Minecraft Modding

---
name: minecraft-modding
description: "Create, modify, debug, or migrate Minecraft mods for current NeoForge or Fabric 26.x, legacy 1.21.x, and Forge 1.20.1. Use for loader-based gameplay code and assets; use minecraft-multiloader when one codebase must target both modern loaders."
---

# Minecraft Modding Skill

## Overview

This skill guides Codex through developing open-source Minecraft mods.
Target platforms:

| Platform | MC Version | Java | Build System |
|---|---|---|---|
| **NeoForge** | 26.x current; 1.21.11 examples retained | Java 25 current; Java 21 on 1.21.x | Gradle + ModDevGradle |
| **Forge** | 1.20.1 legacy lane | Java 17 | Gradle + ForgeGradle 6 |
| **Fabric** | 26.x current; 1.21.11 examples retained | Java 25 current; Java 21 on 1.21.x | Gradle + Fabric Loom |
| **Architectury** (multiloader) | 26.x or 1.21.x | Match Minecraft | Gradle + Architectury Loom |

Always confirm the platform and Minecraft version from `gradle.properties` or `build.gradle`
before writing any mod-specific code.

Minecraft 26.1 introduced Java 25 and unobfuscated game executables. For 26.x
projects, start from the current loader generator or example mod and preserve
its build layout. Do not copy the 1.21.11 mapping, Loom plugin, remapping task,
or Java 21 snippets in this skill into a 26.x project. Fabric 26.x uses the
non-remapping Loom path and official names; NeoForge 26.x should start from the
current NeoForge generator. Treat the detailed API references here as the
legacy 1.21.x lane unless a section explicitly says 26.x.

### Routing Boundaries
- `Use when`: the task is Java/Kotlin mod code, registry/event work, networking, datagen wiring, and loader APIs.
- `Do not use when`: the task is command-only vanilla logic (`minecraft-commands-scripting`) or pure datapacks (`minecraft-datapack`).
- `Do not use when`: the task targets Paper/Bukkit plugins (`minecraft-plugin-dev`).

---

## 1. Identifying the Platform

```bash
# NeoForge project signature
grep -r "net.neoforged" gradle.properties build.gradle settings.gradle 2>/dev/null | head -5

# Forge 1.20.1 project signature
grep -r "net.minecraftforge" gradle.properties build.gradle settings.gradle 2>/dev/null | head -5

# Fabric project signature
grep -r "fabric" gradle.properties build.gradle settings.gradle 2>/dev/null | head -5

# Read mod ID and version
cat gradle.properties
```

Key files per platform:

- **NeoForge**: `src/main/resources/META-INF/neoforge.mods.toml`, annotated `@Mod` main class
- **Forge 1.20.1**: `src/main/resources/META-INF/mods.toml`, `net.minecraftforge:forge` dependency
- **Fabric**: `src/main/resources/fabric.mod.json`, class implementing `ModInitializer`
- **Architectury**: `common/`, `fabric/`, `neoforge/` subprojects

---

## 2. Build & Test Commands

```bash
# Build the mod jar
./gradlew build

# Run the Minecraft client to test
./gradlew runClient

# Run a dedicated server to test
./gradlew runServer

# Run game tests (NeoForge JUnit-style game tests)
./gradlew runGameTestServer

# Run data generation (generates JSON assets automatically)
./gradlew runData

# Clean build cache
./gradlew clean

# Check for dependency updates (optional)
./gradlew dependencyUpdates
```

After `./gradlew build`, the mod jar is at:
`build/libs/<mod_id>-<version>.jar`

---

## 3. Project Layout (NeoForge)

```
src/
  main/
    java/<groupId>/<modid>/
      MyMod.java               ← @Mod entry point
      block/
        ModBlocks.java         ← DeferredRegister<Block>
        MyCustomBlock.java
      item/
        ModItems.java          ← DeferredRegister<Item>
      entity/
        ModEntities.java       ← DeferredRegister<EntityType<?>>
      menu/                    ← custom GUI containers
      recipe/
      worldgen/
      datagen/
        ModDataGen.java        ← GatherDataEvent handler
        providers/
    resources/
      META-INF/
        neoforge.mods.toml     ← mod metadata (renamed from mods.toml in NeoForge 1.20.5+)
      assets/<modid>/
        blockstates/           ← JSON blockstate definitions
        models/
          block/               ← block model JSON
          item/                ← item model JSON
        textures/
          block/               ← 16×16 PNG textures
          item/
        lang/
          en_us.json           ← translation strings
      data/<modid>/
        recipes/               ← crafting recipe JSON
        loot_table/
          blocks/              ← per-block loot table JSON
        tags/
          blocks/
          items/
```

