, TemplateDefinition> {
+        val result = linkedMapOf<String, TemplateDefinition>()
+        guiJson.getAsJsonObject("templates")?.entrySet()?.forEach { (name, element) ->
+            val body = element.asJsonObject
+            val inputs = linkedMapOf<String, TemplateInput>()
+            body.getAsJsonObject("inputs")?.entrySet()?.forEach { (inputName, inputElement) ->
+                val input = inputElement.asJsonObject
+                val type = TemplateInputType.fromId(input.get("type")?.asString ?: "string")
+                    ?: throw IllegalArgumentException("unknown template input type at '$name.$inputName'")
+                val default = input.get("default")
+                require(default == null || type.accepts(default)) { "invalid template default at '$name.$inputName'" }
+                inputs[inputName] = TemplateInput(type, default)
+            }
+            result[name] = TemplateDefinition(name, "", inputs.toMap(), body.get("extends")?.asString, body)
+        }
+        return result.toMap()
+    }
+
+    private fun parsePropertyMap(json: JsonObject, path: String) = json.entrySet().associate { (key, value) ->
+        key to parseProperty(value, "$path.$key")
+    }
+
+    private fun parseProperty(value: JsonElement, path: String): WidgetProperty {
+        if (value.isJsonObject && value.asJsonObject.has("molang")) {
+            require(value.asJsonObject.size() == 1) {
+                "expression '$path' must contain only a string 'molang' field"
+            }
+            return WidgetProperty.molang(value.asJsonObject.get("molang").requireUiString("$path.molang"))
+        }
+        return WidgetProperty.literal(jsonToLiteral(value, path))
+    }
+
+    private fun jsonToLiteral(element: JsonElement, path: String): Any = when {
+        element.isJsonPrimitive -> element.asJsonPrimitive.let { when {
+            it.isNumber -> it.asDouble.also { number -> require(number.isFinite()) { "non-finite number at '$path'" } }
+            it.isBoolean -> it.asBoolean
+            else -> it.asString
+        } }
+        element.isJsonArray -> element.asJsonArray.mapIndexed { index, item ->
+            if (item.isJsonObject && item.asJsonObject.has("molang")) {
+                require(item.asJsonObject.size() == 1) { "expression '$path[$index]' contains unknown fields" }
+                WidgetExpression(item.asJsonObject.get("molang").requireUiString("$path[$index].molang"))
+            } else jsonToLiteral(item, "$path[$index]")
+        }.toList()
+        element.isJsonObject -> element.asJsonObject.entrySet().associate { (key, item) -> key to jsonToLiteral(item, "$path.$key") }.toMap()
+        else -> throw IllegalArgumentException("null is not a supported widget value at '$path'")
+    }
+
+    private fun readExpression(element: JsonElement, path: String): String {
+        require(element.isJsonObject && element.asJsonObject.size() == 1) { "client event '$path' must be an explicit molang object" }
+        return element.asJsonObject.get("molang").requireUiString("$path.molang").also {
+            require(!it.contains("send_action")) { "client event '$path' may not dispatch server actions" }
+        }
+    }
+
+    private fun parseServerEvent(element: JsonElement, path: String): ServerWidgetEvent {
+        if (element.isJsonPrimitive && element.asJsonPrimitive.isString) {
+            val action = element.asString
+            require(action.startsWith("action@") && action.length > 7) { "server event '$path' must declare exactly one action" }
+            return ServerWidgetEvent(ActionKey(action.substringAfter("action@")))
+        }
+        require(element.isJsonObject) { "server event '$path' must be an object" }
+        val obj = element.asJsonObject
+        val action = ActionKey(obj.get("action")?.asString
+            ?: throw IllegalArgumentException("server event '$path' is missing action"))
+        val parameters = obj.getAsJsonObject("parameters")?.entrySet()?.associate { (name, value) ->
+            name to parseProperty(value, "$path.parameters.$name")
+        }?.toMap() ?: emptyMap()
+        require(obj.keySet().all { it == "action" || it == "parameters" }) { "server event '$path' contains unknown fields" }
+        return ServerWidgetEvent(action, parameters)
+    }
+
+    private fun validateGeometry(id: String, type: WidgetType, properties: Map<String, WidgetProperty>) {
+        listOf("size", "offset", "grid_dimensions", "slot_size", "slot_spacing").forEach { key ->
+            val raw = properties[key]?.raw ?: return@forEach
+            require(raw is List<*> && raw.size == 2) { "'$id.$key' must contain exactly two values" }
+            raw.filterIsInstance<Double>().forEach { require(it.isFinite()) { "non-finite geometry at '$id.$key'" } }
+        }
+        listOf("size", "offset").forEach { key ->
+            (properties[key]?.raw as? List<*>)?.forEach { SizeExpression.parse(requireNotNull(it)) }
+        }
+        properties["padding"]?.raw?.let { padding ->
+            require(padding is List<*> && padding.size == 4 && padding.all { it is Number || it is WidgetExpression }) {
+                "'$id.padding' must contain four numbers or explicit Molang expressions"
+            }
+            padding.filterIsInstance<Number>().forEach {
+                require(it.toFloat().isFinite() && it.toFloat() >= 0f) { "invalid padding at '$id'" }
+            }
+        }
+        if (type == WidgetType.SLOT_GRID) {
+            val dimensions = properties["grid_dimensions"]?.raw as? List<*>
+                ?: throw IllegalArgumentException("slot grid '$id' is missing grid_dimensions")
+            require(dimensions.all { it is Double && it in 1.0..16.0 && it == it.toInt().toDouble() }) {
+                "slot grid '$id' must have integer dimensions in 1..16"
+            }
+            val collectionName = properties["collection_name"]?.raw
+            require(collectionName is String && collectionName.isNotBlank()) { "slot grid '$id' is missing collection_name" }
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/api/vending/VendingMachineCatalogs.kt

@@ -0,0 +1,97 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.vending
+
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.data.JsonDataRegistry
+import com.cobblemon.mod.common.api.molangui.ActionKey
+import com.cobblemon.mod.common.api.molangui.MolangUiActionContext
+import com.cobblemon.mod.common.api.molangui.MolangUiActionHandler
+import com.cobblemon.mod.common.api.molangui.ParameterValue
+import com.cobblemon.mod.common.api.reactive.SimpleObservable
+import com.cobblemon.mod.common.util.adapters.IdentifierAdapter
+import com.cobblemon.mod.common.util.cobblemonResource
+import com.google.gson.GsonBuilder
+import com.google.gson.reflect.TypeToken
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.server.packs.PackType
+
+data class CatalogEntry(val item: ResourceLocation, val price: Int)
+
+data class VendingMachineCatalog(val entries: List<CatalogEntry> = emptyList()) {
+
+    val pageCount: Int get() = if (entries.isEmpty()) 1 else (entries.size + PAGE_SIZE - 1) / PAGE_SIZE
+
+    fun clampPage(page: Int): Int = page.coerceIn(0, pageCount - 1)
+
+    fun pageParameters(page: Int): Map<String, ParameterValue> {
+        val safePage = clampPage(page)
+        val parameters = linkedMapOf<String, ParameterValue>(
+            "page" to ParameterValue.NumberValue(safePage.toDouble()),
+            "page_count" to ParameterValue.NumberValue(pageCount.toDouble()),
+            "page_text" to ParameterValue.StringValue("${safePage + 1}/$pageCount")
+        )
+        for (row in 0 until PAGE_SIZE) {
+            val entry = entries.getOrNull(safePage * PAGE_SIZE + row)
+            parameters["row${row}_item"] = ParameterValue.StringValue(entry?.item?.toString() ?: "")
+            parameters["row${row}_price"] = ParameterValue.StringValue(entry?.price?.toString() ?: "")
+        }
+        return parameters
+    }
+
+    companion object { const val PAGE_SIZE = 3 }
+}
+
+object VendingMachinePageHandler : MolangUiActionHandler {
+    val ACTION = ActionKey("vending_page")
+
+    override fun handle(context: MolangUiActionContext) {
+        val requested = (context.parameters["page"] as? ParameterValue.NumberValue)?.value ?: return
+        val catalog = VendingMachineCatalogs.catalog(VendingMachineCatalogs.DEFAULT)
+        context.session.updateParameters(catalog.pageParameters(requested.toInt()))
+    }
+}
+
+object VendingMachineCatalogs : JsonDataRegistry<VendingMachineCatalog> {
+    override val id = cobblemonResource("vending_machine")
+    override val type = PackType.SERVER_DATA
+    override val observable = SimpleObservable<VendingMachineCatalogs>()
+    override val typeToken: TypeToken<VendingMachineCatalog> = TypeToken.get(VendingMachineCatalog::class.java)
+    override val resourcePath = "vending_machine"
+    override val gson = GsonBuilder()
+        .registerTypeAdapter(ResourceLocation::class.java, IdentifierAdapter)
+        .create()
+
+    val DEFAULT = cobblemonResource("default")
+
+    private val catalogs = mutableMapOf<ResourceLocation, VendingMachineCatalog>()
+
+    fun catalog(id: ResourceLocation): VendingMachineCatalog = catalogs[id] ?: VendingMachineCatalog()
+
+    override fun reload(data: Map<ResourceLocation, VendingMachineCatalog>) {
+        catalogs.clear()
+        data.forEach { (id, catalog) -> catalogs[id] = validate(id, catalog) }
+        Cobblemon.LOGGER.info("Loaded ${catalogs.size} vending machine catalogs")
+        observable.emit(this)
+    }
+
+    override fun sync(player: ServerPlayer) = Unit
+
+    private fun validate(id: ResourceLocation, catalog: VendingMachineCatalog): VendingMachineCatalog {
+        val kept = catalog.entries.filter { entry ->
+            val known = BuiltInRegistries.ITEM.containsKey(entry.item)
+            if (!known) Cobblemon.LOGGER.error("Vending catalog '$id' drops unknown item '${entry.item}'")
+            else if (entry.price <= 0) Cobblemon.LOGGER.error("Vending catalog '$id' drops '${entry.item}' with price ${entry.price}")
+            known && entry.price > 0
+        }
+        return VendingMachineCatalog(kept)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/block/VendingMachineBlock.kt

@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.block
+
+import com.cobblemon.mod.common.api.molangui.MolangUiOpenRequest
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.api.vending.VendingMachineCatalogs
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.core.BlockPos
+import net.minecraft.core.Direction
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.world.InteractionResult
+import net.minecraft.world.entity.player.Player
+import net.minecraft.world.item.context.BlockPlaceContext
+import net.minecraft.world.level.Level
+import net.minecraft.world.level.block.Block
+import net.minecraft.world.level.block.Rotation
+import net.minecraft.world.level.block.state.BlockState
+import net.minecraft.world.level.block.state.StateDefinition
+import net.minecraft.world.level.block.state.properties.BlockStateProperties.HORIZONTAL_FACING
+import net.minecraft.world.phys.BlockHitResult
+
+@Suppress("OVERRIDE_DEPRECATION")
+class VendingMachineBlock(settings: Properties) : Block(settings) {
+    init {
+        registerDefaultState(stateDefinition.any().setValue(HORIZONTAL_FACING, Direction.NORTH))
+    }
+
+    override fun getStateForPlacement(context: BlockPlaceContext): BlockState =
+        defaultBlockState().setValue(HORIZONTAL_FACING, context.horizontalDirection.opposite)
+
+    override fun createBlockStateDefinition(builder: StateDefinition.Builder<Block, BlockState>) {
+        builder.add(HORIZONTAL_FACING)
+    }
+
+    override fun rotate(state: BlockState, rotation: Rotation): BlockState =
+        state.setValue(HORIZONTAL_FACING, rotation.rotate(state.getValue(HORIZONTAL_FACING)))
+
+    override fun useWithoutItem(
+        state: BlockState,
+        world: Level,
+        pos: BlockPos,
+        player: Player,
+        hitResult: BlockHitResult
+    ): InteractionResult {
+        if (player !is ServerPlayer) return InteractionResult.SUCCESS
+        val catalog = VendingMachineCatalogs.catalog(VendingMachineCatalogs.DEFAULT)
+        MolangUiService.open(player, MolangUiOpenRequest(UI_ID, catalog.pageParameters(0), pos))
+        return InteractionResult.SUCCESS
+    }
+
+    companion object {
+        private val UI_ID = cobblemonResource("vending_machine")
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/CobblemonClient.kt

@@ -30,6 +30,10 @@ import com.cobblemon.mod.common.client.gui.RideControlsOverlay
 import com.cobblemon.mod.common.client.gui.battle.BattleOverlay
 import com.cobblemon.mod.common.client.gui.cookingpot.CookingPotScreen
 import com.cobblemon.mod.common.client.gui.party.PartyTutorialToasts
+import com.cobblemon.mod.common.api.molangui.MolangUiRegistry
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientManager
+import com.cobblemon.mod.common.client.gui.molangui.renderer.WidgetRendererRegistry
+import com.cobblemon.mod.common.client.net.molangui.MolangUiCompatibilityRequestHandler
 import com.cobblemon.mod.common.client.gui.tmmachine.TMMachineScreen
 import com.cobblemon.mod.common.client.particle.BedrockParticleOptionsRepository
 import com.cobblemon.mod.common.client.render.ClientPlayerIcon
@@ -122,6 +126,8 @@ object CobblemonClient {
     }
 
     fun onLogout() {
+        MolangUiClientManager.cleanup()
+        MolangUiCompatibilityRequestHandler.reset()
         storage.onLogout()
         battle = null
         battleOverlay.onLogout()
@@ -163,6 +169,8 @@ object CobblemonClient {
         }
 
         PlatformEvents.CLIENT_ENTITY_UNLOAD.subscribe { event -> EntitySoundTracker.clear(event.entity.id) }
+        WidgetRendererRegistry.registerDefaults()
+        PlatformEvents.CLIENT_TICK_POST.subscribe { MolangUiClientManager.tick() }
         PlatformEvents.CLIENT_TICK_POST.subscribe { event ->
             val player = event.client.player
             if (player != null) {
@@ -423,12 +431,15 @@ object CobblemonClient {
     fun beforeChatRender(context: GuiGraphics, partialDeltaTicks: Float) {
         val partialDeltaTicks = Minecraft.getInstance().timer // Checking that this even works
 //        ClientTaskTracker.update(partialDeltaTicks / 20f)
-        if (battle == null) {
-            overlay.render(context, partialDeltaTicks)
-        } else {
-            battleOverlay.render(context, partialDeltaTicks)
+        if (!MolangUiClientManager.hasExclusiveOverlay()) {
+            if (battle == null) {
+                overlay.render(context, partialDeltaTicks)
+            } else {
+                battleOverlay.render(context, partialDeltaTicks)
+            }
+            rideControlsOverlay.render(context, partialDeltaTicks)
         }
-        rideControlsOverlay.render(context, partialDeltaTicks)
+        MolangUiClientManager.renderOverlays(context, partialDeltaTicks.getGameTimeDeltaPartialTick(false))
     }
 
     @Suppress("UNCHECKED_CAST")
@@ -442,6 +453,9 @@ object CobblemonClient {
 
     private fun registerMenuScreens() {
         MenuScreens.register(CobblemonMenuType.COOKING_POT, ::CookingPotScreen)
+        MenuScreens.register(CobblemonMenuType.MOLANG_UI) { menu, playerInventory, _ ->
+            MolangUiClientManager.createContainerScreen(menu, playerInventory)
+        }
         MenuScreens.register(CobblemonMenuType.TM_MACHINE, ::TMMachineScreen)
     }
 
@@ -510,6 +524,8 @@ object CobblemonClient {
 
         BerryModelRepository.reload(resourceManager)
         MiscModelRepository.reload(resourceManager)
+        MolangUiRegistry.reload(resourceManager)
+        MolangUiClientManager.onResourceReload()
         LOGGER.info("Loaded assets")
     }
 

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiClientManager.kt

@@ -0,0 +1,193 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.molangui.GuiMode
+import com.cobblemon.mod.common.api.molangui.MolangUiDefinition
+import com.cobblemon.mod.common.api.molangui.MolangUiParameters
+import com.cobblemon.mod.common.api.molangui.MolangUiRegistry
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
+import com.cobblemon.mod.common.api.molangui.container.SlotLayout
+import com.cobblemon.mod.common.client.net.molangui.MolangUiCompatibilityRequestHandler
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiClosedPacket
+import net.minecraft.client.Minecraft
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.client.gui.screens.Screen
+import net.minecraft.client.gui.screens.inventory.AbstractContainerScreen
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.world.entity.player.Inventory
+import java.util.UUID
+
+object MolangUiClientManager {
+    private class Session(
+        var definition: MolangUiDefinition,
+        val tree: MolangUiWidgetTree,
+        val slotLayouts: Map<String, SlotLayout>,
+        val preparedAtNanos: Long = System.nanoTime()
+    ) {
+        var screen: Screen? = null
+        val overlay = if (definition.mode == GuiMode.OVERLAY) {
+            MolangUiOverlay(tree, definition.exclusive)
+        } else null
+    }
+
+    private val sessions = linkedMapOf<UUID, Session>()
+    private var pendingContainerId: UUID? = null
+
+    fun open(
+        sessionId: UUID, guiId: ResourceLocation, contractVersion: String, mode: GuiMode, args: Map<String, String>,
+        exclusive: Boolean = false,
+        hasContainer: Boolean = false, slotLayouts: Map<String, SlotLayout> = emptyMap()
+    ) {
+        if (sessionId in sessions) return
+        tick()
+        val definition = MolangUiRegistry.getDefinition(guiId)
+        if (definition == null || definition.contractVersion != contractVersion || definition.mode != mode ||
+            definition.exclusive != exclusive ||
+            (hasContainer && mode != GuiMode.SCREEN)) {
+            Cobblemon.LOGGER.error("Rejected incompatible Molang UI open for '{}'", guiId)
+            notifyClosed(sessionId)
+            return
+        }
+        if (mode == GuiMode.OVERLAY && sessions.values.count { it.overlay != null } >= 8) {
+            notifyClosed(sessionId)
+            return
+        }
+        val tree = try {
+            MolangUiWidgetTree(guiId, sessionId, definition.root,
+                MolangUiParameters.decode(definition.parameters, args), definition.variables)
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.error("Failed to prepare Molang UI '{}'", guiId, error)
+            notifyClosed(sessionId)
+            return
+        }
+        if (mode == GuiMode.SCREEN) {
+            sessions.filterValues { it.definition.mode == GuiMode.SCREEN }.keys.toList().forEach { close(it, true) }
+        }
+        val session = Session(definition, tree, slotLayouts.toMap())
+        sessions[sessionId] = session
+        if (mode == GuiMode.SCREEN) {
+            if (hasContainer) {
+                pendingContainerId = sessionId
+            } else {
+                val screen = MolangUiScreen(sessionId, tree)
+                session.screen = screen
+                val client = Minecraft.getInstance()
+                client.execute {
+                    if (sessions[sessionId] === session) client.setScreen(screen)
+                }
+            }
+        }
+    }
+
+    fun createContainerScreen(menu: MolangContainerMenu, playerInventory: Inventory): AbstractContainerScreen<MolangContainerMenu> {
+        tick()
+        val id = pendingContainerId
+        pendingContainerId = null
+        val session = id?.let(sessions::get)
+            ?: return RejectedMolangUiContainerScreen(menu, playerInventory)
+        return try {
+            menu.initializeClient(session.slotLayouts)
+            MolangUiContainerScreen(menu, playerInventory, session.tree.sessionId, session.tree).also { session.screen = it }
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.error("Failed to prepare Molang UI container '{}'", session.tree.guiId, error)
+            close(session.tree.sessionId, true)
+            RejectedMolangUiContainerScreen(menu, playerInventory)
+        }
+    }
+
+    fun close(sessionId: UUID) = close(sessionId, false)
+
+    private fun close(sessionId: UUID, notifyServer: Boolean) {
+        val session = sessions.remove(sessionId) ?: return
+        if (pendingContainerId == sessionId) pendingContainerId = null
+        session.tree.cleanup()
+        if (notifyServer) notifyClosed(sessionId)
+        val screen = session.screen ?: return
+        val client = Minecraft.getInstance()
+        client.execute {
+            if (client.screen === screen) {
+                if (screen is MolangUiContainerScreen && client.player?.containerMenu === screen.menu) {
+                    if (notifyServer) client.player?.closeContainer() else client.player?.clientSideCloseContainer()
+                } else client.setScreen(null)
+            }
+        }
+    }
+
+    internal fun onScreenRemoved(sessionId: UUID, screen: Screen, tree: MolangUiWidgetTree) {
+        tree.cleanup()
+        if (sessions[sessionId]?.screen !== screen) return
+        sessions.remove(sessionId)
+        notifyClosed(sessionId)
+    }
+
+    fun updateArgs(sessionId: UUID, args: Map<String, String>, variables: Map<String, Double> = emptyMap()) {
+        val session = sessions[sessionId] ?: return
+        try {
+            val parameters = MolangUiParameters.decode(session.definition.parameters, args)
+            MolangUiParameters.validateVariables(variables)
+            if (variables.isNotEmpty()) session.tree.updateVariables(variables)
+            if (parameters.isNotEmpty()) session.tree.updateParameters(parameters)
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.warn("Rejected invalid Molang UI update for '{}'", session.tree.guiId, error)
+            close(sessionId, true)
+        }
+    }
+
+    fun renderOverlays(guiGraphics: GuiGraphics, delta: Float) {
+        sessions.values.toList().forEach { it.overlay?.render(guiGraphics, delta) }
+    }
+
+    fun cleanup() {
+        val old = sessions.values.toList()
+        sessions.clear()
+        pendingContainerId = null
+        old.forEach { it.tree.cleanup() }
+    }
+
+    fun hasExclusiveOverlay(): Boolean = sessions.values.any { it.overlay?.exclusive == true }
+
+    fun onResourceReload() {
+        for ((id, session) in sessions.toMap()) {
+            val previous = session.definition
+            val definition = MolangUiRegistry.getDefinition(session.tree.guiId)
+            if (definition == null || definition.contractVersion != previous.contractVersion ||
+                definition.mode != previous.mode ||
+                definition.exclusive != previous.exclusive || definition.parameters != previous.parameters ||
+                definition.actions != previous.actions) {
+                close(id, true)
+                continue
+            }
+            try {
+                session.tree.rebuildFromDefinition(definition)
+                session.definition = definition
+            } catch (error: Exception) {
+                Cobblemon.LOGGER.error("Failed to reload Molang UI '{}'", session.tree.guiId, error)
+                close(id, true)
+            }
+        }
+        MolangUiCompatibilityRequestHandler.respond()
+    }
+
+    fun tick(nowNanos: Long = System.nanoTime()) {
+        val id = pendingContainerId ?: return
+        val session = sessions[id] ?: return
+        if (nowNanos - session.preparedAtNanos > 5_000_000_000L) close(id, true)
+    }
+
+    private fun notifyClosed(sessionId: UUID) {
+        if (Minecraft.getInstance().connection == null) return
+        try {
+            MolangUiClosedPacket(sessionId).sendToServer()
+        } catch (error: Exception) {
+            Cobblemon.LOGGER.warn("Failed to notify server of closed Molang UI '{}'", sessionId, error)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiClientRuntimeFactory.kt

