Minecraft Modding Skill for Codex

Minecraft Modding Skill

Overview

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

Target platforms:

| Platform | MC Version | Java | Build System |

|---|---|---|---|

| NeoForge | 1.21.x with 1.21.11 examples | Java 21 | Gradle + ModDevGradle |

| Forge | 1.20.1 legacy lane | Java 17 | Gradle + ForgeGradle 6 |

| Fabric | 1.21.x with 1.21.11 examples | Java 21 | Gradle + Fabric Loom |

| Architectury (multiloader) | 1.21.x | Java 21 | Gradle + Architectury Loom |

Always confirm the platform and Minecraft version from gradle.properties or build.gradle

before writing any mod-specific code.

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

# 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

# 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:

// 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.

// 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
    }
}

// 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.

// 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
    }
}

// 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.

// 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();
    }
}

// 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.

// 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