## 4. Project Layout (Forge 1.20.1)

Use this layout only when `minecraft_version=1.20.1` and the project depends on
`net.minecraftforge:forge`. Forge 1.20.1 is not NeoForge: keep `mods.toml`,
`net.minecraftforge.*` imports, Java 17, and ForgeGradle 6 patterns.

```
src/
  main/
    java/<groupId>/<modid>/
      MyMod.java               <- @Mod entry point
      block/
        ModBlocks.java         <- DeferredRegister<Block>
      item/
        ModItems.java          <- DeferredRegister<Item>
      datagen/
        ModDataGen.java        <- GatherDataEvent handler
    resources/
      META-INF/
        mods.toml              <- Forge metadata
      assets/<modid>/          <- client assets
      data/<modid>/            <- server data using 1.20.1 paths
```

See `references/forge-1.20.1-api.md` before editing Forge 1.20.1 projects.

## 5. Project Layout (Fabric)

```
src/
  main/
    java/<groupId>/<modid>/
      MyMod.java               ← implements ModInitializer
      client/
        MyModClient.java       ← implements ClientModInitializer
      block/
      item/
      mixin/                   ← Mixin classes
    resources/
      fabric.mod.json
      assets/<modid>/          ← same as NeoForge
      data/<modid>/            ← same as NeoForge
      <modid>.mixins.json      ← mixin configuration
```

---

## 6. Core Concepts Cheatsheet

### Sides
- **Physical client** – the game client JAR (has rendering code)
- **Physical server** – the dedicated server JAR (no rendering)
- **Logical client** – the client thread (handles rendering, input)
- **Logical server** – the server thread (handles world simulation)
- Code decorated with `@OnlyIn(Dist.CLIENT)` (NeoForge) or `@Environment(EnvType.CLIENT)` (Fabric)
  must NEVER run on the server.

### Registries
Everything in Minecraft lives in a registry. Always register objects; never
construct them at field initializer time outside a registry call. Use the
mapping-appropriate registry constants for the loader you are editing:

| Type | NeoForge / Mojang mappings | Fabric / Yarn mappings |
|------|-----------------------------|-------------------------|
| Blocks | `BuiltInRegistries.BLOCK` | `Registries.BLOCK` |
| Items | `BuiltInRegistries.ITEM` | `Registries.ITEM` |
| Entity types | `BuiltInRegistries.ENTITY_TYPE` | `Registries.ENTITY_TYPE` |
| Block entity types | `BuiltInRegistries.BLOCK_ENTITY_TYPE` | `Registries.BLOCK_ENTITY_TYPE` |
| Menu / screen-handler types | `BuiltInRegistries.MENU` | `Registries.SCREEN_HANDLER` |
| Sound events | `BuiltInRegistries.SOUND_EVENT` | `Registries.SOUND_EVENT` |
| Biomes | `Registries.BIOME` registry keys | `RegistryKeys.BIOME` registry keys |

Do not copy older `Registry.BLOCK` / `Registry.ITEM` constants into 1.21.x code;
those names are stale for the examples in this skill.