@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.api.molang.function.GeneralMoLangFunctions
+import com.cobblemon.mod.common.api.molangui.MolangUiParameters
+import com.cobblemon.mod.common.api.molangui.ParameterValue
+import com.cobblemon.mod.common.api.molangui.asMoLangValue
+import net.minecraft.client.resources.language.I18n
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+
+internal object MolangUiClientRuntimeFactory {
+    fun create(guiId: ResourceLocation, parameters: Map<String, ParameterValue>, variables: Map<String, Double>) =
+        MoLangRuntime().also { runtime ->
+            GeneralMoLangFunctions.functions.filterKeys { it in MolangUiParameters.GENERAL_FUNCTIONS }.forEach { (name, function) ->
+                runtime.environment.query.addFunction(name) { function(it) }
+            }
+            runtime.environment.query.addFunction("gui_id") { StringValue(guiId.toString()) }
+            runtime.environment.query.addFunction("translate") { params ->
+                StringValue(I18n.get(params.getString(0), *params.params.drop(1).map { it.asString() }.toTypedArray()))
+            }
+            runtime.environment.query.addFunction("item_name") { params ->
+                val id = ResourceLocation.tryParse(params.getString(0))
+                val item = id?.takeIf(BuiltInRegistries.ITEM::containsKey)?.let(BuiltInRegistries.ITEM::get)
+                StringValue(item?.description?.string ?: "")
+            }
+            updateParameters(runtime, parameters)
+            updateVariables(runtime, variables)
+        }
+
+    fun updateFrame(runtime: MoLangRuntime, guiTime: Float, delta: Float, width: Int, height: Int, mouseX: Double, mouseY: Double) {
+        runtime.environment.query.addFunction("gui_time") { DoubleValue(guiTime.toDouble()) }
+        runtime.environment.query.addFunction("delta") { DoubleValue(delta.toDouble()) }
+        runtime.environment.query.addFunction("screen_width") { DoubleValue(width.toDouble()) }
+        runtime.environment.query.addFunction("screen_height") { DoubleValue(height.toDouble()) }
+        runtime.environment.query.addFunction("mouse_x") { DoubleValue(mouseX) }
+        runtime.environment.query.addFunction("mouse_y") { DoubleValue(mouseY) }
+    }
+
+    fun updateParameters(runtime: MoLangRuntime, parameters: Map<String, ParameterValue>) {
+        MolangUiParameters.validate(parameters)
+        parameters.forEach { (key, value) ->
+            val resolved = value.asMoLangValue()
+            runtime.environment.query.addFunction(key) { resolved }
+        }
+    }
+
+    fun updateVariables(runtime: MoLangRuntime, variables: Map<String, Double>) {
+        MolangUiParameters.validateVariables(variables)
+        variables.forEach { (key, value) ->
+            runtime.environment.setSimpleVariable(key.removePrefix("v."), DoubleValue(value))
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiContainerScreen.kt

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.client.gui.screens.inventory.AbstractContainerScreen
+import net.minecraft.network.chat.Component
+import net.minecraft.world.entity.player.Inventory
+import java.util.UUID
+
+class MolangUiContainerScreen(
+    menu: MolangContainerMenu,
+    playerInventory: Inventory,
+    val sessionId: UUID,
+    val widgetTree: MolangUiWidgetTree
+) : AbstractContainerScreen<MolangContainerMenu>(menu, playerInventory, Component.empty()) {
+    private val widgetButtons = mutableSetOf<Int>()
+
+    init {
+        titleLabelX = Int.MAX_VALUE
+        titleLabelY = Int.MAX_VALUE
+        inventoryLabelX = Int.MAX_VALUE
+        inventoryLabelY = Int.MAX_VALUE
+    }
+
+    override fun render(guiGraphics: GuiGraphics, mouseX: Int, mouseY: Int, delta: Float) {
+        widgetTree.render(guiGraphics, width, height, mouseX.toDouble(), mouseY.toDouble(), delta)
+        super.render(guiGraphics, mouseX, mouseY, delta)
+    }
+
+    override fun renderBg(guiGraphics: GuiGraphics, delta: Float, mouseX: Int, mouseY: Int) {
+    }
+
+    override fun mouseClicked(mouseX: Double, mouseY: Double, button: Int): Boolean {
+        if (menu.slots.any { it.isActive && isHovering(it.x, it.y, 16, 16, mouseX, mouseY) }) {
+            return super.mouseClicked(mouseX, mouseY, button)
+        }
+        if (widgetTree.onClick(mouseX, mouseY, button)) {
+            widgetButtons.add(button)
+            return true
+        }
+        return super.mouseClicked(mouseX, mouseY, button)
+    }
+
+    override fun mouseReleased(mouseX: Double, mouseY: Double, button: Int): Boolean =
+        widgetButtons.remove(button) || super.mouseReleased(mouseX, mouseY, button)
+
+    override fun mouseDragged(mouseX: Double, mouseY: Double, button: Int, dragX: Double, dragY: Double): Boolean =
+        button in widgetButtons || super.mouseDragged(mouseX, mouseY, button, dragX, dragY)
+
+    override fun mouseScrolled(mouseX: Double, mouseY: Double, horizontalAmount: Double, verticalAmount: Double): Boolean {
+        val handled = widgetTree.onScroll(mouseX, mouseY, verticalAmount)
+        return handled || super.mouseScrolled(mouseX, mouseY, horizontalAmount, verticalAmount)
+    }
+
+    override fun keyPressed(keyCode: Int, scanCode: Int, modifiers: Int): Boolean {
+        val handled = widgetTree.onKeyPress(keyCode, scanCode, modifiers)
+        if (handled) return true
+        return super.keyPressed(keyCode, scanCode, modifiers)
+    }
+
+    override fun removed() {
+        try {
+            super.removed()
+        } finally {
+            widgetButtons.clear()
+            MolangUiClientManager.onScreenRemoved(sessionId, this, widgetTree)
+        }
+    }
+
+    override fun isPauseScreen(): Boolean = false
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiOverlay.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import net.minecraft.client.Minecraft
+import net.minecraft.client.gui.GuiGraphics
+
+class MolangUiOverlay(
+    private val widgetTree: MolangUiWidgetTree,
+    val exclusive: Boolean
+) {
+    fun render(guiGraphics: GuiGraphics, delta: Float) {
+        val mc = Minecraft.getInstance()
+        val screenWidth = mc.window.guiScaledWidth
+        val screenHeight = mc.window.guiScaledHeight
+        if (screenWidth <= 0 || screenHeight <= 0 || mc.window.screenWidth <= 0 || mc.window.screenHeight <= 0) return
+        val mouseX = (mc.mouseHandler.xpos() * screenWidth / mc.window.screenWidth)
+        val mouseY = (mc.mouseHandler.ypos() * screenHeight / mc.window.screenHeight)
+
+        widgetTree.render(guiGraphics, screenWidth, screenHeight, mouseX, mouseY, delta)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiScreen.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.client.gui.screens.Screen
+import net.minecraft.network.chat.Component
+import java.util.UUID
+
+class MolangUiScreen(
+    val sessionId: UUID,
+    val widgetTree: MolangUiWidgetTree
+) : Screen(Component.empty()) {
+
+    override fun render(guiGraphics: GuiGraphics, mouseX: Int, mouseY: Int, delta: Float) {
+        widgetTree.render(guiGraphics, width, height, mouseX.toDouble(), mouseY.toDouble(), delta)
+    }
+
+    override fun mouseClicked(mouseX: Double, mouseY: Double, button: Int): Boolean {
+        val handled = widgetTree.onClick(mouseX, mouseY, button)
+        return handled || super.mouseClicked(mouseX, mouseY, button)
+    }
+
+    override fun mouseScrolled(mouseX: Double, mouseY: Double, horizontalAmount: Double, verticalAmount: Double): Boolean {
+        val handled = widgetTree.onScroll(mouseX, mouseY, verticalAmount)
+        return handled || super.mouseScrolled(mouseX, mouseY, horizontalAmount, verticalAmount)
+    }
+
+    override fun keyPressed(keyCode: Int, scanCode: Int, modifiers: Int): Boolean {
+        val handled = widgetTree.onKeyPress(keyCode, scanCode, modifiers)
+        if (handled) return true
+        return super.keyPressed(keyCode, scanCode, modifiers)
+    }
+
+    override fun removed() {
+        try {
+            super.removed()
+        } finally {
+            MolangUiClientManager.onScreenRemoved(sessionId, this, widgetTree)
+        }
+    }
+
+    override fun isPauseScreen(): Boolean = false
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/MolangUiWidgetTree.kt

@@ -0,0 +1,316 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.mojang.blaze3d.systems.RenderSystem
+import com.cobblemon.mod.common.client.render.RenderAlphaScope
+import com.cobblemon.mod.common.Cobblemon
+import com.cobblemon.mod.common.api.molangui.MolangUiDefinition
+import com.cobblemon.mod.common.api.molangui.ParameterValue
+import com.cobblemon.mod.common.api.molangui.event.ClientEventProcessor
+import com.cobblemon.mod.common.api.molangui.event.WidgetEvent
+import com.cobblemon.mod.common.api.molangui.event.WidgetEventType
+import com.cobblemon.mod.common.api.molangui.layout.LayoutEngine
+import com.cobblemon.mod.common.api.molangui.widget.WidgetDefinition
+import com.cobblemon.mod.common.api.molangui.widget.WidgetExpansion
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.cobblemon.mod.common.api.molangui.widget.WidgetType
+import com.cobblemon.mod.common.client.gui.molangui.renderer.NumberInputRenderer
+import com.cobblemon.mod.common.client.gui.molangui.renderer.WidgetRendererRegistry
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.client.Minecraft
+import com.cobblemon.mod.common.util.lang
+import net.minecraft.resources.ResourceLocation
+import org.lwjgl.glfw.GLFW.GLFW_KEY_ESCAPE
+import java.util.UUID
+
+class MolangUiWidgetTree(
+    val guiId: ResourceLocation,
+    val sessionId: UUID,
+    var rootDefinition: WidgetDefinition,
+    parameters: Map<String, ParameterValue>,
+    variables: Map<String, Double>,
+    private val nanoTime: () -> Long = System::nanoTime
+) {
+    private val startedAt = nanoTime()
+    private var previousFrame = startedAt
+    private val currentParameters = parameters.toMutableMap()
+    val runtime: MoLangRuntime = MolangUiClientRuntimeFactory.create(guiId, parameters, variables)
+    var root: WidgetInstance = WidgetExpansion.refresh(rootDefinition, runtime)
+        private set
+    private var hoveredPath: List<WidgetInstance> = emptyList()
+    private var hoverRevision = 0L
+    private var expansionError: String? = null
+    private var closed = false
+    private var lastWidth = 0
+    private var lastHeight = 0
+
+    fun updateParameters(parameters: Map<String, ParameterValue>) {
+        MolangUiClientRuntimeFactory.updateParameters(runtime, parameters)
+        currentParameters.putAll(parameters)
+    }
+
+    fun updateVariables(variables: Map<String, Double>) {
+        MolangUiClientRuntimeFactory.updateVariables(runtime, variables)
+    }
+
+    fun rebuildFromDefinition(newDefinition: MolangUiDefinition) {
+        this.rootDefinition = newDefinition.root
+
+        val newVariables = newDefinition.variables.filterKeys { it.removePrefix("v.") !in runtime.environment.variable.map }
+        MolangUiClientRuntimeFactory.updateVariables(runtime, newVariables)
+
+        MolangUiClientRuntimeFactory.updateParameters(runtime, currentParameters)
+
+        prepare(lastWidth, lastHeight)
+    }
+
+    internal fun prepare(width: Int, height: Int): Boolean {
+        if (closed) return false
+        lastWidth = width
+        lastHeight = height
+        try {
+            root = WidgetExpansion.refresh(rootDefinition, runtime, root)
+            fun visibility(widget: WidgetInstance, parentEnabled: Boolean) {
+                widget.withItemContext(runtime) {
+                    widget.visible = widget.definition.getProperty("visible")?.resolveBoolean(runtime) ?: true
+                    widget.enabled = parentEnabled && (widget.definition.getProperty("enabled")?.resolveBoolean(runtime) ?: true)
+                }
+                widget.children.forEach { visibility(it, widget.enabled) }
+            }
+            visibility(root, true)
+            LayoutEngine.layout(root, width.toFloat(), height.toFloat(), runtime)
+            clampScrollOffsets(root)
+            expansionError = null
+            return true
+        } catch (error: Exception) {
+            clearHover()
+            val reason = error.message ?: error.javaClass.simpleName
+            if (reason != expansionError) Cobblemon.LOGGER.warn("Invalid Molang UI {}: {}", guiId, reason)
+            expansionError = reason
+            return false
+        }
+    }
+
+    fun render(guiGraphics: GuiGraphics, screenWidth: Int, screenHeight: Int, mouseX: Double, mouseY: Double, delta: Float) {
+        if (closed) return
+        val now = nanoTime()
+        val elapsed = ((now - startedAt).coerceAtLeast(0) / 1_000_000_000.0).toFloat()
+        val frameDuration = ((now - previousFrame).coerceAtLeast(0) / 1_000_000_000.0).toFloat()
+        previousFrame = now
+        MolangUiClientRuntimeFactory.updateFrame(runtime, elapsed, frameDuration, screenWidth, screenHeight, mouseX, mouseY)
+        if (!prepare(screenWidth, screenHeight)) {
+            if (!closed) guiGraphics.drawString(Minecraft.getInstance().font, lang("molang_ui.invalid_view"), 8, 8, 0xFFFFFF)
+            return
+        }
+
+        if (!updateHover(mouseX.toFloat(), mouseY.toFloat())) return
+
+        renderWidget(root, guiGraphics, mouseX.toFloat(), mouseY.toFloat(), delta)
+    }
+
+    private fun updateHover(mouseX: Float, mouseY: Float): Boolean {
+        val target = ClientEventProcessor.hitTest(root, mouseX, mouseY)
+        val next = generateSequence(target) { it.parent }.toList().asReversed()
+        if (next == hoveredPath) return true
+        val previous = hoveredPath
+        val shared = previous.zip(next).takeWhile { (old, new) -> old === new }.size
+        val exited = previous.drop(shared).asReversed()
+        val entered = next.drop(shared)
+        exited.forEach { it.hovered = false }
+        entered.forEach { it.hovered = true }
+        hoveredPath = next
+        val revision = ++hoverRevision
+        var handled = false
+        for (widget in exited) {
+            handled = dispatchHover(widget, WidgetEventType.HOVER_EXIT) || handled
+            if (closed || hoverRevision != revision) return false
+        }
+        for (widget in entered) {
+            handled = dispatchHover(widget, WidgetEventType.HOVER_ENTER) || handled
+            if (closed || hoverRevision != revision) return false
+        }
+        return !handled || prepare(lastWidth, lastHeight)
+    }
+
+    private fun dispatchHover(widget: WidgetInstance, type: WidgetEventType): Boolean {
+        val definition = widget.definition
+        if (type.id !in definition.events && type.clientId !in definition.clientEvents) return false
+        widget.withItemContext(runtime) {
+            ClientEventProcessor.processEvent(WidgetEvent(type, definition.id), definition, runtime, sessionId)
+        }
+        return true
+    }
+
+    private fun clearHover() {
+        hoveredPath.forEach { it.hovered = false }
+        hoveredPath = emptyList()
+        hoverRevision++
+    }
+
+    fun onClick(mouseX: Double, mouseY: Double, button: Int): Boolean {
+        if (!prepare(lastWidth, lastHeight)) return false
+        val hit = ClientEventProcessor.hitTest(root, mouseX.toFloat(), mouseY.toFloat()) ?: return false
+
+        if (button == 0 && hit.definition.type == WidgetType.NUMBER_INPUT) {
+            var contentY = mouseY.toFloat()
+            var parent = hit.parent
+            while (parent != null) {
+                if (parent.definition.type == WidgetType.SCROLL_PANEL) contentY -= parent.scrollOffset
+                parent = parent.parent
+            }
+            if (hit.withItemContext(runtime) {
+                NumberInputRenderer.handleClick(hit, runtime, mouseX.toFloat(), contentY) { value ->
+                    bubbleEvent(hit, WidgetEventType.CHANGE, mapOf("value" to value.toDouble()))
+                }
+            }) {
+                return true
+            }
+        }
+
+        val data = mapOf("button" to button.toDouble())
+        return bubbleEvent(hit, WidgetEventType.CLICK, data)
+    }
+
+    fun onScroll(mouseX: Double, mouseY: Double, amount: Double): Boolean {
+        if (!amount.isFinite() || !prepare(lastWidth, lastHeight)) return false
+        val hit = ClientEventProcessor.hitTest(root, mouseX.toFloat(), mouseY.toFloat()) ?: return false
+
+        val scrollPanel = findParentOfType(hit, WidgetType.SCROLL_PANEL)
+        if (scrollPanel != null) {
+            scrollPanel.scrollOffset = (scrollPanel.scrollOffset.toDouble() + amount * 10.0)
+                .coerceIn(minimumScroll(scrollPanel).toDouble(), 0.0).toFloat()
+            bubbleEvent(hit, WidgetEventType.SCROLL, mapOf("amount" to amount))
+            return true
+        }
+
+        val data = mapOf("amount" to amount)
+        return bubbleEvent(hit, WidgetEventType.SCROLL, data)
+    }
+
+    private fun findParentOfType(widget: WidgetInstance, type: WidgetType): WidgetInstance? {
+        var current: WidgetInstance? = widget
+        while (current != null) {
+            if (current.definition.type == type) return current
+            current = current.parent
+        }
+        return null
+    }
+
+    private fun bubbleEvent(target: WidgetInstance, type: WidgetEventType, data: Map<String, Double>): Boolean {
+        var current: WidgetInstance? = target
+        while (current != null) {
+            val def = current.definition
+            val hasServer = def.events.containsKey(type.id)
+            val hasClient = def.clientEvents.containsKey(type.clientId)
+            if (hasServer || hasClient) {
+                val event = WidgetEvent(type, target.definition.id, data)
+                target.withItemContext(runtime) { ClientEventProcessor.processEvent(event, def, runtime, sessionId) }
+                return true
+            }
+            current = current.parent
+        }
+        return false
+    }
+
+    fun onKeyPress(keyCode: Int, scanCode: Int, modifiers: Int): Boolean {
+        if (keyCode == GLFW_KEY_ESCAPE || !prepare(lastWidth, lastHeight) || !root.visible || !root.enabled) return false
+        val target = root
+        val hasHandler = target.definition.events.containsKey("on_key") || target.definition.clientEvents.containsKey("on_key_client")
+        if (!hasHandler) return false
+        val event = WidgetEvent(WidgetEventType.KEY, target.definition.id,
+            mapOf("key" to keyCode.toDouble(), "scan" to scanCode.toDouble(), "modifiers" to modifiers.toDouble()))
+        target.withItemContext(runtime) { ClientEventProcessor.processEvent(event, target.definition, runtime, sessionId) }
+        return true
+    }
+
+    fun cleanup() {
+        closed = true
+        clearHover()
+        root.children.clear()
+        root.rendererState = null
+    }
+
+    private fun renderWidget(widget: WidgetInstance, guiGraphics: GuiGraphics, mouseX: Float, mouseY: Float, delta: Float): Unit = widget.withItemContext(runtime) {
+        if (!widget.visible) return
+        val opacity = widget.definition.getProperty("opacity")?.resolveFloat(runtime) ?: 1f
+        if (!opacity.isFinite() || opacity <= 0f) return
+        if (opacity >= 1f) {
+            renderContents(widget, guiGraphics, mouseX, mouseY, delta)
+            return
+        }
+        val color = RenderSystem.getShaderColor().copyOf()
+        try {
+            guiGraphics.setColor(color[0], color[1], color[2], color[3] * opacity)
+            renderContents(widget, guiGraphics, mouseX, mouseY, delta)
+        } finally {
+            guiGraphics.setColor(color[0], color[1], color[2], color[3])
+        }
+    }
+
+    private fun renderContents(widget: WidgetInstance, guiGraphics: GuiGraphics, mouseX: Float, mouseY: Float, delta: Float) {
+        val renderer = WidgetRendererRegistry.get(widget.definition.type)
+        if (widget.definition.type == WidgetType.POKEMON_RENDER || widget.definition.type == WidgetType.ITEM_SLOT) {
+            RenderAlphaScope.render { renderer?.render(widget, guiGraphics, runtime, mouseX, mouseY, delta) }
+        } else {
+            renderer?.render(widget, guiGraphics, runtime, mouseX, mouseY, delta)
+        }
+        val isScrollPanel = widget.definition.type == WidgetType.SCROLL_PANEL
+        val scrollOffset = widget.scrollOffset
+        if (isScrollPanel) {
+            val x = widget.absoluteX.toInt()
+            val y = widget.absoluteY.toInt()
+            val w = widget.computedWidth.toInt()
+            val h = widget.computedHeight.toInt()
+            guiGraphics.enableScissor(x, y, x + w, y + h)
+        }
+
+        try {
+            if (isScrollPanel) applyScrollOffset(widget, scrollOffset)
+            for (child in widget.children) {
+                renderWidget(child, guiGraphics, mouseX, mouseY, delta)
+            }
+        } finally {
+            if (isScrollPanel) {
+                applyScrollOffset(widget, -scrollOffset)
+                guiGraphics.disableScissor()
+            }
+        }
+    }
+
+    private fun applyScrollOffset(widget: WidgetInstance, offset: Float) {
+        for (child in widget.children) {
+            child.absoluteY += offset
+            applyScrollOffset(child, offset)
+        }
+    }
+
+    private fun clampScrollOffsets(widget: WidgetInstance) {
+        if (!widget.visible) return
+        if (widget.definition.type == WidgetType.SCROLL_PANEL) {
+            widget.scrollOffset = if (widget.scrollOffset.isFinite()) {
+                widget.scrollOffset.coerceIn(minimumScroll(widget), 0f)
+            } else 0f
+        }
+        widget.children.forEach(::clampScrollOffsets)
+    }
+
+    private fun minimumScroll(widget: WidgetInstance): Float {
+        val viewportBottom = widget.absoluteY.toDouble() + widget.computedHeight
+        var contentBottom = viewportBottom
+        fun collect(node: WidgetInstance) {
+            if (!node.visible) return
+            contentBottom = maxOf(contentBottom, node.absoluteY.toDouble() + node.computedHeight)
+            if (node.definition.type != WidgetType.SCROLL_PANEL) node.children.forEach(::collect)
+        }
+        widget.children.forEach(::collect)
+        return (viewportBottom - contentBottom).toFloat()
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/RejectedMolangUiContainerScreen.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui
+
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
+import com.cobblemon.mod.common.util.lang
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.client.gui.screens.inventory.AbstractContainerScreen
+import net.minecraft.world.entity.player.Inventory
+
+internal class RejectedMolangUiContainerScreen(menu: MolangContainerMenu, inventory: Inventory) :
+    AbstractContainerScreen<MolangContainerMenu>(menu, inventory, lang("molang_ui.invalid_view")) {
+    override fun containerTick() {
+        minecraft?.player?.closeContainer()
+    }
+
+    override fun render(graphics: GuiGraphics, mouseX: Int, mouseY: Int, delta: Float) {
+        graphics.drawString(font, title, 8, 8, 0xFFFFFF)
+    }
+
+    override fun renderBg(graphics: GuiGraphics, delta: Float, mouseX: Int, mouseY: Int) {}
+    override fun mouseClicked(mouseX: Double, mouseY: Double, button: Int) = true
+    override fun mouseReleased(mouseX: Double, mouseY: Double, button: Int) = true
+    override fun mouseDragged(mouseX: Double, mouseY: Double, button: Int, dragX: Double, dragY: Double) = true
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/ButtonRenderer.kt

