Agent skill

add-custom-mob

Use when adding new mobs or entities. Covers entity class, model, renderer, client registration, spawn config, loot tables.

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Install this agent skill to your Project

npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/add-custom-mob

SKILL.md

Add Custom Mob to ChronoDawn

Purpose: Complete guide for adding a new mob entity to ChronoDawn with full multi-version and multi-loader support.

Quick Checklist

When adding a new mob, complete ALL of the following:

Java Classes (per module: 1.20.1, 1.21.1, 1.21.2, 1.21.4)

Note: 1.21.3 uses 1.21.2 modules (no separate directory)

  • Entity class (entities/mobs/MobNameEntity.java)
  • Model class (client/model/MobNameModel.java)
  • Renderer class (client/renderer/mobs/MobNameRenderer.java)
  • RenderState class (client/renderer/mobs/MobNameRenderState.java) - 1.21.2 and 1.21.4 only

Registration (per version)

  • ModEntities.java - Entity type registration
  • ModItems.java - Spawn egg registration (see custom-mob-spawn-egg skill)

Platform-Specific Registration (CRITICAL - Common source of crashes)

  • Fabric ChronoDawnFabric.java - Attribute & spawn placement registration
  • Fabric ChronoDawnClientFabric.java - Import statements for Model and Renderer
  • Fabric ChronoDawnClientFabric.java - Model layer registration (EntityModelLayerRegistry.registerModelLayer)
  • Fabric ChronoDawnClientFabric.java - Renderer registration (EntityRendererRegistry.register)
  • NeoForge ChronoDawnNeoForge.java - Attribute & spawn placement registration
  • NeoForge ChronoDawnClientNeoForge.java - Model layer & renderer registration
  • NeoForge ChronoDawnClientNeoForge.java - Spawn egg color handler

Resource Files (per version)

  • Texture file (textures/entity/mobs/mob_name.png)
  • Loot table (loot_table/entities/mob_name.json or loot_tables/ for 1.20.1)
  • Spawn egg model (models/item/mob_name_spawn_egg.json)
  • Spawn egg item definition (items/mob_name_spawn_egg.json) - 1.21.4 only
  • Language files (lang/en_us.json, lang/ja_jp.json)
  • Biome spawn configuration (if replacing vanilla mob)

Vanilla Mob Reference Parameters

When creating a mob based on a vanilla mob, use these reference values:

Base Mob sized(w, h) Health Attack Speed Other Attributes
Iron Golem 1.4f, 2.7f 100 7.5-21 0.25 Knockback Resistance 1.0
Enderman 0.6f, 2.9f 40 7 0.3 Follow Range 64
Zombie 0.6f, 1.95f 20 3 0.23 -
Spider 1.4f, 0.9f 16 2 0.3 Wall climbing
Skeleton 0.6f, 1.99f 20 - 0.25 Bow attack
Chicken 0.4f, 0.7f 4 - 0.25 -
Pig 0.9f, 0.9f 10 - 0.25 -
Rabbit 0.4f, 0.5f 3 - 0.3 -

Tip: For hostile versions of friendly mobs, consider increasing Health and adding Attack damage.

1. Entity Class

Create in common-{version}/src/main/java/com/chronodawn/entities/mobs/

1.21.2 Template

java
package com.chronodawn.entities.mobs;

import net.minecraft.world.entity.EntityType;
import net.minecraft.world.entity.ai.attributes.AttributeSupplier;
import net.minecraft.world.entity.ai.attributes.Attributes;
import net.minecraft.world.entity.monster.Monster;
import net.minecraft.world.level.Level;

public class MobNameEntity extends Monster {

    public MobNameEntity(EntityType<? extends MobNameEntity> entityType, Level level) {
        super(entityType, level);
    }

    public static AttributeSupplier.Builder createAttributes() {
        return Monster.createMonsterAttributes()
                .add(Attributes.MAX_HEALTH, 20.0D)
                .add(Attributes.ATTACK_DAMAGE, 3.0D)
                .add(Attributes.MOVEMENT_SPEED, 0.3D)
                .add(Attributes.FOLLOW_RANGE, 35.0D);
    }