### ResourceLocation / Identifier
Every registry entry needs a namespaced ID:
```java
// NeoForge / vanilla Java
ResourceLocation id = ResourceLocation.fromNamespaceAndPath("mymod", "my_block");

// Fabric with Yarn mappings
Identifier id = Identifier.of("mymod", "my_block");
```

---

## 7. NeoForge Quick Patterns

See full patterns in `references/neoforge-api.md`.

```java
// Main mod class
@Mod(MyMod.MOD_ID)
public class MyMod {
    public static final String MOD_ID = "mymod";

    public MyMod(IEventBus modEventBus) {
        ModBlocks.BLOCKS.register(modEventBus);
        ModItems.ITEMS.register(modEventBus);
        modEventBus.addListener(this::commonSetup);
    }

    private void commonSetup(FMLCommonSetupEvent event) {
        // runs after all mods are registered
    }
}
```

```java
// Block registration
public class ModBlocks {
    public static final DeferredRegister<Block> BLOCKS =
        DeferredRegister.create(BuiltInRegistries.BLOCK, MyMod.MOD_ID);

    public static final DeferredBlock<Block> MY_BLOCK =
        BLOCKS.registerSimpleBlock("my_block",
            BlockBehaviour.Properties.of()
                .mapColor(MapColor.STONE)
                .strength(1.5f, 6.0f)
                .sound(SoundType.STONE)
                .requiresCorrectToolForDrops());
}
```

---

## 8. Forge 1.20.1 Quick Patterns

See full patterns in `references/forge-1.20.1-api.md`.

```java
// Main mod class
@Mod(MyMod.MOD_ID)
public class MyMod {
    public static final String MOD_ID = "mymod";

    public MyMod(FMLJavaModLoadingContext context) {
        IEventBus modEventBus = context.getModEventBus();
        ModBlocks.BLOCKS.register(modEventBus);
        ModItems.ITEMS.register(modEventBus);
        modEventBus.addListener(this::commonSetup);
        MinecraftForge.EVENT_BUS.register(this);
    }

    private void commonSetup(FMLCommonSetupEvent event) {
        // runs after registries are prepared
    }
}
```

```java
// Block registration
public class ModBlocks {
    public static final DeferredRegister<Block> BLOCKS =
        DeferredRegister.create(ForgeRegistries.BLOCKS, MyMod.MOD_ID);

    public static final RegistryObject<Block> MY_BLOCK =
        BLOCKS.register("my_block", () -> new Block(
            BlockBehaviour.Properties.of()
                .mapColor(MapColor.STONE)
                .strength(1.5f, 6.0f)
                .sound(SoundType.STONE)
                .requiresCorrectToolForDrops()));
}
```

---
## 9. Fabric Quick Patterns

See full patterns in `references/fabric-api.md`.

```java
// Main mod class
public class MyMod implements ModInitializer {
    public static final String MOD_ID = "mymod";
    public static final Logger LOGGER = LoggerFactory.getLogger(MOD_ID);

    @Override
    public void onInitialize() {
        ModBlocks.register();
        ModItems.register();
    }
}
```

```java
// Block registration
public class ModBlocks {
    public static final Block MY_BLOCK = new Block(
        AbstractBlock.Settings.create()
            .mapColor(MapColor.STONE)
            .strength(1.5f, 6.0f)
            .sounds(BlockSoundGroup.STONE)
            .requiresTool()
    );

    public static void register() {
        Registry.register(Registries.BLOCK,
            Identifier.of(MyMod.MOD_ID, "my_block"), MY_BLOCK);
    }
}
```

---

## 10. JSON Asset Templates

Always provide matching JSON assets for every registered block/item.
Codex should generate or update these files alongside Java code.
For Forge 1.20.1, check `references/forge-1.20.1-api.md` for legacy server-data
directory names before creating loot tables or tags.

See `references/common-patterns.md` for full JSON templates for:
- Blockstate JSON
- Block model JSON (cube, slab, stairs, fence, door, trapdoor, etc.)
- Item model JSON
- Loot table JSON
- Recipe JSON (crafting_shaped, crafting_shapeless, smelting, blasting, stonecutting)
- Language file (`en_us.json`) entries
- Tag JSON

---

## 11. Data Generation

Prefer data generation over hand-authored JSON for maintainability.

```java
// NeoForge – register data gen providers in GatherDataEvent
@SubscribeEvent
public static void gatherData(GatherDataEvent event) {
    DataGenerator gen = event.getGenerator();
    PackOutput output = gen.getPackOutput();
    ExistingFileHelper helper = event.getExistingFileHelper();
    CompletableFuture<HolderLookup.Provider> lookupProvider = event.getLookupProvider();

    gen.addProvider(event.includeClient(), new ModBlockStateProvider(output, helper));
    gen.addProvider(event.includeClient(), new ModItemModelProvider(output, helper));
    gen.addProvider(event.includeServer(), new ModRecipeProvider(output, lookupProvider));
    gen.addProvider(event.includeServer(), new ModLootTableProvider(output, lookupProvider));
    gen.addProvider(event.includeServer(), new ModBlockTagsProvider(output, lookupProvider, helper));
}
```

Run data generation with `./gradlew runData`, then commit the generated files.
For Forge 1.20.1, use the mod-event-bus registration, `GatherDataEvent`
signature, provider classes, and legacy output paths from
`references/forge-1.20.1-api.md`.