@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.Minecraft
+import net.minecraft.client.gui.GuiGraphics
+
+object ButtonRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val enabled = widget.enabled
+        val baseColor = widget.definition.getProperty("color")?.resolveInt(runtime) ?: 0xFF555555.toInt()
+
+        val color = when {
+            !enabled -> 0xFF333333.toInt()
+            widget.hovered -> widget.definition.hoverOverrides["color"]?.resolveInt(runtime) ?: 0xFF777777.toInt()
+            else -> baseColor
+        }
+
+        guiGraphics.fill(
+            widget.absoluteX.toInt(), widget.absoluteY.toInt(),
+            (widget.absoluteX + widget.computedWidth).toInt(),
+            (widget.absoluteY + widget.computedHeight).toInt(),
+            color
+        )
+
+        val text = widget.definition.getProperty("text")?.resolveString(runtime) ?: return
+        val contentWidth = (widget.computedWidth.toInt() - 4).coerceAtLeast(0)
+        if (text.isEmpty() || contentWidth == 0) return
+        val font = Minecraft.getInstance().font
+        if (widget.computedHeight < font.lineHeight + 2) return
+        val displayed = if (font.width(text) <= contentWidth) text else {
+            val suffix = "?"
+            val suffixWidth = font.width(suffix)
+            if (suffixWidth > contentWidth) return
+            font.plainSubstrByWidth(text, contentWidth - suffixWidth) + suffix
+        }
+        val textX = widget.absoluteX + (widget.computedWidth - font.width(displayed)) / 2f
+        val textY = widget.absoluteY + (widget.computedHeight - font.lineHeight) / 2f
+        val textColor = if (enabled) 0xFFFFFF else 0xAAAAAA
+        guiGraphics.enableScissor(widget.absoluteX.toInt(), widget.absoluteY.toInt(),
+            (widget.absoluteX + widget.computedWidth).toInt(), (widget.absoluteY + widget.computedHeight).toInt())
+        try {
+            guiGraphics.drawString(font, displayed, textX.toInt(), textY.toInt(), textColor, true)
+        } finally {
+            guiGraphics.disableScissor()
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/FillRenderer.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+
+object FillRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val color = widget.definition.getProperty("color")?.resolveInt(runtime) ?: 0xFFFFFFFF.toInt()
+        guiGraphics.fill(
+            widget.absoluteX.toInt(), widget.absoluteY.toInt(),
+            (widget.absoluteX + widget.computedWidth).toInt(),
+            (widget.absoluteY + widget.computedHeight).toInt(),
+            color
+        )
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/ImageRenderer.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.cobblemon.mod.common.client.render.withStandardBlend
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.resources.ResourceLocation
+
+object ImageRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val textureStr = widget.definition.getProperty("texture")?.resolveString(runtime) ?: return
+        val texture = ResourceLocation.tryParse(textureStr) ?: return
+        if (!widget.absoluteX.isFinite() || !widget.absoluteY.isFinite()) return
+        val width = pixelSize(widget.computedWidth) ?: return
+        val height = pixelSize(widget.computedHeight) ?: return
+        val uvProperty = widget.definition.getProperty("uv")
+        val uv = uvProperty?.raw as? List<*>
+        if (uvProperty != null && uv == null) return
+        val sizeProperty = widget.definition.getProperty("texture_size")
+        val textureSize = sizeProperty?.raw as? List<*>
+        if (sizeProperty != null && textureSize == null) return
+        val texW = pixelSize(component(textureSize, 0, if (uv == null) width.toFloat() else 256f)) ?: return
+        val texH = pixelSize(component(textureSize, 1, if (uv == null) height.toFloat() else 256f)) ?: return
+        val u = component(uv, 0, 0f) ?: return
+        val v = component(uv, 1, 0f) ?: return
+        val uvW = pixelSize(component(uv, 2, if (uv == null) texW.toFloat() else width.toFloat())) ?: return
+        val uvH = pixelSize(component(uv, 3, if (uv == null) texH.toFloat() else height.toFloat())) ?: return
+        withStandardBlend {
+            guiGraphics.blit(
+                texture,
+                widget.absoluteX.toInt(), widget.absoluteY.toInt(), width, height,
+                u, v, uvW, uvH, texW, texH
+            )
+        }
+    }
+
+    private fun component(values: List<*>?, index: Int, default: Float): Float? {
+        if (values == null || index >= values.size) return default
+        return (values[index] as? Number)?.toFloat()?.takeIf { it.isFinite() }
+    }
+
+    private fun pixelSize(value: Float?): Int? = value?.takeIf {
+        it.isFinite() && it >= 1f && it.toDouble() <= Int.MAX_VALUE.toDouble()
+    }?.toInt()
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/ItemSlotRenderer.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.world.item.ItemStack
+
+object ItemSlotRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val itemId = widget.definition.getProperty("item")?.resolveString(runtime) ?: return
+        val resourceLocation = ResourceLocation.tryParse(itemId) ?: return
+        val item = BuiltInRegistries.ITEM.get(resourceLocation)
+        val stack = ItemStack(item)
+
+        val x = widget.absoluteX.toInt()
+        val y = widget.absoluteY.toInt()
+
+        guiGraphics.renderItem(stack, x, y)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/LabelRenderer.kt

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.Minecraft
+import net.minecraft.client.gui.GuiGraphics
+
+object LabelRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val font = Minecraft.getInstance().font
+        val text = widget.definition.getProperty("text")?.resolveString(runtime) ?: ""
+        val color = widget.definition.getProperty("color")?.resolveInt(runtime) ?: 0xFFFFFF
+        val shadow = widget.definition.getProperty("shadow")?.resolveBoolean(runtime) ?: false
+        val alignment = widget.definition.getStringProperty("alignment") ?: "left"
+        val scale = widget.definition.getProperty("scale")?.resolveFloat(runtime) ?: 1f
+        if (!scale.isFinite() || scale <= 0f) return
+        val textWidth = font.width(text) * scale
+
+        val x = when (alignment) {
+            "center" -> widget.absoluteX + widget.computedWidth / 2f - textWidth / 2f
+            "right" -> widget.absoluteX + widget.computedWidth - textWidth
+            else -> widget.absoluteX
+        }
+
+        if (!textWidth.isFinite() || !x.isFinite() || !widget.absoluteY.isFinite()) return
+        val pose = guiGraphics.pose()
+        pose.pushPose()
+        try {
+            pose.translate(x, widget.absoluteY, 0f)
+            pose.scale(scale, scale, 1f)
+            guiGraphics.drawString(font, text, 0, 0, color, shadow)
+        } finally {
+            pose.popPose()
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/NumberInputRenderer.kt

@@ -0,0 +1,101 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.cobblemon.mod.common.api.molangui.MolangUiParameters
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import net.minecraft.client.Minecraft
+import net.minecraft.client.gui.GuiGraphics
+
+object NumberInputRenderer : WidgetRenderer {
+    private const val BUTTON_WIDTH = 20
+
+    private data class Values(val min: Int, val max: Int, val step: Int, val current: Int)
+    private data class Bounds(val x: Int, val y: Int, val width: Int, val height: Int) {
+        val buttonWidth = minOf(BUTTON_WIDTH, width / 3)
+        val right get() = x + width
+        val bottom get() = y + height
+
+        fun direction(mouseX: Float, mouseY: Float): Int = when {
+            !mouseX.isFinite() || !mouseY.isFinite() || mouseY < y || mouseY >= bottom -> 0
+            mouseX >= x && mouseX < x + buttonWidth -> -1
+            mouseX >= right - buttonWidth && mouseX < right -> 1
+            else -> 0
+        }
+    }
+
+    override fun render(widget: WidgetInstance, guiGraphics: GuiGraphics, runtime: MoLangRuntime, mouseX: Float, mouseY: Float, delta: Float) {
+        val bounds = bounds(widget) ?: return
+        val values = values(widget, runtime) ?: return
+        val font = Minecraft.getInstance().font
+        val textColor = widget.definition.getProperty("color")?.resolveInt(runtime) ?: 0xFFFFFF
+        val buttonColor = widget.definition.getProperty("button_color")?.resolveInt(runtime) ?: 0xFF555555.toInt()
+        val hoverColor = 0xFF777777.toInt()
+        val hovered = if (widget.enabled) bounds.direction(mouseX, mouseY) else 0
+        guiGraphics.fill(bounds.x, bounds.y, bounds.x + bounds.buttonWidth, bounds.bottom,
+            if (hovered == -1 && values.current > values.min) hoverColor else buttonColor)
+        guiGraphics.fill(bounds.right - bounds.buttonWidth, bounds.y, bounds.right, bounds.bottom,
+            if (hovered == 1 && values.current < values.max) hoverColor else buttonColor)
+
+        fun drawText(text: String, left: Int, width: Int) {
+            val textWidth = font.width(text)
+            if (textWidth > width || font.lineHeight > bounds.height) return
+            guiGraphics.drawString(font, text, left + (width - textWidth) / 2,
+                bounds.y + (bounds.height - font.lineHeight) / 2, textColor, false)
+        }
+        drawText("?", bounds.x, bounds.buttonWidth)
+        drawText(values.current.toString(), bounds.x + bounds.buttonWidth, bounds.width - 2 * bounds.buttonWidth)
+        drawText("+", bounds.right - bounds.buttonWidth, bounds.buttonWidth)
+    }
+
+    fun handleClick(widget: WidgetInstance, runtime: MoLangRuntime, mouseX: Float, mouseY: Float, onChange: (Int) -> Unit): Boolean {
+        if (!widget.visible || !widget.enabled) return false
+        val direction = bounds(widget)?.direction(mouseX, mouseY) ?: return false
+        if (direction == 0) return false
+        val values = values(widget, runtime) ?: return false
+        val variable = widget.definition.getStringProperty("variable") ?: return false
+        val name = try { MolangUiParameters.requireVariableName(variable) } catch (_: IllegalArgumentException) { return false }
+        val newValue = (values.current.toLong() + direction * values.step.toLong())
+            .coerceIn(values.min.toLong(), values.max.toLong()).toInt()
+        if (newValue != values.current) {
+            runtime.environment.setSimpleVariable(name, DoubleValue(newValue.toDouble()))
+            onChange(newValue)
+        }
+        return true
+    }
+
+    private fun values(widget: WidgetInstance, runtime: MoLangRuntime): Values? {
+        fun integer(key: String, default: Double): Int? {
+            val value = widget.definition.getProperty(key)?.resolveDouble(runtime) ?: default
+            return value.takeIf { it.isFinite() && it >= Int.MIN_VALUE && it <= Int.MAX_VALUE && it % 1.0 == 0.0 }?.toInt()
+        }
+        val min = integer("min", 1.0) ?: return null
+        val max = integer("max", 99.0) ?: return null
+        val step = integer("step", 1.0) ?: return null
+        val variable = widget.definition.getStringProperty("variable")?.removePrefix("v.")
+        val boundValue = variable?.let { runtime.environment.variable.map[it]?.asDouble() } ?: min.toDouble()
+        val current = integer("value", boundValue) ?: return null
+        if (min > max || step <= 0) return null
+        return Values(min, max, step, current)
+    }
+
+    private fun bounds(widget: WidgetInstance): Bounds? {
+        if (!widget.absoluteX.isFinite() || !widget.absoluteY.isFinite() ||
+            !widget.computedWidth.isFinite() || !widget.computedHeight.isFinite()) return null
+        val x = widget.absoluteX.toDouble()
+        val y = widget.absoluteY.toDouble()
+        val width = widget.computedWidth.toDouble()
+        val height = widget.computedHeight.toDouble()
+        if (width < 3 || height < 1 || width > Int.MAX_VALUE || height > Int.MAX_VALUE ||
+            x < Int.MIN_VALUE || y < Int.MIN_VALUE || x + width > Int.MAX_VALUE || y + height > Int.MAX_VALUE) return null
+        return Bounds(x.toInt(), y.toInt(), width.toInt(), height.toInt())
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/PanelRenderer.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+
+object PanelRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val color = widget.definition.getProperty("color")
+        if (color != null) {
+            guiGraphics.fill(
+                widget.absoluteX.toInt(), widget.absoluteY.toInt(),
+                (widget.absoluteX + widget.computedWidth).toInt(),
+                (widget.absoluteY + widget.computedHeight).toInt(),
+                color.resolveInt(runtime)
+            )
+        }
+        ImageRenderer.render(widget, guiGraphics, runtime, mouseX, mouseY, delta)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/PokemonRenderRenderer.kt