    @Override
    protected void registerGoals() {
        // Add AI goals here
    }

    // 1.21.2: Use EntitySpawnReason, SynchedEntityData.Builder
    // Example for synched data:
    // @Override
    // protected void defineSynchedData(SynchedEntityData.Builder builder) {
    //     super.defineSynchedData(builder);
    //     builder.define(DATA_FLAGS, (byte)0);
    // }
}

1.21.1 Template

Key differences from 1.21.2:

  • Use MobSpawnType instead of EntitySpawnReason
  • SynchedEntityData.Builder pattern same as 1.21.2

1.20.1 Template

Key differences:

  • Use MobSpawnType instead of EntitySpawnReason
  • defineSynchedData() has no parameters, use this.entityData.define() directly
java
@Override
protected void defineSynchedData() {
    super.defineSynchedData();
    this.entityData.define(DATA_FLAGS, (byte)0);
}

2. Model Class

Create in common-{version}/src/main/java/com/chronodawn/client/model/

1.21.2 Template (RenderState pattern)

java
package com.chronodawn.client.model;

import com.chronodawn.client.renderer.mobs.MobNameRenderState;
import net.minecraft.client.model.EntityModel;
import net.minecraft.client.model.geom.ModelPart;
import net.minecraft.client.model.geom.PartPose;
import net.minecraft.client.model.geom.builders.*;

public class MobNameModel extends EntityModel<MobNameRenderState> {

    private final ModelPart body;

    public MobNameModel(ModelPart root) {
        super(root);
        this.body = root.getChild("body");
    }

    public static LayerDefinition createBodyLayer() {
        MeshDefinition meshdefinition = new MeshDefinition();
        PartDefinition partdefinition = meshdefinition.getRoot();

        partdefinition.addOrReplaceChild("body",
            CubeListBuilder.create()
                .texOffs(0, 0)
                .addBox(-4.0F, -8.0F, -4.0F, 8.0F, 8.0F, 8.0F),
            PartPose.offset(0.0F, 24.0F, 0.0F));

        return LayerDefinition.create(meshdefinition, 64, 32);
    }

    @Override
    public void setupAnim(MobNameRenderState state) {
        super.setupAnim(state);
        // Animation logic using state.walkAnimationPos, state.walkAnimationSpeed
    }
}

1.21.1 / 1.20.1 Template (Entity pattern)

java
package com.chronodawn.client.model;

import com.chronodawn.entities.mobs.MobNameEntity;
import net.minecraft.client.model.EntityModel;
import net.minecraft.client.model.geom.ModelPart;
// ... other imports

public class MobNameModel extends EntityModel<MobNameEntity> {

    public MobNameModel(ModelPart root) {
        // 1.21.1/1.20.1: No super(root) call
        this.body = root.getChild("body");
    }

    // Same createBodyLayer() as 1.21.2

    @Override
    public void setupAnim(MobNameEntity entity, float limbSwing, float limbSwingAmount,
                          float ageInTicks, float netHeadYaw, float headPitch) {
        // Animation logic
    }

    // 1.21.1: renderToBuffer with int color
    @Override
    public void renderToBuffer(PoseStack poseStack, VertexConsumer buffer,
                               int packedLight, int packedOverlay, int color) {
        body.render(poseStack, buffer, packedLight, packedOverlay, color);
    }

    // 1.20.1: renderToBuffer with float RGBA
    @Override
    public void renderToBuffer(PoseStack poseStack, VertexConsumer buffer,
                               int packedLight, int packedOverlay,
                               float red, float green, float blue, float alpha) {
        body.render(poseStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
    }
}