---

## 12. Common Tasks Checklist

When adding a **new block**:
- [ ] `Block` subclass (or use vanilla Block with properties)
- [ ] Register in `ModBlocks.BLOCKS` / `Registries.BLOCK`
- [ ] Register `BlockItem` in `ModItems.ITEMS` / `Registries.ITEM`
- [ ] Blockstate JSON → `assets/<modid>/blockstates/<name>.json`
- [ ] Block model JSON → `assets/<modid>/models/block/<name>.json`
- [ ] Item model JSON → `assets/<modid>/models/item/<name>.json` (or inherits from block)
- [ ] Texture PNG → `assets/<modid>/textures/block/<name>.png`
- [ ] Loot table JSON -> 1.21.x: `data/<modid>/loot_table/blocks/<name>.json`; Forge 1.20.1: `data/<modid>/loot_tables/blocks/<name>.json`
- [ ] Tags -> 1.21.x: `data/<modid>/tags/block/` and `tags/item/`; Forge 1.20.1: `data/<modid>/tags/blocks/` and `tags/items/`
- [ ] Language entry in `en_us.json`
- [ ] Mine-with-correct-tool tag if hardness > 0
- [ ] Do not mix Forge 1.20.1 plural server-data paths with 1.21.x singular server-data paths

When adding a **new item**:
- [ ] `Item` subclass (or use `new Item(properties)`)
- [ ] Register in `ModItems` / `Registries.ITEM`
- [ ] Item model JSON
- [ ] Texture PNG
- [ ] Language entry
- [ ] Creative tab registration (NeoForge/Forge: `BuildCreativeModeTabContentsEvent`; Fabric: `ItemGroupEvents`)
- [ ] Recipe JSON if craftable

When adding a **new entity**:
- [ ] Entity class (extends appropriate base: `Mob`, `Animal`, `TamableAnimal`, etc.)
- [ ] `EntityType` registration
- [ ] Renderer class (`@OnlyIn(Dist.CLIENT)`)
- [ ] Model class (`@OnlyIn(Dist.CLIENT)`)
- [ ] Register renderer in `EntityRenderersEvent.RegisterRenderers` (NeoForge) or
      `EntityModelLayerRegistry` (Fabric)
- [ ] Spawn egg item (optional)
- [ ] Spawn rules / biome modifier

---

## 13. Open-Source Conventions

- **License**: MIT or LGPL-3.0 — include `LICENSE` file and `SPDX-License-Identifier` header
- **Versioning**: `{mod_version}+{mc_version}` (e.g., `2.0.0+1.21.11`)
- **Changelog**: Keep `CHANGELOG.md` up to date with semver notes
- **Publishing**: Use `gradle-modrinth` or `curseforgegradle` plugins for CurseForge / Modrinth
- **CI**: GitHub Actions with `./gradlew build` and `./gradlew runGameTestServer`
- **PR conventions**: Keep PRs scoped to a single feature; include asset files with Java changes

---

## 14. References

- NeoForge API patterns and event system: `./references/neoforge-api.md`
- Forge 1.20.1 API patterns and MDK workflow: `./references/forge-1.20.1-api.md`
- Fabric API patterns and mixin guide: `./references/fabric-api.md`
- Blocks, items, recipes, commands, GUIs, datagen: `./references/common-patterns.md`
- NeoForge official docs: https://docs.neoforged.net/
- Fabric developer docs: https://docs.fabricmc.net/develop/
- Architectury (multiloader): https://docs.architectury.dev/
- Minecraft Wiki (data formats): https://minecraft.wiki/w/Java_Edition_data_values