@@ -0,0 +1,76 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.cobblemon.mod.common.client.gui.drawProfilePokemon
+import com.cobblemon.mod.common.client.render.models.blockbench.FloatingState
+import com.cobblemon.mod.common.pokemon.RenderablePokemon
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+import org.joml.Quaternionf
+import org.joml.Vector3f
+import com.cobblemon.mod.common.util.math.fromEulerXYZDegrees
+import com.cobblemon.mod.common.api.pokemon.PokemonSpecies
+
+object PokemonRenderRenderer : WidgetRenderer {
+
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val speciesProp = widget.definition.getProperty("species")
+        if (speciesProp == null) return
+        val speciesName = speciesProp.resolveString(runtime)
+        if (speciesName.isEmpty() || speciesName == "0" || speciesName == "0.0") return
+        val species = PokemonSpecies.getByName(speciesName)
+        if (species == null) return
+        val aspectsStr = widget.definition.getProperty("aspects")?.resolveString(runtime) ?: ""
+        val aspects = if (aspectsStr.isNotEmpty()) aspectsStr.split(",").map { it.trim() }.toSet() else emptySet()
+        val scale = widget.definition.getProperty("scale")?.resolveFloat(runtime) ?: 2.5f
+        val rotY = widget.definition.getProperty("rotation_y")?.resolveFloat(runtime) ?: 35f
+        val rotX = widget.definition.getProperty("rotation_x")?.resolveFloat(runtime) ?: 13f
+        if (!scale.isFinite() || scale <= 0f || !rotX.isFinite() || !rotY.isFinite()) return
+
+        val renderablePokemon = RenderablePokemon(species, aspects)
+
+        val state = (widget.rendererState as? FloatingState) ?: FloatingState().also { widget.rendererState = it }
+
+        val x = widget.absoluteX
+        val y = widget.absoluteY
+        val w = widget.computedWidth
+        val h = widget.computedHeight
+        if (!x.isFinite() || !y.isFinite() || !w.isFinite() || !h.isFinite() || w <= 0f || h <= 0f) return
+
+        val matrices = guiGraphics.pose()
+        guiGraphics.enableScissor(x.toInt(), y.toInt(), (x + w).toInt(), (y + h).toInt())
+        try {
+            matrices.pushPose()
+            try {
+                matrices.translate((x + w * 0.5).toDouble(), y.toDouble(), 0.0)
+                matrices.scale(scale, scale, scale)
+                drawProfilePokemon(
+                    renderablePokemon = renderablePokemon,
+                    matrixStack = matrices,
+                    rotation = Quaternionf().fromEulerXYZDegrees(Vector3f(rotX, rotY, 0f)),
+                    state = state,
+                    partialTicks = delta
+                )
+            } finally {
+                matrices.popPose()
+            }
+        } finally {
+            guiGraphics.disableScissor()
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/ProgressBarRenderer.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+
+object ProgressBarRenderer : WidgetRenderer {
+    override fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    ) {
+        val value = widget.definition.getProperty("value")?.resolveDouble(runtime) ?: 0.0
+        val max = widget.definition.getProperty("max")?.resolveDouble(runtime) ?: 100.0
+        val bgColor = widget.definition.getProperty("background_color")?.resolveInt(runtime) ?: 0xFF333333.toInt()
+        val barColor = widget.definition.getProperty("color")?.resolveInt(runtime) ?: 0xFF00FF00.toInt()
+        val direction = widget.definition.getStringProperty("direction") ?: "left_to_right"
+
+        val x = widget.absoluteX.toInt()
+        val y = widget.absoluteY.toInt()
+        val w = widget.computedWidth.toInt()
+        val h = widget.computedHeight.toInt()
+        val fraction = if (max > 0) (value / max).coerceIn(0.0, 1.0) else 0.0
+
+        guiGraphics.fill(x, y, x + w, y + h, bgColor)
+        when (direction) {
+            "left_to_right" -> guiGraphics.fill(x, y, x + (w * fraction).toInt(), y + h, barColor)
+            "right_to_left" -> guiGraphics.fill(x + w - (w * fraction).toInt(), y, x + w, y + h, barColor)
+            "bottom_to_top" -> guiGraphics.fill(x, y + h - (h * fraction).toInt(), x + w, y + h, barColor)
+            "top_to_bottom" -> guiGraphics.fill(x, y, x + w, y + (h * fraction).toInt(), barColor)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/WidgetRenderer.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetInstance
+import com.bedrockk.molang.runtime.MoLangRuntime
+import net.minecraft.client.gui.GuiGraphics
+
+interface WidgetRenderer {
+    fun render(
+        widget: WidgetInstance,
+        guiGraphics: GuiGraphics,
+        runtime: MoLangRuntime,
+        mouseX: Float,
+        mouseY: Float,
+        delta: Float
+    )
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/gui/molangui/renderer/WidgetRendererRegistry.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.gui.molangui.renderer
+
+import com.cobblemon.mod.common.api.molangui.widget.WidgetType
+
+object WidgetRendererRegistry {
+    private val renderers = mutableMapOf<WidgetType, WidgetRenderer>()
+
+    fun register(type: WidgetType, renderer: WidgetRenderer) {
+        renderers[type] = renderer
+    }
+
+    fun get(type: WidgetType): WidgetRenderer? = renderers[type]
+
+    fun registerDefaults() {
+        register(WidgetType.PANEL, PanelRenderer)
+        register(WidgetType.LABEL, LabelRenderer)
+        register(WidgetType.IMAGE, ImageRenderer)
+        register(WidgetType.FILL, FillRenderer)
+        register(WidgetType.BUTTON, ButtonRenderer)
+        register(WidgetType.PROGRESS_BAR, ProgressBarRenderer)
+        register(WidgetType.NUMBER_INPUT, NumberInputRenderer)
+        register(WidgetType.ITEM_SLOT, ItemSlotRenderer)
+        register(WidgetType.SCROLL_PANEL, PanelRenderer)
+        register(WidgetType.POKEMON_RENDER, PokemonRenderRenderer)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/net/molangui/MolangUiCloseHandler.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.net.molangui
+
+import com.cobblemon.mod.common.api.net.ClientNetworkPacketHandler
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientManager
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiClosePacket
+import net.minecraft.client.Minecraft
+
+object MolangUiCloseHandler : ClientNetworkPacketHandler<MolangUiClosePacket> {
+    override fun handle(packet: MolangUiClosePacket, client: Minecraft) {
+        MolangUiClientManager.close(packet.sessionId)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/net/molangui/MolangUiCompatibilityRequestHandler.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.net.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiRegistry
+import com.cobblemon.mod.common.api.net.ClientNetworkPacketHandler
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiCompatibilityRequestPacket
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiCompatibilityResponsePacket
+import net.minecraft.client.Minecraft
+import net.minecraft.resources.ResourceLocation
+
+object MolangUiCompatibilityRequestHandler : ClientNetworkPacketHandler<MolangUiCompatibilityRequestPacket> {
+    private var requested: Set<ResourceLocation> = emptySet()
+    override fun handle(packet: MolangUiCompatibilityRequestPacket, client: Minecraft) {
+        requested = packet.contracts.keys.toSet()
+        respond()
+    }
+    fun respond() {
+        if (requested.isEmpty()) return
+        val available = MolangUiRegistry.getVersionMap().filterKeys { it in requested }
+        MolangUiCompatibilityResponsePacket(available).sendToServer()
+    }
+    fun reset() { requested = emptySet() }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/net/molangui/MolangUiOpenHandler.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.net.molangui
+
+import com.cobblemon.mod.common.api.net.ClientNetworkPacketHandler
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientManager
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiOpenPacket
+import net.minecraft.client.Minecraft
+
+object MolangUiOpenHandler : ClientNetworkPacketHandler<MolangUiOpenPacket> {
+    override fun handle(packet: MolangUiOpenPacket, client: Minecraft) {
+        MolangUiClientManager.open(packet.sessionId, packet.guiId, packet.contractVersion, packet.mode, packet.args, packet.exclusive, packet.hasContainer, packet.slotLayouts)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/net/molangui/MolangUiUpdateHandler.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.net.molangui
+
+import com.cobblemon.mod.common.api.net.ClientNetworkPacketHandler
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientManager
+import com.cobblemon.mod.common.net.messages.client.molangui.MolangUiUpdatePacket
+import net.minecraft.client.Minecraft
+
+object MolangUiUpdateHandler : ClientNetworkPacketHandler<MolangUiUpdatePacket> {
+    override fun handle(packet: MolangUiUpdatePacket, client: Minecraft) {
+        MolangUiClientManager.updateArgs(packet.sessionId, packet.args, packet.variables)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/render/RenderAlphaScope.kt

@@ -0,0 +1,73 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.render
+
+import com.mojang.blaze3d.systems.RenderSystem
+import org.lwjgl.opengl.GL11
+import org.lwjgl.opengl.GL14
+import kotlin.math.sqrt
+
+object RenderAlphaScope {
+    private class MaterialState(val color: FloatArray, val blendColor: FloatArray?)
+
+    private class Scope {
+        val materials = ArrayDeque<MaterialState>()
+    }
+
+    private val current = ThreadLocal<Scope>()
+
+    fun render(draw: () -> Unit) {
+        val color = RenderSystem.getShaderColor().copyOf()
+        if (!color[3].isFinite() || color[3] >= 1f) {
+            draw()
+            return
+        }
+        val blend = RenderBlendState.capture()
+        val previous = current.get()
+        current.set(Scope())
+        try {
+            draw()
+        } finally {
+            if (previous == null) current.remove() else current.set(previous)
+            RenderSystem.setShaderColor(color[0], color[1], color[2], color[3])
+            blend.restore()
+        }
+    }
+
+    @JvmStatic
+    fun afterMaterialSetup() {
+        val scope = current.get() ?: return
+        val color = RenderSystem.getShaderColor().copyOf()
+        val blended = GL11.glIsEnabled(GL11.GL_BLEND)
+        scope.materials.addLast(MaterialState(color, if (blended) null else RenderBlendState.readColor()))
+        val opacity = color[3].coerceIn(0f, 1f)
+        if (!blended) {
+            RenderSystem.enableBlend()
+            // Cutout texels are opaque above the discard threshold; fade their coverage uniformly.
+            GL14.glBlendColor(0f, 0f, 0f, opacity)
+            RenderSystem.blendFuncSeparate(GL14.GL_CONSTANT_ALPHA, GL14.GL_ONE_MINUS_CONSTANT_ALPHA, GL11.GL_ONE, GL11.GL_ZERO)
+        } else {
+            val factor = when (GL11.glGetInteger(GL14.GL_BLEND_SRC_RGB)) {
+                GL11.GL_ONE -> opacity
+                // SRC_COLOR squares the source RGB, so sqrt(alpha) fades glint linearly.
+                GL11.GL_SRC_COLOR -> sqrt(opacity)
+                else -> 1f
+            }
+            if (factor != 1f) RenderSystem.setShaderColor(color[0] * factor, color[1] * factor, color[2] * factor, color[3])
+        }
+    }
+
+    @JvmStatic
+    fun afterMaterialClear() {
+        val material = current.get()?.materials?.removeLastOrNull() ?: return
+        val color = material.color
+        RenderSystem.setShaderColor(color[0], color[1], color[2], color[3])
+        material.blendColor?.let { RenderBlendState.restoreColor(it) }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/client/render/RenderBlendState.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.client.render
+
+import com.mojang.blaze3d.systems.RenderSystem
+import org.lwjgl.opengl.ARBImaging.GL_BLEND_COLOR
+import org.lwjgl.opengl.GL11
+import org.lwjgl.opengl.GL14
+
+internal class RenderBlendState private constructor(
+    private val enabled: Boolean,
+    private val sourceRgb: Int,
+    private val destinationRgb: Int,
+    private val sourceAlpha: Int,
+    private val destinationAlpha: Int,
+    private val color: FloatArray
+) {
+    fun restore() {
+        RenderSystem.blendFuncSeparate(sourceRgb, destinationRgb, sourceAlpha, destinationAlpha)
+        restoreColor(color)
+        if (enabled) RenderSystem.enableBlend() else RenderSystem.disableBlend()
+    }
+
+    companion object {
+        fun capture() = RenderBlendState(
+            GL11.glIsEnabled(GL11.GL_BLEND),
+            GL11.glGetInteger(GL14.GL_BLEND_SRC_RGB), GL11.glGetInteger(GL14.GL_BLEND_DST_RGB),
+            GL11.glGetInteger(GL14.GL_BLEND_SRC_ALPHA), GL11.glGetInteger(GL14.GL_BLEND_DST_ALPHA), readColor()
+        )
+
+        fun readColor() = FloatArray(4).also { GL11.glGetFloatv(GL_BLEND_COLOR, it) }
+
+        fun restoreColor(color: FloatArray) = GL14.glBlendColor(color[0], color[1], color[2], color[3])
+    }
+}
+
+internal inline fun withStandardBlend(draw: () -> Unit) {
+    val state = RenderBlendState.capture()
+    try {
+        RenderSystem.enableBlend()
+        RenderSystem.defaultBlendFunc()
+        draw()
+    } finally {
+        state.restore()
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/command/MolangUiCommand.kt

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.command
+
+import com.cobblemon.mod.common.api.molangui.MolangUiContractRegistry
+import com.cobblemon.mod.common.api.molangui.MolangUiOpenRequest
+import com.cobblemon.mod.common.api.molangui.MolangUiOpenResult
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.util.lang
+import com.mojang.brigadier.Command
+import com.mojang.brigadier.CommandDispatcher
+import net.minecraft.commands.CommandSourceStack
+import net.minecraft.commands.Commands
+import net.minecraft.commands.arguments.ResourceLocationArgument
+
+object MolangUiCommand {
+    private const val UI_ID = "ui"
+    fun register(dispatcher: CommandDispatcher<CommandSourceStack>) {
+        dispatcher.register(Commands.literal("molangui").requires { it.hasPermission(2) }
+            .then(Commands.literal("open").then(Commands.argument(UI_ID, ResourceLocationArgument.id())
+                .suggests { _, builder -> MolangUiContractRegistry.getAllContracts().keys.forEach { builder.suggest(it.toString()) }; builder.buildFuture() }
+                .executes { context ->
+                    val id = ResourceLocationArgument.getId(context, UI_ID)
+                    when (val result = MolangUiService.open(context.source.playerOrException, MolangUiOpenRequest(id))) {
+                        is MolangUiOpenResult.Opened -> {
+                            context.source.sendSuccess({ lang("molang_ui.command.opened", id.toString(), result.session.sessionId.toString()) }, false)
+                            Command.SINGLE_SUCCESS
+                        }
+                        is MolangUiOpenResult.Rejected -> {
+                            context.source.sendFailure(lang("molang_ui.command.rejected", id.toString(),
+                                lang("molang_ui.rejection.${result.reason.name.lowercase()}")))
+                            0
+                        }
+                    }
+                }))
+            .then(Commands.literal("list").executes { context ->
+                val contracts = MolangUiContractRegistry.getAllContracts()
+                context.source.sendSuccess({ lang("molang_ui.command.list", contracts.size, contracts.keys.joinToString()) }, false)
+                Command.SINGLE_SUCCESS
+            }))
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/data/CobblemonDataProvider.kt

@@ -28,6 +28,9 @@ import com.cobblemon.mod.common.api.item.HeldItems
 import com.cobblemon.mod.common.api.mark.Marks
 import com.cobblemon.mod.common.api.moves.Moves
 import com.cobblemon.mod.common.api.moves.animations.ActionEffects
+import com.cobblemon.mod.common.api.molangui.MolangUiContractRegistry
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.api.vending.VendingMachineCatalogs
 import com.cobblemon.mod.common.api.npc.NPCClasses
 import com.cobblemon.mod.common.api.npc.NPCPresets
 import com.cobblemon.mod.common.api.pokeball.PokeBalls
@@ -69,6 +72,14 @@ object CobblemonDataProvider : DataProvider {
 
     fun registerDefaults() {
         this.register(CobblemonScripts, reloadable = true)
+        this.register(MolangUiContractRegistry, reloadable = true)
+        this.register(VendingMachineCatalogs, reloadable = true)
+        MolangUiContractRegistry.observable.subscribe {
+            server()?.execute {
+                MolangUiService.closeAll()
+                server()?.playerList?.players?.forEach(MolangUiService::requestCompatibility)
+            }
+        }
         this.register(SpeciesFeatures, reloadable = false)
         this.register(GlobalSpeciesFeatures, reloadable = false)
         this.register(SpeciesFeatureAssignments, reloadable = false)

common/src/main/kotlin/com/cobblemon/mod/common/item/group/CobblemonItemGroups.kt

@@ -330,6 +330,7 @@ object CobblemonItemGroups {
 
 //        entries.accept(CobblemonItems.INCENSE_SWEET) // TODO after 1.7
         entries.accept(CobblemonItems.DISPLAY_CASE)
+        entries.accept(CobblemonItems.VENDING_MACHINE)
         entries.accept(CobblemonItems.APRICORN_LOG)
         entries.accept(CobblemonItems.APRICORN_WOOD)
         entries.accept(CobblemonItems.STRIPPED_APRICORN_LOG)

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/client/molangui/MolangUiClosePacket.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.client.molangui
+
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import java.util.UUID
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+
+class MolangUiClosePacket(
+    val sessionId: UUID
+) : NetworkPacket<MolangUiClosePacket> {
+
+    override val id = ID
+
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        buffer.writeUUID(sessionId)
+    }
+
+    companion object {
+        val ID = cobblemonResource("molang_ui_close")
+
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiClosePacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            return MolangUiClosePacket(buffer.readUUID())
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/client/molangui/MolangUiCompatibilityRequestPacket.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.client.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import net.minecraft.resources.ResourceLocation
+
+class MolangUiCompatibilityRequestPacket(val contracts: Map<ResourceLocation, String>) : NetworkPacket<MolangUiCompatibilityRequestPacket> {
+    override val id = ID
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        require(contracts.size <= MolangUiPacketLimits.MAX_CONTRACTS)
+        buffer.writeVarInt(contracts.size)
+        contracts.forEach { (id, version) ->
+            buffer.writeUtf(id.toString(), MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            buffer.writeUtf(version, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+        }
+    }
+    companion object {
+        val ID = cobblemonResource("molang_ui_compatibility_request")
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiCompatibilityRequestPacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            val count = buffer.readVarInt()
+            require(count in 0..MolangUiPacketLimits.MAX_CONTRACTS)
+            val contracts = linkedMapOf<ResourceLocation, String>()
+            repeat(count) {
+                val id = ResourceLocation.tryParse(buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH))
+                    ?: throw IllegalArgumentException("invalid Molang UI id")
+                contracts[id] = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            }
+            return MolangUiCompatibilityRequestPacket(contracts)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/client/molangui/MolangUiOpenPacket.kt