3. Renderer Class

Create in common-{version}/src/main/java/com/chronodawn/client/renderer/mobs/

1.21.2 Template

java
package com.chronodawn.client.renderer.mobs;

import com.chronodawn.ChronoDawn;
import com.chronodawn.client.model.MobNameModel;
import com.chronodawn.entities.mobs.MobNameEntity;
import net.minecraft.client.renderer.entity.EntityRendererProvider;
import net.minecraft.client.renderer.entity.MobRenderer;
import net.minecraft.resources.ResourceLocation;

public class MobNameRenderer extends MobRenderer<MobNameEntity, MobNameRenderState, MobNameModel> {

    private static final ResourceLocation TEXTURE =
        ResourceLocation.fromNamespaceAndPath(ChronoDawn.MOD_ID, "textures/entity/mobs/mob_name.png");

    public MobNameRenderer(EntityRendererProvider.Context context) {
        super(context, new MobNameModel(context.bakeLayer(MobNameModel.LAYER_LOCATION)), 0.5f);
    }

    @Override
    public ResourceLocation getTextureLocation(MobNameRenderState state) {
        return TEXTURE;
    }

    @Override
    public MobNameRenderState createRenderState() {
        return new MobNameRenderState();
    }
}

1.21.1 / 1.20.1 Template

java
// MobRenderer has 2 type parameters: <Entity, Model>
public class MobNameRenderer extends MobRenderer<MobNameEntity, MobNameModel> {

    @Override
    public ResourceLocation getTextureLocation(MobNameEntity entity) {
        return TEXTURE;
    }

    // No createRenderState() method needed
}

4. RenderState Class (1.21.2 and 1.21.4)

Create in common-1.21.2/ and common-1.21.4/ under src/main/java/com/chronodawn/client/renderer/mobs/

java
package com.chronodawn.client.renderer.mobs;

import net.minecraft.client.renderer.entity.state.LivingEntityRenderState;

public class MobNameRenderState extends LivingEntityRenderState {
    // Add custom state fields if needed for animations
}

5. Entity Registration (ModEntities.java)

java
public static final RegistrySupplier<EntityType<MobNameEntity>> MOB_NAME = ENTITIES.register(
    "mob_name",
    () -> EntityType.Builder.of(MobNameEntity::new, MobCategory.MONSTER)
        .sized(0.6f, 1.8f)  // width, height
        .clientTrackingRange(8)
        .updateInterval(3)
        .build(ResourceKey.create(Registries.ENTITY_TYPE,
            CompatResourceLocation.create(ChronoDawn.MOD_ID, "mob_name")))
);

1.20.1 difference: Use .build("mob_name") instead of .build(ResourceKey...)

6. Platform-Specific Registration

Fabric - ChronoDawnFabric.java

java
// Attributes
FabricDefaultAttributeRegistry.register(ModEntities.MOB_NAME.get(), MobNameEntity.createAttributes());

// Spawn placement (1.21.1/1.21.2)
SpawnPlacements.register(
    ModEntities.MOB_NAME.get(),
    SpawnPlacementTypes.ON_GROUND,
    Heightmap.Types.MOTION_BLOCKING_NO_LEAVES,
    Monster::checkMonsterSpawnRules
);

// Spawn placement (1.20.1)
SpawnPlacements.register(
    ModEntities.MOB_NAME.get(),
    SpawnPlacements.Type.ON_GROUND,  // Note: different enum
    Heightmap.Types.MOTION_BLOCKING_NO_LEAVES,
    Monster::checkMonsterSpawnRules
);

Fabric - ChronoDawnClientFabric.java

IMPORTANT: Don't forget to add imports at the top of the file!

java
// Required imports (add to import section)
import com.chronodawn.client.model.MobNameModel;
import com.chronodawn.client.renderer.mobs.MobNameRenderer;

// In registerEntityModelLayers() method - Model layer registration
EntityModelLayerRegistry.registerModelLayer(MobNameModel.LAYER_LOCATION, MobNameModel::createBodyLayer);

// In registerEntityRenderers() method - Renderer registration
EntityRendererRegistry.register(ModEntities.MOB_NAME.get(), MobNameRenderer::new);

Common Mistake: Forgetting either the imports OR the registrations will cause a NullPointerException crash when spawning the entity.

