Minecraft Modding Skill

Overview

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

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

# 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

# Remove this project's generated build outputs before a fresh rebuild
./gradlew clean

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

./gradlew build runs the project's configured build tasks. Candidate mod jars are usually under build/libs/, but task names and file names are project-specific. Treat the build output as compilation evidence, then identify the intended distributable before publishing it.

3. Project Layout (NeoForge)

src/
  main/
    java/<groupId>/<modid>/
      MyMod.java               ← @Mod entry point
      block/
        ModBlocks.java         ← DeferredRegister.Blocks
        MyCustomBlock.java
      item/
        ModItems.java          ← DeferredRegister.Items
      entity/
        ModEntities.java       ← DeferredRegister.Entities
      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
        items/                 ← 1.21.x item-definition JSON
        textures/
          block/               ← 16×16 PNG textures
          item/
        lang/
          en_us.json           ← translation strings
      data/<modid>/
        recipe/                ← crafting recipe JSON (26.x)
        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.Blocks
      item/
        ModItems.java          <- DeferredRegister.Items
      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:

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 (26.x)

For 26.x, use the explicitly labelled 26.x sections in references/common-patterns.md and select the project's exact version in the NeoForge documentation. references/neoforge-api.md contains legacy 1.21.x / Java 21 patterns only; do not copy its dependency pins into a 26.x project.

// 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.Blocks BLOCKS =
        DeferredRegister.createBlocks(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

Match the project's Minecraft version and mappings in the Fabric documentation. references/fabric-api.md contains legacy 1.21.x / Java 21 patterns only. The explicitly labelled 26.x sections in references/common-patterns.md use NeoForge syntax; adapt them against the exact Fabric API rather than copying loader-specific classes or legacy dependency pins.

// 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.initialize();
        ModItems.register();
    }
}

// Fabric 26.x with official Mojang mappings. Create the key before the object
// so its properties receive the required id during construction.
public final class ModBlocks {
    public static final ResourceKey<Block> MY_BLOCK_KEY = ResourceKey.create(
        Registries.BLOCK,
        Identifier.fromNamespaceAndPath(MyMod.MOD_ID, "my_block")
    );

    public static final Block MY_BLOCK = register(
        MY_BLOCK_KEY,
        Block::new,
        BlockBehaviour.Properties.of()
            .mapColor(MapColor.STONE)
            .strength(1.5f, 6.0f)
            .sound(SoundType.STONE)
            .requiresCorrectToolForDrops()
    );

    private static Block register(ResourceKey<Block> key,
            Function<BlockBehaviour.Properties, Block> factory,
            BlockBehaviour.Properties properties) {
        Block block = factory.apply(properties.setId(key));
        return Registry.register(BuiltInRegistries.BLOCK, key, block);
    }

    public static void initialize() {}
}

Call ModBlocks.initialize() from the Fabric initializer. The Yarn-named 1.21.11 examples remain in references/fabric-api.md; do not mix those names with this 26.x pattern.

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 (craftingshaped, craftingshapeless, 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)
• 1.21.x item definition → assets/<modid>/items/<name>.json, pointing at the item or block model
• Texture PNG → assets/<modid>/textures/block/<name>.png
• Loot table JSON -> 1.21.x: data/<modid>/loottable/blocks/<name>.json; Forge 1.20.1: data/<modid>/loottables/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
• 1.21.x item definition → assets/<modid>/items/<name>.json
• Texture PNG
• Language entry
• Creative tab registration (NeoForge/Forge: BuildCreativeModeTabContentsEvent; Fabric: ItemGroupEvents)
• Recipe JSON if craftable (data/<modid>/recipe/ for 26.x; see the

version-specific recipe reference before using a 1.21.x project)

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: {modversion}+{mcversion} (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/JavaEditiondata_values