@@ -0,0 +1,91 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.client.molangui
+
+import com.cobblemon.mod.common.api.molangui.GuiMode
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.molangui.MolangUiParameters
+import com.cobblemon.mod.common.api.molangui.container.SlotLayout
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import net.minecraft.resources.ResourceLocation
+import java.util.UUID
+
+class MolangUiOpenPacket(
+    val sessionId: UUID, val guiId: ResourceLocation, val contractVersion: String, val mode: GuiMode,
+    val args: Map<String, String>, val exclusive: Boolean,
+    val hasContainer: Boolean = false, val slotLayouts: Map<String, SlotLayout> = emptyMap()
+) : NetworkPacket<MolangUiOpenPacket> {
+    init {
+        MolangUiPacketLimits.requireIdentifier(guiId.toString(), "UI id")
+        MolangUiPacketLimits.requireIdentifier(contractVersion, "contract version")
+        require(args.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS) { "too many UI parameters" }
+        args.forEach { (name, value) ->
+            MolangUiParameters.requireName(name)
+            MolangUiPacketLimits.requireValue(value, "parameter '$name'")
+        }
+        require(hasContainer == slotLayouts.isNotEmpty()) { "container flag and slot layouts disagree" }
+        require(!hasContainer || mode == GuiMode.SCREEN) { "overlay views cannot have containers" }
+        SlotLayout.validateCollections(slotLayouts)
+    }
+
+    override val id = ID
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        val startIndex = buffer.writerIndex()
+        buffer.writeUUID(sessionId)
+        buffer.writeUtf(guiId.toString(), MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+        buffer.writeUtf(contractVersion, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+        buffer.writeEnum(mode)
+        buffer.writeVarInt(args.size)
+        args.forEach { (key, value) -> buffer.writeUtf(key, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH).writeUtf(value, MolangUiPacketLimits.MAX_VALUE_LENGTH) }
+        buffer.writeBoolean(exclusive).writeBoolean(hasContainer)
+        if (hasContainer) {
+            buffer.writeVarInt(slotLayouts.size)
+            slotLayouts.forEach { (name, layout) ->
+                buffer.writeUtf(name, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+                listOf(layout.offsetX, layout.offsetY, layout.columns, layout.rows, layout.slotWidth, layout.slotHeight, layout.spacingX, layout.spacingY)
+                    .forEach(buffer::writeVarInt)
+            }
+        }
+        require(buffer.writerIndex() - startIndex <= MolangUiPacketLimits.MAX_PACKET_BYTES) { "UI open packet is too large" }
+    }
+    companion object {
+        val ID = cobblemonResource("molang_ui_open")
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiOpenPacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            val sessionId = buffer.readUUID()
+            val guiId = ResourceLocation.tryParse(buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)) ?: error("invalid UI id")
+            val version = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            val mode = buffer.readEnum(GuiMode::class.java)
+            val argCount = buffer.readVarInt()
+            require(argCount in 0..MolangUiPacketLimits.MAX_ACTION_FIELDS)
+            val args = linkedMapOf<String, String>()
+            repeat(argCount) {
+                val name = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+                require(name !in args) { "duplicate UI parameter '$name'" }
+                args[name] = buffer.readUtf(MolangUiPacketLimits.MAX_VALUE_LENGTH)
+            }
+            val exclusive = buffer.readBoolean()
+            val hasContainer = buffer.readBoolean()
+            val layouts = linkedMapOf<String, SlotLayout>()
+            if (hasContainer) {
+                val count = buffer.readVarInt()
+                require(count in 0..MolangUiPacketLimits.MAX_CONTRACTS)
+                repeat(count) {
+                    val name = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+                    require(name !in layouts) { "duplicate container collection '$name'" }
+                    layouts[name] = SlotLayout(buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt(), buffer.readVarInt())
+                }
+            }
+            require(!buffer.isReadable) { "unexpected trailing UI open data" }
+            return MolangUiOpenPacket(sessionId, guiId, version, mode, args, exclusive, hasContainer, layouts)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/client/molangui/MolangUiUpdatePacket.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.client.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import java.util.UUID
+
+class MolangUiUpdatePacket(val sessionId: UUID, val args: Map<String, String>, val variables: Map<String, Double> = emptyMap()) : NetworkPacket<MolangUiUpdatePacket> {
+    override val id = ID
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        require(args.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS && variables.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS)
+        buffer.writeUUID(sessionId).writeVarInt(args.size)
+        args.forEach { (key, value) -> buffer.writeUtf(key, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH).writeUtf(value, MolangUiPacketLimits.MAX_VALUE_LENGTH) }
+        buffer.writeVarInt(variables.size)
+        variables.forEach { (key, value) -> require(value.isFinite()); buffer.writeUtf(key, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH).writeDouble(value) }
+    }
+    companion object {
+        val ID = cobblemonResource("molang_ui_update")
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiUpdatePacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            val sessionId = buffer.readUUID()
+            val argCount = buffer.readVarInt(); require(argCount in 0..MolangUiPacketLimits.MAX_ACTION_FIELDS)
+            val args = linkedMapOf<String, String>()
+            repeat(argCount) { args[buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)] = buffer.readUtf(MolangUiPacketLimits.MAX_VALUE_LENGTH) }
+            val variableCount = buffer.readVarInt(); require(variableCount in 0..MolangUiPacketLimits.MAX_ACTION_FIELDS)
+            val variables = linkedMapOf<String, Double>()
+            repeat(variableCount) {
+                val key = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+                val value = buffer.readDouble(); require(value.isFinite()); variables[key] = value
+            }
+            return MolangUiUpdatePacket(sessionId, args, variables)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/server/molangui/MolangUiActionPacket.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.server.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import java.util.UUID
+
+class MolangUiActionPacket(val sessionId: UUID, val action: String, val data: Map<String, String>) : NetworkPacket<MolangUiActionPacket> {
+    override val id = ID
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        require(data.size <= MolangUiPacketLimits.MAX_ACTION_FIELDS)
+        buffer.writeUUID(sessionId)
+        buffer.writeUtf(action, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+        buffer.writeVarInt(data.size)
+        data.forEach { (key, value) ->
+            buffer.writeUtf(key, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            buffer.writeUtf(value, MolangUiPacketLimits.MAX_VALUE_LENGTH)
+        }
+    }
+    companion object {
+        val ID = cobblemonResource("molang_ui_action")
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiActionPacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            val sessionId = buffer.readUUID()
+            val action = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            val count = buffer.readVarInt()
+            require(count in 0..MolangUiPacketLimits.MAX_ACTION_FIELDS)
+            val data = linkedMapOf<String, String>()
+            repeat(count) { data[buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)] = buffer.readUtf(MolangUiPacketLimits.MAX_VALUE_LENGTH) }
+            return MolangUiActionPacket(sessionId, action, data)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/server/molangui/MolangUiClosedPacket.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.server.molangui
+
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import java.util.UUID
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+
+class MolangUiClosedPacket(
+    val sessionId: UUID
+) : NetworkPacket<MolangUiClosedPacket> {
+
+    override val id = ID
+
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        buffer.writeUUID(sessionId)
+    }
+
+    companion object {
+        val ID = cobblemonResource("molang_ui_closed")
+
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiClosedPacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            return MolangUiClosedPacket(buffer.readUUID())
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/messages/server/molangui/MolangUiCompatibilityResponsePacket.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.messages.server.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiPacketLimits
+import com.cobblemon.mod.common.api.net.NetworkPacket
+import com.cobblemon.mod.common.util.cobblemonResource
+import net.minecraft.network.RegistryFriendlyByteBuf
+import net.minecraft.resources.ResourceLocation
+
+class MolangUiCompatibilityResponsePacket(val views: Map<ResourceLocation, String>) : NetworkPacket<MolangUiCompatibilityResponsePacket> {
+    override val id = ID
+    override fun encode(buffer: RegistryFriendlyByteBuf) {
+        require(views.size <= MolangUiPacketLimits.MAX_CONTRACTS)
+        buffer.writeVarInt(views.size)
+        views.forEach { (id, version) ->
+            buffer.writeUtf(id.toString(), MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            buffer.writeUtf(version, MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+        }
+    }
+    companion object {
+        val ID = cobblemonResource("molang_ui_compatibility_response")
+        fun decode(buffer: RegistryFriendlyByteBuf): MolangUiCompatibilityResponsePacket {
+            require(buffer.readableBytes() <= MolangUiPacketLimits.MAX_PACKET_BYTES)
+            val count = buffer.readVarInt()
+            require(count in 0..MolangUiPacketLimits.MAX_CONTRACTS)
+            val views = linkedMapOf<ResourceLocation, String>()
+            repeat(count) {
+                val id = ResourceLocation.tryParse(buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH))
+                    ?: throw IllegalArgumentException("invalid Molang UI id")
+                views[id] = buffer.readUtf(MolangUiPacketLimits.MAX_IDENTIFIER_LENGTH)
+            }
+            return MolangUiCompatibilityResponsePacket(views)
+        }
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/serverhandling/molangui/MolangUiActionHandler.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.serverhandling.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.api.net.ServerNetworkPacketHandler
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiActionPacket
+import net.minecraft.server.MinecraftServer
+import net.minecraft.server.level.ServerPlayer
+
+object MolangUiActionHandler : ServerNetworkPacketHandler<MolangUiActionPacket> {
+    override fun handle(packet: MolangUiActionPacket, server: MinecraftServer, player: ServerPlayer) {
+        MolangUiService.handleAction(packet.sessionId, player, packet.action, packet.data)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/serverhandling/molangui/MolangUiClosedHandler.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.serverhandling.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.api.net.ServerNetworkPacketHandler
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiClosedPacket
+import net.minecraft.server.MinecraftServer
+import net.minecraft.server.level.ServerPlayer
+
+object MolangUiClosedHandler : ServerNetworkPacketHandler<MolangUiClosedPacket> {
+    override fun handle(packet: MolangUiClosedPacket, server: MinecraftServer, player: ServerPlayer) {
+        MolangUiService.handleClosed(packet.sessionId, player)
+    }
+}

common/src/main/kotlin/com/cobblemon/mod/common/net/serverhandling/molangui/MolangUiCompatibilityResponseHandler.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.net.serverhandling.molangui
+
+import com.cobblemon.mod.common.api.molangui.MolangUiService
+import com.cobblemon.mod.common.api.net.ServerNetworkPacketHandler
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiCompatibilityResponsePacket
+import net.minecraft.server.MinecraftServer
+import net.minecraft.server.level.ServerPlayer
+
+object MolangUiCompatibilityResponseHandler : ServerNetworkPacketHandler<MolangUiCompatibilityResponsePacket> {
+    override fun handle(packet: MolangUiCompatibilityResponsePacket, server: MinecraftServer, player: ServerPlayer) {
+        MolangUiService.applyCompatibility(player, packet.views)
+    }
+}

common/src/main/resources/assets/cobblemon/blockstates/vending_machine.json

@@ -0,0 +1,19 @@
+{
+  "variants": {
+    "facing=north": {
+      "model": "cobblemon:block/vending_machine"
+    },
+    "facing=east": {
+      "model": "cobblemon:block/vending_machine",
+      "y": 90
+    },
+    "facing=south": {
+      "model": "cobblemon:block/vending_machine",
+      "y": 180
+    },
+    "facing=west": {
+      "model": "cobblemon:block/vending_machine",
+      "y": 270
+    }
+  }
+}

common/src/main/resources/assets/cobblemon/lang/en_us.json

@@ -940,6 +940,7 @@
   "block.cobblemon.green_gilded_chest": "Green Gilded Chest",
   "block.cobblemon.white_gilded_chest": "White Gilded Chest",
   "block.cobblemon.display_case": "Display Case",
+  "block.cobblemon.vending_machine": "Vending Machine",
   "block.cobblemon.campfire": "Campfire",
   "block.cobblemon.campfire_pot_black": "Black Campfire Pot",
   "block.cobblemon.campfire_pot_blue": "Blue Campfire Pot",

common/src/main/resources/assets/cobblemon/models/block/vending_machine.json

@@ -0,0 +1,8 @@
+{
+  "parent": "minecraft:block/orientable",
+  "textures": {
+    "front": "cobblemon:block/functional/vending_machine_front",
+    "side": "cobblemon:block/functional/vending_machine_side",
+    "top": "cobblemon:block/functional/vending_machine_top"
+  }
+}

common/src/main/resources/assets/cobblemon/models/item/vending_machine.json

@@ -0,0 +1,3 @@
+{
+  "parent": "cobblemon:block/vending_machine"
+}

common/src/main/resources/assets/cobblemon/molang_ui/battle_intro.json

@@ -0,0 +1,120 @@
+{
+  "format_version": 1,
+  "contract_version": "1.0.0",
+  "mode": "overlay",
+  "parameters": {
+    "player_name": "string",
+    "opponent_name": "string",
+    "player_party_count": "number",
+    "opponent_party_count": "number"
+  },
+  "actions": [],
+  "root": {
+    "type": "panel",
+    "size": ["100%", "100%"],
+    "controls": [
+      { "top_bar": {
+          "type": "fill",
+          "anchor": "top_center",
+          "size": ["100%", 30],
+          "color": "0xFF000000",
+          "opacity": { "molang": "math.clamp(q.gui_time / 0.4, 0, 1)" }
+      }},
+      { "bottom_bar": {
+          "type": "fill",
+          "anchor": "bottom_center",
+          "size": ["100%", 30],
+          "color": "0xFF000000",
+          "opacity": { "molang": "math.clamp(q.gui_time / 0.4, 0, 1)" }
+      }},
+      { "player_panel": {
+          "type": "fill",
+          "anchor": "center_left",
+          "size": ["45%", 60],
+          "color": "0xCC2244AA",
+          "offset": [{ "molang": "math.lerp(-q.screen_width, 0, math.clamp((q.gui_time - 0.2) / 0.6, 0, 1))" }, -10],
+          "controls": [
+            { "player_name": {
+                "type": "label",
+                "anchor": "center_left",
+                "offset": [{ "molang": "math.lerp(-200, 10, math.clamp((q.gui_time - 0.8) / 2.2, 0, 1))" }, 0],
+                "text": { "molang": "q.player_name == '' ? q.translate('cobblemon.molang_ui.battle_intro.player_fallback') : q.player_name" },
+                "shadow": true
+            }},
+            { "player_balls": {
+                "type": "list",
+                "source": { "molang": "q.player_party_count" },
+                "source_type": "count",
+                "layout": "stack_horizontal",
+                "spacing": 4,
+                "anchor": "bottom_left",
+                "offset": [10, -4],
+                "item_template": {
+                  "type": "fill",
+                  "size": [8, 8],
+                  "color": "0xFFAABBFF",
+                  "opacity": { "molang": "math.clamp((q.gui_time - (0.6 + q.item.index * 0.15)) / 0.3, 0, 1)" }
+                }
+            }}
+          ]
+      }},
+      { "opponent_panel": {
+          "type": "fill",
+          "anchor": "center_right",
+          "size": ["45%", 60],
+          "color": "0xCCAA2244",
+          "offset": [{ "molang": "math.lerp(q.screen_width, 0, math.clamp((q.gui_time - 0.4) / 0.6, 0, 1))" }, 10],
+          "controls": [
+            { "opponent_name": {
+                "type": "label",
+                "anchor": "center_right",
+                "offset": [{ "molang": "math.lerp(200, -10, math.clamp((q.gui_time - 1.0) / 2.2, 0, 1))" }, 0],
+                "text": { "molang": "q.opponent_name == '' ? q.translate('cobblemon.molang_ui.battle_intro.opponent_fallback') : q.opponent_name" },
+                "alignment": "right",
+                "shadow": true
+            }},
+            { "opponent_balls": {
+                "type": "list",
+                "source": { "molang": "q.opponent_party_count" },
+                "source_type": "count",
+                "layout": "stack_horizontal",
+                "spacing": 4,
+                "anchor": "bottom_right",
+                "offset": [-10, -4],
+                "item_template": {
+                  "type": "fill",
+                  "size": [8, 8],
+                  "color": "0xFFFFAABB",
+                  "opacity": { "molang": "math.clamp((q.gui_time - (0.9 + q.item.index * 0.15)) / 0.3, 0, 1)" }
+                }
+            }}
+          ]
+      }},
+      { "vs_text": {
+          "type": "label",
+          "anchor": "center",
+          "text": {"molang": "q.translate('cobblemon.molang_ui.battle_intro.versus')"},
+          "color": "0xFFFFDD44",
+          "shadow": true,
+          "opacity": { "molang": "math.clamp((q.gui_time - 0.8) / 0.5, 0, 1)" },
+          "scale": { "molang": "math.lerp(2.0, 1.0, math.clamp((q.gui_time - 0.8) / 0.5, 0, 1))" }
+      }},
+      { "player_accent": {
+          "type": "fill",
+          "anchor": "top_left",
+          "offset": [0, 30],
+          "size": ["45%", 2],
+          "color": "0xFF4466FF",
+          "opacity": { "molang": "math.clamp((q.gui_time - 0.2) / 0.6, 0, 1)" }
+      }},
+      { "opponent_accent": {
+          "type": "fill",
+          "anchor": "bottom_right",
+          "offset": [0, -30],
+          "size": ["45%", 2],
+          "color": "0xFFFF4466",
+          "opacity": { "molang": "math.clamp((q.gui_time - 0.4) / 0.6, 0, 1)" }
+      }}
+    ]
+  }
+}

common/src/main/resources/assets/cobblemon/molang_ui/templates/common.json

@@ -0,0 +1,25 @@
+{
+  "templates": {
+    "panel": { "inputs": {}, "type": "panel", "color": "0x00000000" },
+    "label": {
+      "inputs": {
+        "text": { "type": "string", "default": "" },
+        "color": { "type": "string", "default": "0xFFFFFFFF" }
+      },
+      "type": "label", "text": "$text", "color": "$color", "shadow": false
+    },
+    "button": {
+      "inputs": { "text": { "type": "string", "default": "" } },
+      "type": "button", "size": [120, 20], "text": "$text", "color": "0xFF555555",
+      "hover": { "color": "0xFF777777" }
+    },
+    "progress_bar": {
+      "inputs": {
+        "value": { "type": "expression", "default": { "molang": "0" } },
+        "max": { "type": "expression", "default": { "molang": "100" } },
+        "color": { "type": "string", "default": "0xFF00FF00" }
+      },
+      "type": "progress_bar", "value": "$value", "max": "$max", "color": "$color", "background_color": "0xFF333333"
+    }
+  }
+}

common/src/main/resources/assets/cobblemon/molang_ui/vending_machine.json

@@ -0,0 +1,86 @@
+{
+  "format_version": 1,
+  "contract_version": "1.1.0",
+  "mode": "screen",
+  "actions": ["buy", "vending_page"],
+  "parameters": {
+    "page": "number",
+    "page_count": "number",
+    "page_text": "string",
+    "row0_item": "string",
+    "row1_item": "string",
+    "row2_item": "string",
+    "row0_price": "string",
+    "row1_price": "string",
+    "row2_price": "string"
+  },
+  "variables": { "v.choice": 0, "v.result": 0 },
+  "root": {
+    "type": "panel", "size": [176, 166], "anchor": "center", "texture": "cobblemon:textures/gui/vending_machine.png",
+    "controls": [
+      { "title": { "type": "label", "offset": [8, 4], "text": "Vending Machine", "color": "0xFF404040" } },
+      { "result_ok": { "type": "label", "offset": [96, 4], "size": [72, 10], "alignment": "right", "text": "Purchased!", "color": "0xFF2E7D32", "visible": { "molang": "v.result == 1" } } },
+      { "result_fail": { "type": "label", "offset": [96, 4], "size": [72, 10], "alignment": "right", "text": "Not enough", "color": "0xFF8B1A1A", "visible": { "molang": "v.result == 2" } } },
+
+      { "row0": {
+        "type": "button", "offset": [6, 16], "size": [118, 18], "color": "0x33000000", "hover": { "color": "0x55FFFFFF" },
+        "visible": { "molang": "q.row0_item != ''" },
+        "on_click_client": { "molang": "v.choice = 0; v.result = 0;" },
+        "controls": [
+          { "row0_selected": { "type": "fill", "offset": [0, 0], "size": ["100%", "100%"], "color": "0x66FFD54F", "visible": { "molang": "v.choice == 0" } } },
+          { "row0_icon": { "type": "item_slot", "offset": [2, 1], "item": { "molang": "q.row0_item" } } },
+          { "row0_name": { "type": "label", "offset": [22, 5], "text": { "molang": "q.item_name(q.row0_item)" } } },
+          { "row0_price_text": { "type": "label", "offset": [84, 5], "size": [16, 10], "alignment": "right", "text": { "molang": "q.row0_price" }, "color": "0xFFFFE066" } },
+          { "row0_coin": { "type": "item_slot", "offset": [100, 1], "item": "cobblemon:relic_coin" } }
+        ]
+      } },
+
+      { "row1": {
+        "type": "button", "offset": [6, 35], "size": [118, 18], "color": "0x33000000", "hover": { "color": "0x55FFFFFF" },
+        "visible": { "molang": "q.row1_item != ''" },
+        "on_click_client": { "molang": "v.choice = 1; v.result = 0;" },
+        "controls": [
+          { "row1_selected": { "type": "fill", "offset": [0, 0], "size": ["100%", "100%"], "color": "0x66FFD54F", "visible": { "molang": "v.choice == 1" } } },
+          { "row1_icon": { "type": "item_slot", "offset": [2, 1], "item": { "molang": "q.row1_item" } } },
+          { "row1_name": { "type": "label", "offset": [22, 5], "text": { "molang": "q.item_name(q.row1_item)" } } },
+          { "row1_price_text": { "type": "label", "offset": [84, 5], "size": [16, 10], "alignment": "right", "text": { "molang": "q.row1_price" }, "color": "0xFFFFE066" } },
+          { "row1_coin": { "type": "item_slot", "offset": [100, 1], "item": "cobblemon:relic_coin" } }
+        ]
+      } },
+
+      { "row2": {
+        "type": "button", "offset": [6, 54], "size": [118, 18], "color": "0x33000000", "hover": { "color": "0x55FFFFFF" },
+        "visible": { "molang": "q.row2_item != ''" },
+        "on_click_client": { "molang": "v.choice = 2; v.result = 0;" },
+        "controls": [
+          { "row2_selected": { "type": "fill", "offset": [0, 0], "size": ["100%", "100%"], "color": "0x66FFD54F", "visible": { "molang": "v.choice == 2" } } },
+          { "row2_icon": { "type": "item_slot", "offset": [2, 1], "item": { "molang": "q.row2_item" } } },
+          { "row2_name": { "type": "label", "offset": [22, 5], "text": { "molang": "q.item_name(q.row2_item)" } } },
+          { "row2_price_text": { "type": "label", "offset": [84, 5], "size": [16, 10], "alignment": "right", "text": { "molang": "q.row2_price" }, "color": "0xFFFFE066" } },
+          { "row2_coin": { "type": "item_slot", "offset": [100, 1], "item": "cobblemon:relic_coin" } }
+        ]
+      } },
+
+      { "page_prev": {
+        "type": "button", "offset": [6, 72], "size": [12, 10], "text": "<", "color": "0x55000000", "hover": { "color": "0x77FFFFFF" },
+        "visible": { "molang": "q.page > 0" },
+        "on_click_client": { "molang": "v.choice = 0; v.result = 0;" },
+        "on_click": { "action": "vending_page", "parameters": { "page": { "molang": "q.page - 1" } } }
+      } },
+      { "page_label": { "type": "label", "offset": [20, 73], "size": [90, 10], "alignment": "center", "text": { "molang": "q.page_text" }, "color": "0xFFCCCCCC" } },
+      { "page_next": {
+        "type": "button", "offset": [112, 72], "size": [12, 10], "text": ">", "color": "0x55000000", "hover": { "color": "0x77FFFFFF" },
+        "visible": { "molang": "q.page + 1 < q.page_count" },
+        "on_click_client": { "molang": "v.choice = 0; v.result = 0;" },
+        "on_click": { "action": "vending_page", "parameters": { "page": { "molang": "q.page + 1" } } }
+      } },
+
+      { "payment_slot": { "type": "slot_grid", "offset": [141, 22], "collection_name": "payment", "grid_dimensions": [1, 1] } },
+      { "buy": { "type": "button", "offset": [129, 44], "size": [40, 14], "text": "Buy", "color": "0xFFB71C1C", "hover": { "color": "0xFFE53935" }, "on_click": { "action": "buy", "parameters": { "choice": { "molang": "v.choice" } } } } },
+      { "output_slot": { "type": "slot_grid", "offset": [141, 62], "collection_name": "output", "grid_dimensions": [1, 1] } },
+
+      { "player_inv": { "type": "slot_grid", "offset": [8, 84], "collection_name": "player_inventory", "grid_dimensions": [9, 3] } },
+      { "player_hotbar": { "type": "slot_grid", "offset": [8, 142], "collection_name": "player_hotbar", "grid_dimensions": [9, 1] } }
+    ]
+  }
+}

common/src/main/resources/data/cobblemon/loot_table/blocks/vending_machine.json

@@ -0,0 +1,20 @@
+{
+  "type": "minecraft:block",
+  "pools": [
+    {
+      "rolls": 1.0,
+      "bonus_rolls": 0.0,
+      "entries": [
+        {
+          "type": "minecraft:item",
+          "name": "cobblemon:vending_machine"
+        }
+      ],
+      "conditions": [
+        {
+          "condition": "minecraft:survives_explosion"
+        }
+      ]
+    }
+  ]
+}

common/src/main/resources/data/cobblemon/molang/ui/vending_machine/vending_machine_buy.molang

@@ -0,0 +1,8 @@
+t.item = '';
+t.price = 0;
+v.choice == 0 ? { t.item = q.parameter('row0_item'); t.price = q.to_number(q.parameter('row0_price')); } : 0;
+v.choice == 1 ? { t.item = q.parameter('row1_item'); t.price = q.to_number(q.parameter('row1_price')); } : 0;
+v.choice == 2 ? { t.item = q.parameter('row2_item'); t.price = q.to_number(q.parameter('row2_price')); } : 0;
+t.bought = 0;
+t.item != '' && t.price > 0 && q.slot('payment', 0) == 'cobblemon:relic_coin' ? { t.bought = q.replace_slot('payment', 0, t.price, 'output', 0, t.item, 1); } : 0;
+q.update_variable('v.result', t.bought ? 1 : 2);

common/src/main/resources/data/cobblemon/molang_ui/battle_intro.json

@@ -0,0 +1,12 @@
+{
+  "format_version": 1,
+  "contract_version": "1.0.0",
+  "mode": "overlay",
+  "parameters": {
+    "player_name": { "type": "string" },
+    "opponent_name": { "type": "string" },
+    "player_party_count": { "type": "number" },
+    "opponent_party_count": { "type": "number" }
+  },
+  "actions": {}
+}

common/src/main/resources/data/cobblemon/molang_ui/vending_machine.json

@@ -0,0 +1,30 @@
+{
+  "format_version": 1,
+  "contract_version": "1.1.0",
+  "mode": "screen",
+  "parameters": {
+    "page": { "type": "number", "required": false, "default": 0 },
+    "page_count": { "type": "number", "required": false, "default": 1 },
+    "page_text": { "type": "string", "required": false, "default": "1/1" },
+    "row0_item": { "type": "string", "required": false, "default": "" },
+    "row1_item": { "type": "string", "required": false, "default": "" },
+    "row2_item": { "type": "string", "required": false, "default": "" },
+    "row0_price": { "type": "string", "required": false, "default": "" },
+    "row1_price": { "type": "string", "required": false, "default": "" },
+    "row2_price": { "type": "string", "required": false, "default": "" }
+  },
+  "collections": {
+    "payment": { "size": 1, "allowed_items": ["cobblemon:relic_coin"], "layout": { "offset": [141, 22], "grid_dimensions": [1, 1] } },
+    "output": { "size": 1, "take_only": true, "layout": { "offset": [141, 62], "grid_dimensions": [1, 1] } }
+  },
+  "shift_click_rules": [
+    { "from": ["output"], "to": ["player_inventory", "player_hotbar"] },
+    { "from": ["player_inventory", "player_hotbar"], "to": ["payment"] },
+    { "from": ["payment"], "to": ["player_inventory", "player_hotbar"] }
+  ],
+  "on_close_items": "return",
+  "actions": {
+    "buy": { "handler": "cobblemon:vending_machine_buy", "parameters": { "choice": "number" } },
+    "vending_page": { "parameters": { "page": "number" } }
+  }
+}

common/src/main/resources/data/cobblemon/vending_machine/default.json

@@ -0,0 +1,12 @@
+{
+  "entries": [
+    { "item": "cobblemon:poke_ball", "price": 5 },
+    { "item": "cobblemon:great_ball", "price": 10 },
+    { "item": "cobblemon:ultra_ball", "price": 20 },
+    { "item": "cobblemon:potion", "price": 8 },
+    { "item": "cobblemon:super_potion", "price": 15 },
+    { "item": "cobblemon:hyper_potion", "price": 30 },
+    { "item": "cobblemon:revive", "price": 40 },
+    { "item": "cobblemon:antidote", "price": 6 }
+  ]
+}

common/src/main/resources/data/minecraft/tags/block/mineable/pickaxe.json

@@ -100,6 +100,7 @@
     "cobblemon:thunder_stone_block",
     "cobblemon:water_stone_block",
     "cobblemon:display_case",
+    "cobblemon:vending_machine",
     "cobblemon:white_plaque",
     "cobblemon:light_gray_plaque",
     "cobblemon:gray_plaque",

common/src/main/resources/mixins.cobblemon-common.json

@@ -38,6 +38,7 @@
     "client.RecipeBookComponentMixin",
     "client.RecipeBookTabButtonMixin",
     "client.RecipeCollectionMixin",
+    "client.RenderStateShardMixin",
     "client.SearchTreeMixin",
     "client.StringRenderOutputMixin",
     "client.snapshots.SnapshotWarningMixin",

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/ClientEventProcessorTest.kt

@@ -0,0 +1,112 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.CobblemonNetwork
+import com.cobblemon.mod.common.api.molangui.event.ClientEventProcessor
+import com.cobblemon.mod.common.api.molangui.event.WidgetEvent
+import com.cobblemon.mod.common.api.molangui.event.WidgetEventType
+import com.cobblemon.mod.common.api.molangui.widget.ServerWidgetEvent
+import com.cobblemon.mod.common.api.molangui.widget.WidgetDefinition
+import com.cobblemon.mod.common.api.molangui.widget.WidgetProperty
+import com.cobblemon.mod.common.api.molangui.widget.WidgetTree
+import com.cobblemon.mod.common.api.molangui.widget.WidgetType
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiWidgetTree
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.cobblemon.mod.common.net.messages.server.molangui.MolangUiActionPacket
+import com.cobblemon.mod.common.util.asExpression
+import com.cobblemon.mod.common.util.resolve
+import com.google.gson.JsonParser
+import io.mockk.*
+import net.minecraft.resources.ResourceLocation
+import org.junit.jupiter.api.AfterEach
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import org.lwjgl.glfw.GLFW.GLFW_KEY_ESCAPE
+import java.util.UUID
+
+@BootstrapMinecraft
+class ClientEventProcessorTest {
+    private val runtime = MoLangRuntime()
+    private val session = UUID.randomUUID()
+    private val sent = mutableListOf<MolangUiActionPacket>()
+
+    @BeforeEach
+    fun network() {
+        mockkStatic(CobblemonNetwork::class)
+        every { CobblemonNetwork.sendToServer(any()) } answers { sent += firstArg<MolangUiActionPacket>() }
+    }
+
+    @AfterEach
+    fun cleanup() = unmockkStatic(CobblemonNetwork::class)
+
+    private fun definition(client: String = "v.amount = q.event.button + 10;", parameters: Map<String, WidgetProperty> = mapOf(
+        "value" to WidgetProperty.molang("v.amount"), "target" to WidgetProperty.molang("q.event.widget_id")
+    )) = WidgetDefinition("parent", WidgetType.PANEL,
+        clientEvents = mapOf("on_click_client" to client),
+        events = mapOf("on_click" to ServerWidgetEvent(ActionKey("choose"), parameters)))
+
+    @Test
+    fun `client mutation and event context are available when server arguments are evaluated`() {
+        runtime.environment.query.addFunction("event") { StringValue("outside") }
+        ClientEventProcessor.processEvent(WidgetEvent(WidgetEventType.CLICK, "child", mapOf("button" to 2.0)), definition(), runtime, session)
+        assertEquals(12.0, sent.single().data.getValue("value").toDouble())
+        assertEquals("child", sent.single().data.getValue("target"))
+        assertEquals(session, sent.single().sessionId)
+        assertEquals("outside", runtime.resolve("q.event".asExpression()).asString())
+    }
+
+    @Test
+    fun `nested events restore the outer context even when sending fails`() {
+        runtime.environment.query.addFunction("nested") {
+            ClientEventProcessor.processEvent(WidgetEvent(WidgetEventType.CLICK, "inner", mapOf("button" to 7.0)), definition(), runtime, session)
+            runtime.resolve("q.event.widget_id".asExpression())
+        }
+        every { CobblemonNetwork.sendToServer(any()) } throws IllegalStateException("disconnected")
+        assertDoesNotThrow {
+            ClientEventProcessor.processEvent(WidgetEvent(WidgetEventType.CLICK, "outer", mapOf("button" to 3.0)),
+                definition("v.outer = q.nested; v.after = q.event.button;"), runtime, session)
+        }
+        assertEquals("outer", runtime.resolve("v.outer".asExpression()).asString())
+        assertEquals(3.0, runtime.resolve("v.after".asExpression()).asDouble())
+        assertFalse("event" in runtime.environment.query.functions)
+    }
+
+    @Test
+    fun `failed handlers arguments and oversized values do not dispatch partial actions`() {
+        runtime.environment.query.addFunction("fail") { throw IllegalStateException("failed expression") }
+        val failure = WidgetProperty.molang("q.fail")
+        for (widget in listOf(definition("q.fail;"), definition(parameters = mapOf("value" to failure)),
+            definition(parameters = mapOf("value" to WidgetProperty.literal("x".repeat(MolangUiPacketLimits.MAX_VALUE_LENGTH + 1)))))) {
+            assertDoesNotThrow { ClientEventProcessor.processEvent(WidgetEvent(WidgetEventType.CLICK, "child"), widget, runtime, session) }
+            assertFalse("event" in runtime.environment.query.functions)
+        }
+        assertTrue(sent.isEmpty())
+        verify(exactly = 0) { CobblemonNetwork.sendToServer(any()) }
+    }
+
+    @Test
+    fun `key handlers can inspect key data but cannot swallow escape`() {
+        val tree = MolangUiWidgetTree(ResourceLocation.fromNamespaceAndPath("cobblemon", "key_test"), session,
+            WidgetTree.parseWidget("root", JsonParser.parseString("""{"size":[100,100],
+                "on_key_client":{"molang":"v.key = q.event.key; v.scan = q.event.scan; v.modifiers = q.event.modifiers;"}}""").asJsonObject),
+            emptyMap(), emptyMap())
+        assertTrue(tree.prepare(100, 100))
+        assertTrue(tree.onKeyPress(65, 30, 2))
+        assertEquals(65.0, tree.runtime.resolve("v.key".asExpression()).asDouble())
+        assertEquals(30.0, tree.runtime.resolve("v.scan".asExpression()).asDouble())
+        assertEquals(2.0, tree.runtime.resolve("v.modifiers".asExpression()).asDouble())
+        assertFalse(tree.onKeyPress(GLFW_KEY_ESCAPE, 1, 0))
+        assertEquals(65.0, tree.runtime.resolve("v.key".asExpression()).asDouble())
+        assertThrows(IllegalArgumentException::class.java) { MolangUiParameters.requireName("event") }
+    }
+}

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/MolangContainerMenuTest.kt

@@ -0,0 +1,252 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.CobblemonMenuType
+import com.cobblemon.mod.common.api.molangui.container.CloseItemsPolicy
+import com.cobblemon.mod.common.api.molangui.container.CollectionConfig
+import com.cobblemon.mod.common.api.molangui.container.CollectionSource
+import com.cobblemon.mod.common.api.molangui.container.ItemFilterEntry
+import com.cobblemon.mod.common.api.molangui.container.MolangContainerMenu
+import com.cobblemon.mod.common.api.molangui.container.ServerCollectionSetup
+import com.cobblemon.mod.common.api.molangui.container.ShiftClickRule
+import com.cobblemon.mod.common.api.molangui.container.SlotLayout
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.cobblemon.mod.common.util.asExpression
+import com.cobblemon.mod.common.util.resolve
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.mockkStatic
+import io.mockk.unmockkStatic
+import io.mockk.verify
+import net.minecraft.resources.ResourceLocation
+import net.minecraft.server.level.ServerPlayer
+import net.minecraft.world.Container
+import net.minecraft.world.SimpleContainer
+import net.minecraft.world.entity.player.Inventory
+import net.minecraft.world.flag.FeatureFlags
+import net.minecraft.world.inventory.AbstractContainerMenu
+import net.minecraft.world.inventory.MenuType
+import net.minecraft.world.item.ItemStack
+import net.minecraft.world.item.Items
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertNull
+import org.junit.jupiter.api.Assertions.assertThrows
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.BeforeAll
+import org.junit.jupiter.api.Test
+
+@BootstrapMinecraft
+class MolangContainerMenuTest {
+    companion object {
+        @JvmStatic
+        @BeforeAll
+        fun initializeMenuTypes() {
+            // The test bootstrap freezes vanilla registries before Cobblemon's menu initializer runs.
+            mockkStatic(MenuType::class)
+            try {
+                every { MenuType.register(any(), any<MenuType.MenuSupplier<AbstractContainerMenu>>()) } answers {
+                    MenuType(secondArg<MenuType.MenuSupplier<AbstractContainerMenu>>(), FeatureFlags.DEFAULT_FLAGS)
+                }
+                CobblemonMenuType.MOLANG_UI
+            } finally {
+                unmockkStatic(MenuType::class)
+            }
+        }
+    }
+
+    private val player = mockk<ServerPlayer>(relaxed = true)
+    private val inventory = Inventory(player)
+    private val session = mockk<MolangUiSession>(relaxed = true)
+
+    @Test
+    fun `menu removal closes its session even when item cleanup fails`() {
+        val container = object : SimpleContainer(1) {
+            override fun clearContent() { throw IllegalStateException("cleanup failed") }
+        }
+        val menu = MolangContainerMenu(1, inventory)
+        menu.initializeServer(listOf(setup("input", container)), sessionOwned = setOf("input"),
+            closeItemsPolicy = CloseItemsPolicy.DESTROY, owner = player, session = session)
+
+        assertThrows(IllegalStateException::class.java) { menu.removed(player) }
+
+        verify(exactly = 1) { session.closeFromMenu() }
+    }
+
+    @Test
+    fun `scripts address collection slots through their block mapping`() {
+        val block = SimpleContainer(5)
+        block.setItem(0, ItemStack(Items.EMERALD, 7))
+        block.setItem(3, ItemStack(Items.DIAMOND, 4))
+        val menu = menu(listOf(setup("input", block, mapping = listOf(3)), setup("output", block, mapping = listOf(4))))
+        assertEquals(4, menu.getStack("input", 0)?.count)
+        assertNull(menu.getStack("input", 3))
+        assertNull(menu.getStack("input", Int.MAX_VALUE))
+        assertTrue(menu.replaceSlotAtomically("input", 0, 2, "output", 0, "minecraft:gold_ingot", 3))
+        assertEquals(2, block.getItem(3).count)
+        assertTrue(block.getItem(4).`is`(Items.GOLD_INGOT))
+        assertEquals(3, block.getItem(4).count)
+        assertEquals(7, block.getItem(0).count)
+    }
+
+    @Test
+    fun `rejected oversized outputs leave both slots unchanged`() {
+        val input = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 4)) }
+        val output = SimpleContainer(1)
+        val menu = menu(listOf(setup("input", input), setup("output", output)))
+        for (count in listOf(65, Int.MAX_VALUE)) {
+            assertFalse(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:gold_ingot", count))
+            assertEquals(4, input.getItem(0).count)
+            assertTrue(output.getItem(0).isEmpty)
+        }
+        output.setItem(0, ItemStack(Items.GOLD_INGOT, 63))
+        assertFalse(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:gold_ingot", Int.MAX_VALUE))
+        assertEquals(63, output.getItem(0).count)
+        assertEquals(4, input.getItem(0).count)
+    }
+
+    @Test
+    fun `scripted output honors container limits and filters on take-only collections`() {
+        val input = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 4)) }
+        val output = object : SimpleContainer(1) {
+            override fun getMaxStackSize(stack: ItemStack) = 3
+        }
+        val allowed = listOf(ItemFilterEntry.Item(ResourceLocation.withDefaultNamespace("gold_ingot")))
+        val menu = menu(listOf(setup("input", input), setup("output", output, takeOnly = true, allowed = allowed)))
+        assertFalse(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:gold_ingot", 4))
+        assertFalse(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:emerald", 1))
+        assertEquals(4, input.getItem(0).count)
+        assertTrue(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:gold_ingot", 3))
+        assertEquals(3, output.getItem(0).count)
+    }
+
+    @Test
+    fun `aliased input and output slots transform only after validating the remainder`() {
+        val block = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 4)) }
+        val menu = menu(listOf(setup("input", block), setup("output", block)))
+        assertFalse(menu.replaceSlotAtomically("input", 0, 1, "output", 0, "minecraft:gold_ingot", 1))
+        assertEquals(4, block.getItem(0).count)
+        assertTrue(menu.replaceSlotAtomically("input", 0, 4, "output", 0, "minecraft:gold_ingot", 2))
+        assertTrue(block.getItem(0).`is`(Items.GOLD_INGOT))
+        assertEquals(2, block.getItem(0).count)
+    }
+
+    @Test
+    fun `inventory and hotbar use their own shift click rules`() {
+        val main = SimpleContainer(1)
+        val hotbar = SimpleContainer(1)
+        val menu = menu(listOf(setup("main", main), setup("hotbar", hotbar)), listOf(
+            ShiftClickRule(listOf(ShiftClickRule.PLAYER_INVENTORY), listOf("main")),
+            ShiftClickRule(listOf(ShiftClickRule.PLAYER_HOTBAR), listOf("hotbar"))
+        ))
+        inventory.setItem(9, ItemStack(Items.DIAMOND, 3))
+        inventory.setItem(0, ItemStack(Items.EMERALD, 5))
+        menu.quickMoveStack(player, 2)
+        menu.quickMoveStack(player, 29)
+        assertEquals(3, main.getItem(0).count)
+        assertTrue(main.getItem(0).`is`(Items.DIAMOND))
+        assertEquals(5, hotbar.getItem(0).count)
+        assertTrue(hotbar.getItem(0).`is`(Items.EMERALD))
+        assertTrue(inventory.getItem(9).isEmpty)
+        assertTrue(inventory.getItem(0).isEmpty)
+    }
+
+    @Test
+    fun `shift click continues through partially filled destinations`() {
+        val first = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 63)) }
+        val second = SimpleContainer(2)
+        val menu = menu(listOf(setup("first", first), setup("second", second)), listOf(
+            ShiftClickRule(listOf(ShiftClickRule.PLAYER_INVENTORY), listOf("first", "second"))
+        ))
+        inventory.setItem(9, ItemStack(Items.DIAMOND, 10))
+        menu.quickMoveStack(player, 3)
+        assertEquals(64, first.getItem(0).count)
+        assertEquals(9, second.getItem(0).count)
+        assertTrue(inventory.getItem(9).isEmpty)
+    }
+
+    @Test
+    fun `shift click cannot merge into filtered or take-only occupied slots`() {
+        val denied = listOf(ItemFilterEntry.Item(ResourceLocation.withDefaultNamespace("emerald")))
+        val filtered = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 1)) }
+        val takeOnly = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 1)) }
+        val destination = SimpleContainer(1)
+        val menu = menu(listOf(setup("filtered", filtered, allowed = denied), setup("output", takeOnly, takeOnly = true), setup("valid", destination)))
+        inventory.setItem(9, ItemStack(Items.DIAMOND, 5))
+        menu.quickMoveStack(player, 3)
+        assertEquals(1, filtered.getItem(0).count)
+        assertEquals(1, takeOnly.getItem(0).count)
+        assertEquals(5, destination.getItem(0).count)
+    }
+
+    @Test
+    fun `shift click skips aliases of the source slot`() {
+        val block = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 4)) }
+        val destination = SimpleContainer(1)
+        val menu = menu(listOf(setup("source", block), setup("alias", block), setup("destination", destination)), listOf(
+            ShiftClickRule(listOf("source"), listOf("alias", "destination"))
+        ))
+        menu.quickMoveStack(player, 0)
+        assertTrue(block.getItem(0).isEmpty)
+        assertEquals(4, destination.getItem(0).count)
+    }
+
+    @Test
+    fun `shift click respects block placement restrictions and fills multiple limited slots`() {
+        val restricted = object : SimpleContainer(1) {
+            override fun canPlaceItem(index: Int, stack: ItemStack) = false
+        }.apply { setItem(0, ItemStack(Items.DIAMOND, 1)) }
+        val limited = object : SimpleContainer(2) {
+            override fun getMaxStackSize(stack: ItemStack) = 2
+        }
+        val menu = menu(listOf(setup("restricted", restricted), setup("limited", limited)))
+        inventory.setItem(9, ItemStack(Items.DIAMOND, 5))
+        menu.quickMoveStack(player, 3)
+        assertEquals(1, restricted.getItem(0).count)
+        assertEquals(2, limited.getItem(0).count)
+        assertEquals(2, limited.getItem(1).count)
+        assertEquals(1, inventory.getItem(9).count)
+    }
+
+    @Test
+    fun `server expressions reject fractional or overflowing slot counts and indices`() {
+        val input = SimpleContainer(1).apply { setItem(0, ItemStack(Items.DIAMOND, 4)) }
+        val output = SimpleContainer(1)
+        val menu = menu(listOf(setup("input", input), setup("output", output)))
+        every { session.containerMenu } returns menu
+        val runtime = MolangUiServerRuntimeFactory.create(session)
+        for (expression in listOf(
+            "q.replace_slot('input', 0.5, 1, 'output', 0, 'minecraft:gold_ingot', 1)",
+            "q.replace_slot('input', 0, 1.5, 'output', 0, 'minecraft:gold_ingot', 1)",
+            "q.replace_slot('input', 0, 1, 'output', 0.5, 'minecraft:gold_ingot', 1)",
+            "q.replace_slot('input', 0, 1, 'output', 0, 'minecraft:gold_ingot', 1.5)",
+            "q.replace_slot('input', 0, 1, 'output', 0, 'minecraft:gold_ingot', 4294967297)"
+        )) {
+            assertEquals(0.0, runtime.resolve(expression.asExpression()).asDouble(), expression)
+        }
+        assertEquals("", runtime.resolve("q.slot('input', 0.5)".asExpression()).asString())
+        assertEquals("minecraft:diamond", runtime.resolve("q.slot('input', 0)".asExpression()).asString())
+        assertEquals(4, input.getItem(0).count)
+        assertTrue(output.getItem(0).isEmpty)
+    }
+
+    private fun setup(name: String, container: Container, mapping: List<Int>? = null, takeOnly: Boolean = false,
+                      allowed: List<ItemFilterEntry> = emptyList()): ServerCollectionSetup {
+        val size = mapping?.size ?: container.containerSize
+        return ServerCollectionSetup(name, CollectionConfig(size, allowed, takeOnly,
+            if (mapping == null) CollectionSource.SESSION else CollectionSource.BLOCK_ENTITY,
+            mapping, SlotLayout(0, 0, size, 1)), container)
+    }
+
+    private fun menu(setups: List<ServerCollectionSetup>, rules: List<ShiftClickRule> = emptyList()) =
+        MolangContainerMenu(1, inventory).also {
+            it.initializeServer(setups, rules, owner = player, session = session)
+        }
+}

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiCodecTest.kt

@@ -0,0 +1,339 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.api.molangui.template.TemplateDefinition
+import com.cobblemon.mod.common.api.molangui.template.TemplateInput
+import com.cobblemon.mod.common.api.molangui.template.TemplateInputType
+import com.cobblemon.mod.common.api.molangui.template.TemplateResolver
+import com.cobblemon.mod.common.api.molangui.widget.WidgetTree
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.google.gson.JsonObject
+import com.google.gson.JsonParser
+import net.minecraft.core.registries.BuiltInRegistries
+import net.minecraft.resources.ResourceLocation
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertNull
+import org.junit.jupiter.api.Assertions.assertNotNull
+import org.junit.jupiter.api.Assertions.assertThrows
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+@BootstrapMinecraft
+class MolangUiCodecTest {
+    private val id = ResourceLocation.fromNamespaceAndPath("cobblemon", "test")
+
+    @Test
+    fun `view codec reads mode and explicit expressions`() {
+        val view = MolangUiRegistry.parse(id, json("""
+            {"format_version":1,"contract_version":"1.2.3","mode":"screen",
+             "parameters":{"title":"string"},"actions":[],"root":{"type":"panel","text":"q.title",
+             "visible":{"molang":"v.enabled"}}}
+        """))
+        assertEquals(GuiMode.SCREEN, view.mode)
+        assertEquals("q.title", view.root.properties.getValue("text").raw)
+        assertNull(view.root.properties.getValue("text").expression)
+        assertNotNull(view.root.properties.getValue("visible").expression)
+    }
+
+    @Test
+    fun `view codec rejects unknown widgets duplicate ids and inferred event scripts`() {
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot("""{"type":"unknown"}""")) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot("""
+            {"type":"panel","controls":[{"same":{"type":"label"}},{"same":{"type":"label"}}]}
+        """)) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot("""
+            {"type":"button","on_click_client":"v.x = 1"}
+        """)) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot("""
+            {"type":"button","on_click":{"action":"undeclared"}}
+        """)) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot("""
+            {"type":"slot_grid","grid_dimensions":[1,1]}
+        """)) }
+    }
+
+    @Test
+    fun `contract codec rejects version type and collection inconsistencies`() {
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, json("""
+            {"format_version":2,"contract_version":"1","mode":"screen"}
+        """)) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, json("""
+            {"format_version":1,"contract_version":"1","mode":"screen","parameters":{"x":{"type":"wat"}}}
+        """)) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, json("""
+            {"format_version":1,"contract_version":"1","mode":"screen","collections":{"x":{"size":2,
+             "layout":{"grid_dimensions":[1,1]}}}}
+        """)) }
+    }
+
+    @Test
+    fun `contract codec preserves literal versus expression defaults`() {
+        val contract = MolangUiContractRegistry.parse(id, json("""
+            {"format_version":1,"contract_version":"1","mode":"overlay","parameters":{
+              "literal":{"type":"string","default":"v.quantity + 1"},
+              "expression":{"type":"number","default":{"molang":"v.quantity + 1"}}
+            }}
+        """))
+        assertEquals(ParameterValue.StringValue("v.quantity + 1"), (contract.parameters.getValue("literal").default as ParameterDefault.Literal).value)
+        assertEquals("v.quantity + 1", (contract.parameters.getValue("expression").default as ParameterDefault.Molang).expression)
+    }
+
+    @Test
+    fun `templates substitute typed JSON values without text injection`() {
+        val body = json("""{"inputs":{},"type":"label","text":"${'$'}text","metadata":"${'$'}metadata"}""")
+        val template = TemplateDefinition("label", "", mapOf(
+            "text" to TemplateInput(TemplateInputType.STRING),
+            "metadata" to TemplateInput(TemplateInputType.OBJECT)
+        ), null, body)
+        val root = json("""
+            {"type":"panel","controls":[{"child@label":{"text":"\"}; malicious() //","metadata":{"nested":{"safe":true}}}}]}
+        """)
+        val resolved = TemplateResolver.resolve(root, mapOf("label" to template))
+        val child = resolved.getAsJsonArray("controls")[0].asJsonObject.getAsJsonObject("child")
+        assertEquals("\"}; malicious() //", child.get("text").asString)
+        assertTrue(child.getAsJsonObject("metadata").getAsJsonObject("nested").get("safe").asBoolean)
+    }
+
+    @Test
+    fun `templates report missing references and inheritance cycles`() {
+        assertThrows(IllegalArgumentException::class.java) {
+            TemplateResolver.resolve(json("""{"type":"panel","controls":[{"x@missing":{}}]}"""), emptyMap())
+        }
+        val a = TemplateDefinition("a", "", emptyMap(), "b", json("""{"type":"panel"}"""))
+        val b = TemplateDefinition("b", "", emptyMap(), "a", json("""{"type":"panel"}"""))
+        assertThrows(IllegalArgumentException::class.java) {
+            TemplateResolver.resolve(json("""{"type":"panel","controls":[{"x@a":{}}]}"""), mapOf("a" to a, "b" to b))
+        }
+    }
+
+    @Test
+    fun `tree limits depth and node count`() {
+        var nested = "{\"type\":\"panel\"}"
+        repeat(WidgetTree.MAX_DEPTH) { index -> nested = "{\"type\":\"panel\",\"controls\":[{\"n$index\":$nested}]}" }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot(nested)) }
+
+        val controls = (0..WidgetTree.MAX_NODES).joinToString(",") { index ->
+            "{\"n$index\":{\"type\":\"label\"}}"
+        }
+        assertThrows(IllegalArgumentException::class.java) {
+            MolangUiRegistry.parse(id, viewWithRoot("{\"type\":\"panel\",\"controls\":[$controls]}"))
+        }
+    }
+
+    @Test
+    fun `rate limiter allows burst then refills at twenty per second`() {
+        val start = 10_000_000_000L
+        val limiter = MolangUiRateLimiter(start)
+        repeat(40) { assertTrue(limiter.tryAcquire(start)) }
+        assertFalse(limiter.tryAcquire(start))
+        repeat(20) { assertTrue(limiter.tryAcquire(start + 1_000_000_000L)) }
+        assertFalse(limiter.tryAcquire(start + 1_000_000_000L))
+    }
+
+    @Test
+    fun `action registry rejects duplicates and supports explicit unregister`() {
+        val key = ActionKey("test_registration")
+        MolangUiActionRegistry.unregister(key)
+        val handler = MolangUiActionHandler { }
+        MolangUiActionRegistry.register(key, handler)
+        try {
+            assertThrows(IllegalStateException::class.java) { MolangUiActionRegistry.register(key, handler) }
+        } finally {
+            assertTrue(MolangUiActionRegistry.unregister(key))
+        }
+        assertFalse(MolangUiActionRegistry.unregister(key))
+    }
+
+    @Test
+    fun `bundled examples are executable contract fixtures`() {
+        val relicCoin = ResourceLocation.fromNamespaceAndPath("cobblemon", "relic_coin")
+        listOf("battle_intro", "vending_machine").forEach { name ->
+            val resourceId = ResourceLocation.fromNamespaceAndPath("cobblemon", name)
+            val view = MolangUiRegistry.parse(
+                resourceId,
+                resourceJson("assets/cobblemon/molang_ui/$name.json")
+            )
+            val contract = MolangUiContractRegistry.parse(
+                resourceId,
+                resourceJson("data/cobblemon/molang_ui/$name.json")
+            ) { item ->
+                BuiltInRegistries.ITEM.containsKey(item) || item == relicCoin
+            }
+
+            assertEquals(contract.contractVersion, view.contractVersion, name)
+            assertEquals(contract.mode, view.mode, name)
+            assertEquals(contract.parameters.keys, view.parameters.keys, name)
+            assertEquals(contract.actions.keys, view.actions, name)
+        }
+    }
+
+    private fun viewWithRoot(root: String) = json("""
+        {"format_version":1,"contract_version":"1","mode":"screen","root":$root}
+    """)
+
+    @Test
+    fun `format versions cannot be truncated or coerced by either codec`() {
+        listOf("1.5", "4294967297", "1e100", "\"1\"", "true", "null").forEach { version ->
+            val source = json("""{"format_version":$version,"contract_version":"1","mode":"screen","root":{"type":"panel"}}""")
+            assertThrows(IllegalArgumentException::class.java, { MolangUiRegistry.parse(id, source) }, version)
+            assertThrows(IllegalArgumentException::class.java, { MolangUiContractRegistry.parse(id, source) }, version)
+        }
+        listOf("1", "1.0", "1e0").forEach { version ->
+            val source = json("""{"format_version":$version,"contract_version":"1","mode":"screen","root":{"type":"panel"}}""")
+            assertEquals(1, MolangUiRegistry.parse(id, source).formatVersion)
+            assertEquals(1, MolangUiContractRegistry.parse(id, source).formatVersion)
+        }
+    }
+
+    @Test
+    fun `contract defaults enforce declared JSON types and finite components`() {
+        val invalid = mapOf(
+            "string" to listOf("12", "true", "null", "[]"),
+            "number" to listOf("\"12\"", "true", "null", "1e400"),
+            "boolean" to listOf("\"false\"", "0", "null"),
+            "vec3" to listOf("[0,\"1\",2]", "[0,true,2]", "[0,1e400,2]", "[0,1]", "null")
+        )
+        invalid.forEach { (type, values) ->
+            values.forEach { value ->
+                val source = json("""{"format_version":1,"contract_version":"1","mode":"screen",
+                    "parameters":{"value":{"type":"$type","default":$value}}}""")
+                assertThrows(IllegalArgumentException::class.java, { MolangUiContractRegistry.parse(id, source) }, "$type: $value")
+            }
+        }
+        val source = json("""{"format_version":1,"contract_version":"1","mode":"screen","parameters":{
+            "text":{"type":"string","default":"0012"},
+            "number":{"type":"number","default":1.25},
+            "flag":{"type":"boolean","default":false},
+            "position":{"type":"vec3","default":[-1,2.5,3]}}}""")
+        val defaults = MolangUiContractRegistry.parse(id, source).parameters.mapValues {
+            (it.value.default as ParameterDefault.Literal).value
+        }
+        assertEquals(mapOf(
+            "text" to ParameterValue.StringValue("0012"),
+            "number" to ParameterValue.NumberValue(1.25),
+            "flag" to ParameterValue.BooleanValue(false),
+            "position" to ParameterValue.Vec3Value(-1.0, 2.5, 3.0)
+        ), defaults)
+        val text = source.getAsJsonObject("parameters").getAsJsonObject("text")
+        text.addProperty("default", "a".repeat(MolangUiPacketLimits.MAX_VALUE_LENGTH))
+        MolangUiContractRegistry.parse(id, source)
+        text.addProperty("default", "a".repeat(MolangUiPacketLimits.MAX_VALUE_LENGTH + 1))
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, source) }
+    }
+
+    @Test
+    fun `both codecs reject unsupported or malformed flags and blank contract versions`() {
+        listOf("blocks_input", "exclusive").forEach { flag ->
+            val values = listOf("\"true\"", "\"false\"", "1", "null") +
+                if (flag == "blocks_input") listOf("true", "false") else emptyList()
+            values.forEach { value ->
+                val source = viewWithRoot("""{"type":"panel"}""")
+                source.add(flag, JsonParser.parseString(value))
+                assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, source) }
+                assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, source) }
+            }
+        }
+        val source = viewWithRoot("""{"type":"panel"}""")
+        source.addProperty("contract_version", " ")
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, source) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, source) }
+        source.addProperty("contract_version", "1")
+        source.add("parameters", json("""{"x":{"type":"number","required":"false"}}"""))
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, source) }
+    }
+
+    @Test
+    fun `parameter declarations must fit opening packet capacity`() {
+        val view = viewWithRoot("""{"type":"panel"}""")
+        val contract = view.deepCopy()
+        val viewParameters = JsonObject()
+        val contractParameters = JsonObject()
+        repeat(MolangUiPacketLimits.MAX_ACTION_FIELDS) { index ->
+            viewParameters.addProperty("p$index", "number")
+            contractParameters.add("p$index", json("""{"type":"number"}"""))
+        }
+        view.add("parameters", viewParameters)
+        contract.add("parameters", contractParameters)
+        assertEquals(MolangUiPacketLimits.MAX_ACTION_FIELDS, MolangUiRegistry.parse(id, view).parameters.size)
+        assertEquals(MolangUiPacketLimits.MAX_ACTION_FIELDS, MolangUiContractRegistry.parse(id, contract).parameters.size)
+        viewParameters.addProperty("extra", "number")
+        contractParameters.add("extra", json("""{"type":"number"}"""))
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, view) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, contract) }
+    }
+
+    @Test
+    fun `view variables must be finite JSON numbers`() {
+        val source = viewWithRoot("""{"type":"panel"}""")
+        listOf("\"12\"", "\"NaN\"", "true", "null", "1e400").forEach { value ->
+            source.add("variables", json("""{"x":$value}"""))
+            assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, source) }
+        }
+        source.add("variables", json("""{"x":-1.25}"""))
+        assertEquals(-1.25, MolangUiRegistry.parse(id, source).variables.getValue("x"))
+    }
+
+    @Test
+    fun `explicit expressions require strings in defaults properties geometry and events`() {
+        listOf("12", "true", "null", "[]").forEach { value ->
+            val contract = json("""{"format_version":1,"contract_version":"1","mode":"screen",
+                "parameters":{"x":{"type":"number","default":{"molang":$value}}}}""")
+            assertThrows(IllegalArgumentException::class.java) { MolangUiContractRegistry.parse(id, contract) }
+            listOf(
+                """{"type":"label","text":{"molang":$value}}""",
+                """{"type":"panel","size":[{"molang":$value},20]}""",
+                """{"type":"button","on_click_client":{"molang":$value}}"""
+            ).forEach { root ->
+                assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, viewWithRoot(root)) }
+            }
+        }
+    }
+
+    @Test
+    fun `slot grid dimensions cannot silently truncate fractional values`() {
+        listOf("[1.5,1]", "[1,2.1]", "[1e100,1]", "[\"1\",1]", "[0,1]", "[17,1]").forEach { size ->
+            assertThrows(IllegalArgumentException::class.java) {
+                MolangUiRegistry.parse(id, viewWithRoot("""{"type":"slot_grid","collection_name":"items","grid_dimensions":$size}"""))
+            }
+        }
+        MolangUiRegistry.parse(id, viewWithRoot("""{"type":"slot_grid","collection_name":"items","grid_dimensions":[16,16]}"""))
+    }
+
+    @Test
+    fun `unsupported widget declarations fail validation while hover remains supported`() {
+        for (type in listOf("toggle", "slider", "text_input", "move_display")) {
+            assertThrows(IllegalArgumentException::class.java) {
+                MolangUiRegistry.parse(id, viewWithRoot("""{"type":"$type"}"""))
+            }
+        }
+        for (property in listOf("transition", "enter", "exit", "static")) {
+            assertThrows(IllegalArgumentException::class.java) {
+                MolangUiRegistry.parse(id, viewWithRoot("""{"type":"panel","$property":{}}"""))
+            }
+        }
+        val view = MolangUiRegistry.parse(id, viewWithRoot("""{"type":"button","hover":{"color":"0xFF777777"}}"""))
+        assertTrue("color" in view.root.hoverOverrides)
+    }
+
+    @Test
+    fun `unused template import declarations are rejected`() {
+        val view = viewWithRoot("""{"type":"panel"}""").apply { add("imports", JsonParser.parseString("[]")) }
+        assertThrows(IllegalArgumentException::class.java) { MolangUiRegistry.parse(id, view) }
+    }
+
+    private fun json(value: String) = JsonParser.parseString(value.trimIndent()).asJsonObject
+
+    private fun resourceJson(path: String) = MolangUiCodecTest::class.java.classLoader
+        .getResourceAsStream(path)
+        ?.bufferedReader()
+        ?.use { JsonParser.parseReader(it).asJsonObject }
+        ?: error("Missing test fixture resource: $path")
+}

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiLayoutTest.kt

@@ -0,0 +1,129 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.MoLangRuntime
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.cobblemon.mod.common.api.molangui.layout.LayoutEngine
+import com.cobblemon.mod.common.api.molangui.layout.SizeExpression
+import com.cobblemon.mod.common.api.molangui.widget.WidgetExpansion
+import com.cobblemon.mod.common.api.molangui.widget.WidgetTree
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiWidgetTree
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.cobblemon.mod.common.util.asExpression
+import com.cobblemon.mod.common.util.resolve
+import com.google.gson.JsonParser
+import net.minecraft.resources.ResourceLocation
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import java.util.UUID
+
+@BootstrapMinecraft
+class MolangUiLayoutTest {
+    private fun definition(json: String) = WidgetTree.parseWidget("root", JsonParser.parseString(json).asJsonObject)
+
+    @Test
+    fun `size literals reject implicit code unsupported auto and float overflow`() {
+        for (value in listOf("v.width", "v.changed = 1;", "auto", "NaN", "Infinity", "1e100", "9".repeat(100) + "%", true)) {
+            assertThrows(IllegalArgumentException::class.java) { SizeExpression.parse(value) }
+        }
+        assertThrows(IllegalArgumentException::class.java) {
+            definition("""{"size":["v.width",20]}""")
+        }
+        val runtime = MoLangRuntime()
+        assertEquals(90f, SizeExpression.parse("50% - 10").resolve(200f, runtime))
+        assertEquals(-10f, SizeExpression.parse(-10).resolve(200f, runtime))
+    }
+
+    @Test
+    fun `invalid dynamic layout prevents clicks and recovers when the expression becomes valid`() {
+        val tree = MolangUiWidgetTree(ResourceLocation.fromNamespaceAndPath("cobblemon", "layout_test"), UUID.randomUUID(),
+            definition("""{"type":"button","size":[{"molang":"v.width"},20],
+                "on_click_client":{"molang":"v.clicked = v.clicked + 1;"}}"""),
+            emptyMap(), mapOf("width" to 30.0, "clicked" to 0.0))
+        assertTrue(tree.prepare(200, 100))
+        for (value in listOf(Double.NaN, Double.POSITIVE_INFINITY, 1e100, -1.0, Int.MAX_VALUE.toDouble())) {
+            tree.runtime.environment.setSimpleVariable("width", DoubleValue(value))
+            assertFalse(tree.prepare(200, 100))
+            assertFalse(tree.onClick(10.0, 10.0, 0))
+        }
+        tree.updateVariables(mapOf("width" to 30.0))
+        assertTrue(tree.prepare(200, 100))
+        assertTrue(tree.onClick(10.0, 10.0, 0))
+        assertEquals(1.0, tree.runtime.resolve("v.clicked".asExpression()).asDouble())
+    }
+
+    @Test
+    fun `nested and stacked coordinates cannot overflow integer draw endpoints`() {
+        val runtime = MoLangRuntime()
+        for (json in listOf(
+            """{"size":[100,100],"offset":[2000000000,0],"controls":[{"child":{"size":[100,20],"offset":[2000000000,0]}}]}""",
+            """{"size":[100,100],"layout":"stack_horizontal","controls":[{"a":{"size":[2000000000,20]}},{"b":{"size":[2000000000,20]}}]}""",
+            """{"size":[100,100],"controls":[{"child":{"size":[{"molang":"v.invalid"},20]}}]}"""
+        )) {
+            runtime.environment.setSimpleVariable("invalid", DoubleValue(Double.NaN))
+            val root = WidgetExpansion.refresh(definition(json), runtime)
+            assertThrows(IllegalArgumentException::class.java) { LayoutEngine.layout(root, 200f, 100f, runtime) }
+        }
+    }
+
+    @Test
+    fun `fill occupies available space and exhausted grid or padded content has zero width`() {
+        val runtime = MoLangRuntime()
+        val root = WidgetExpansion.refresh(definition("""{"size":["fill","fill"],"layout":"stack_horizontal","spacing":10,
+            "controls":[{"fixed":{"size":[30,20]}},{"flex":{"size":["fill",20]}}]}"""), runtime)
+        LayoutEngine.layout(root, 200f, 100f, runtime)
+        assertEquals(200f, root.computedWidth)
+        assertEquals(160f, root.children[1].computedWidth)
+        assertEquals(40f, root.children[1].absoluteX)
+        for (extra in listOf("\"padding\":[0,60,0,60]", "\"layout\":\"grid\",\"columns\":2,\"spacing\":200")) {
+            val exhausted = WidgetExpansion.refresh(definition("""{"size":[100,20],$extra,
+                "controls":[{"child":{"size":["fill",20]}}]}"""), runtime)
+            LayoutEngine.layout(exhausted, 200f, 100f, runtime)
+            assertEquals(0f, exhausted.children.single().computedWidth)
+        }
+    }
+
+    @Test
+    fun `padding expressions retain row context and reject negative or nonfinite values`() {
+        val runtime = MoLangRuntime()
+        val root = WidgetExpansion.refresh(definition("""{"type":"list","size":[100,100],"source_type":"count","source":2,
+            "layout":"stack_vertical","item_template":{"size":[100,30],"padding":[0,0,0,{"molang":"q.item.index * 10 + v.padding"}],
+                "controls":[{"child":{"size":["100%",20]}}]}}"""), runtime)
+        runtime.environment.setSimpleVariable("padding", DoubleValue(5.0))
+        LayoutEngine.layout(root, 200f, 100f, runtime)
+        assertEquals(5f, root.children[0].children.single().absoluteX)
+        assertEquals(15f, root.children[1].children.single().absoluteX)
+        assertEquals(85f, root.children[1].children.single().computedWidth)
+        assertFalse("item" in runtime.environment.query.functions)
+        for (value in listOf(-1.0, Double.NaN, Double.POSITIVE_INFINITY)) {
+            runtime.environment.setSimpleVariable("padding", DoubleValue(value))
+            assertThrows(IllegalArgumentException::class.java) { LayoutEngine.layout(root, 200f, 100f, runtime) }
+        }
+        for (padding in listOf("[1,2]", "[0,-1,0,0]", "[0,0,0,\"v.padding\"]")) {
+            assertThrows(IllegalArgumentException::class.java) { definition("""{"padding":$padding}""") }
+        }
+    }
+
+    @Test
+    fun `nonfinite spacing rejects layout but hidden descendants do not evaluate geometry`() {
+        val runtime = MoLangRuntime()
+        runtime.environment.setSimpleVariable("invalid", DoubleValue(Double.NaN))
+        val root = WidgetExpansion.refresh(definition("""{"size":[100,100],"spacing":{"molang":"v.spacing"},
+            "controls":[{"hidden":{"size":[{"molang":"v.invalid"},20]}}]}"""), runtime)
+        root.children.single().visible = false
+        runtime.environment.setSimpleVariable("spacing", DoubleValue(Double.NaN))
+        assertThrows(IllegalArgumentException::class.java) { LayoutEngine.layout(root, 200f, 100f, runtime) }
+        runtime.environment.setSimpleVariable("spacing", DoubleValue(0.0))
+        LayoutEngine.layout(root, 200f, 100f, runtime)
+        runtime.environment.setSimpleVariable("invalid", DoubleValue(Double.NaN))
+        root.visible = false
+        LayoutEngine.layout(root, 200f, 100f, runtime)
+    }
+}

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiParametersTest.kt

@@ -0,0 +1,106 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.bedrockk.molang.runtime.value.DoubleValue
+import com.bedrockk.molang.runtime.value.StringValue
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientRuntimeFactory
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.cobblemon.mod.common.util.asExpression
+import com.cobblemon.mod.common.util.resolve
+import com.google.gson.JsonParser
+import net.minecraft.resources.ResourceLocation
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertInstanceOf
+import org.junit.jupiter.api.Assertions.assertThrows
+import org.junit.jupiter.api.Test
+
+@BootstrapMinecraft
+class MolangUiParametersTest {
+    private val id = ResourceLocation.fromNamespaceAndPath("cobblemon", "parameters")
+
+    @Test
+    fun `client queries preserve declared types through updates`() {
+        val types = mapOf("code" to ParameterType.STRING, "amount" to ParameterType.NUMBER,
+            "enabled" to ParameterType.BOOLEAN, "position" to ParameterType.VEC3)
+        val parameters = MolangUiParameters.decode(types,
+            mapOf("code" to "001", "amount" to "12.5", "enabled" to "true", "position" to "1,2,3"))
+        val runtime = MolangUiClientRuntimeFactory.create(id, parameters, emptyMap())
+        assertInstanceOf(StringValue::class.java, runtime.resolve("q.code".asExpression()))
+        assertEquals("001", runtime.resolve("q.code".asExpression()).asString())
+        assertInstanceOf(DoubleValue::class.java, runtime.resolve("q.amount".asExpression()))
+        assertEquals(12.5, runtime.resolve("q.amount".asExpression()).asDouble())
+        assertEquals(1.0, runtime.resolve("q.enabled".asExpression()).asDouble())
+        assertEquals("1.0,2.0,3.0", runtime.resolve("q.position".asExpression()).asString())
+        MolangUiClientRuntimeFactory.updateParameters(runtime,
+            MolangUiParameters.decode(types, mapOf("code" to "NaN", "enabled" to "false")))
+        assertEquals("NaN", runtime.resolve("q.code".asExpression()).asString())
+        assertEquals(0.0, runtime.resolve("q.enabled".asExpression()).asDouble())
+    }
+
+    @Test
+    fun `invalid parameters cannot overwrite runtime queries or partially update values`() {
+        val runtime = MolangUiClientRuntimeFactory.create(id,
+            mapOf("code" to ParameterValue.StringValue("original")), emptyMap())
+        assertThrows(IllegalArgumentException::class.java) {
+            MolangUiClientRuntimeFactory.updateParameters(runtime, linkedMapOf(
+                "code" to ParameterValue.StringValue("changed"), "gui_id" to ParameterValue.StringValue("wrong")))
+        }
+        assertEquals("original", runtime.resolve("q.code".asExpression()).asString())
+        assertEquals(id.toString(), runtime.resolve("q.gui_id".asExpression()).asString())
+        for (invalid in listOf("NaN", "Infinity", "1e400", "text")) {
+            assertThrows(IllegalArgumentException::class.java) {
+                MolangUiParameters.decode(mapOf("amount" to ParameterType.NUMBER), mapOf("amount" to invalid))
+            }
+        }
+        assertThrows(IllegalArgumentException::class.java) {
+            MolangUiParameters.decode(emptyMap(), mapOf("undeclared" to "1"))
+        }
+    }
+
+    @Test
+    fun `both resource codecs reject names that cannot be safe parameter queries`() {
+        for (name in listOf("gui_id", "format", "translate", "mouse_x", "item", "MixedCase", "v.nested", "bad-name")) {
+            val view = JsonParser.parseString("""{"format_version":1,"contract_version":"1","mode":"screen",
+                "parameters":{"$name":"string"},"root":{"type":"panel"}}""").asJsonObject
+            val contract = JsonParser.parseString("""{"format_version":1,"contract_version":"1","mode":"screen",
+                "parameters":{"$name":{"type":"string"}}}""").asJsonObject
+            assertThrows(IllegalArgumentException::class.java, { MolangUiRegistry.parse(id, view) }, name)
+            assertThrows(IllegalArgumentException::class.java, { MolangUiContractRegistry.parse(id, contract) }, name)
+        }
+    }
+
+    @Test
+    fun `variable updates reject invalid names aliases and nonfinite values atomically`() {
+        val runtime = MolangUiClientRuntimeFactory.create(id, emptyMap(), mapOf("v.counter" to 1.0))
+        val invalidUpdates = listOf(
+            linkedMapOf("counter" to 2.0, "v.counter" to 3.0),
+            linkedMapOf("counter" to 2.0, "nested.value" to 3.0),
+            linkedMapOf("counter" to 2.0, "invalid" to Double.NaN),
+            linkedMapOf("counter" to 2.0, "" to 3.0)
+        )
+        for (update in invalidUpdates) {
+            assertThrows(IllegalArgumentException::class.java) {
+                MolangUiClientRuntimeFactory.updateVariables(runtime, update)
+            }
+            assertEquals(1.0, runtime.resolve("v.counter".asExpression()).asDouble())
+        }
+        MolangUiClientRuntimeFactory.updateVariables(runtime, mapOf("counter" to 4.0))
+        assertEquals(4.0, runtime.resolve("v.counter".asExpression()).asDouble())
+    }
+
+    @Test
+    fun `empty string parameters preserve their type and native conditional comparison`() {
+        val runtime = MolangUiClientRuntimeFactory.create(id,
+            mapOf("name" to ParameterValue.StringValue("")), emptyMap())
+        assertEquals("", runtime.resolve("q.name".asExpression()).asString())
+        assertEquals(1.0, runtime.resolve("q.name == '' ? 1 : 0".asExpression()).asDouble())
+        assertEquals(0.0, runtime.resolve("q.name == 'trainer' ? 1 : 0".asExpression()).asDouble())
+    }
+}

common/src/test/kotlin/com/cobblemon/mod/common/api/molangui/MolangUiRuntimeMigrationTest.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2023 Cobblemon Contributors
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package com.cobblemon.mod.common.api.molangui
+
+import com.cobblemon.mod.common.client.gui.molangui.MolangUiClientRuntimeFactory
+import com.cobblemon.mod.common.junit.BootstrapMinecraft
+import com.cobblemon.mod.common.util.asExpression
+import com.cobblemon.mod.common.util.resolve
+import io.mockk.every
+import io.mockk.mockkStatic
+import io.mockk.unmockkStatic
+import net.minecraft.client.resources.language.I18n
+import net.minecraft.resources.ResourceLocation
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+@BootstrapMinecraft
+class MolangUiRuntimeMigrationTest {
+    private fun runtime() = MolangUiClientRuntimeFactory.create(
+        ResourceLocation.fromNamespaceAndPath("cobblemon", "migration"), emptyMap(), emptyMap()
+    )
+
+    @Test fun `format survives migration to the upstream general function holder`() {
+        val runtime = runtime()
+        val result = runtime.resolve("q.format(12.345, 2)".asExpression()).asString()
+        assertEquals(String.format("%.2f", 12.345), result)
+    }
+
+    @Test fun `new general functions do not widen the UI runtime permissions`() {
+        val functions = runtime().environment.query.functions
+        assertTrue("format" in functions)
+        assertTrue("to_number" in functions)
+        assertFalse("run_command" in functions)
+        assertFalse("player" in functions)
+        assertFalse("set_query" in functions)
+    }
+
+    @Test fun `translated expressions pass arguments and follow language changes`() {
+        mockkStatic(I18n::class)
+        try {
+            every { I18n.get("test.total", "7", "emeralds") } returns "Total: 7 emeralds"
+            val runtime = runtime()
+            val expression = "q.translate('test.total', '7', 'emeralds')".asExpression()
+            assertEquals("Total: 7 emeralds", runtime.resolve(expression).asString())
+            every { I18n.get("test.total", "7", "emeralds") } returns "Total : 7 émeraudes"
+            assertEquals("Total : 7 émeraudes", runtime.resolve(expression).asString())
+        } finally {
+            unmockkStatic(I18n::class)
+        }
+    }
+}