NeoForge - ChronoDawnNeoForge.java

java
// In onEntityAttributeCreation event
event.put(ModEntities.MOB_NAME.get(), MobNameEntity.createAttributes().build());

// In onSpawnPlacementRegister event
event.register(
    ModEntities.MOB_NAME.get(),
    SpawnPlacementTypes.ON_GROUND,
    Heightmap.Types.MOTION_BLOCKING_NO_LEAVES,
    Monster::checkMonsterSpawnRules,
    SpawnPlacementRegisterEvent.Operation.AND
);

NeoForge - ChronoDawnClientNeoForge.java

java
// In onRegisterEntityRenderers event
event.registerEntityRenderer(ModEntities.MOB_NAME.get(), MobNameRenderer::new);

// In onRegisterLayerDefinitions event
event.registerLayerDefinition(MobNameModel.LAYER_LOCATION, MobNameModel::createBodyLayer);

7. Resource Files

Loot Table

1.21.1 / 1.21.2 / 1.21.3 (data/chronodawn/loot_table/entities/mob_name.json):

json
{
  "type": "minecraft:entity",
  "pools": [
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:item",
          "name": "minecraft:bone",
          "functions": [
            {
              "function": "minecraft:set_count",
              "count": { "min": 0, "max": 2, "type": "minecraft:uniform" }
            },
            {
              "function": "minecraft:enchanted_count_increase",
              "enchantment": "minecraft:looting",
              "count": { "type": "minecraft:uniform", "min": 0, "max": 1 }
            }
          ]
        }
      ]
    }
  ]
}

1.20.1 (data/chronodawn/loot_tables/entities/mob_name.json):

  • Directory is loot_tables (plural)
  • Use "function": "minecraft:looting_enchant" instead of enchanted_count_increase

Language Files

json
"entity.chronodawn.mob_name": "Mob Name",
"item.chronodawn.mob_name_spawn_egg": "Mob Name Spawn Egg"

Biome Spawn Configuration

To replace a vanilla mob in ChronoDawn biomes, edit worldgen/biome/chronodawn_*.json:

json
{
  "spawners": {
    "monster": [
      { "type": "chronodawn:mob_name", "weight": 100, "minCount": 4, "maxCount": 4 }
    ]
  }
}

8. Spawn Egg

IMPORTANT: See the custom-mob-spawn-egg skill for complete spawn egg implementation checklist.

Quick reminder:

  • Register in ModItems.java
  • Add to initializeSpawnEggs() and populateCreativeTab()
  • Create models/item/mob_name_spawn_egg.json with "parent": "item/template_spawn_egg"
  • Add to NeoForge color handler in ChronoDawnClientNeoForge.java
  • 1.21.4 only: Create items/mob_name_spawn_egg.json with tints array (colors in decimal)

Version API Differences Summary

Feature 1.20.1 1.21.1 1.21.2/1.21.3 1.21.4
Spawn type enum MobSpawnType MobSpawnType EntitySpawnReason EntitySpawnReason
SynchedEntityData defineSynchedData() no params defineSynchedData(Builder) defineSynchedData(Builder) defineSynchedData(Builder)
EntityModel type param EntityModel<Entity> EntityModel<Entity> EntityModel<RenderState> EntityModel<RenderState>
MobRenderer type params 2 (Entity, Model) 2 (Entity, Model) 3 (Entity, RenderState, Model) 3 (Entity, RenderState, Model)
renderToBuffer color float r,g,b,a int color N/A (handled by RenderState) N/A (handled by RenderState)
EntityType.Builder.build() build("name") build(ResourceKey) build(ResourceKey) build(ResourceKey)
SpawnPlacements.Type SpawnPlacements.Type SpawnPlacementTypes SpawnPlacementTypes SpawnPlacementTypes
Loot table directory loot_tables/ loot_table/ loot_table/ loot_table/
Looting enchant function looting_enchant enchanted_count_increase enchanted_count_increase enchanted_count_increase
RenderState class Not needed Not needed Required Required
Damage handling hurt(DamageSource, float) hurt(DamageSource, float) hurtServer(ServerLevel, DamageSource, float) hurtServer(ServerLevel, DamageSource, float)
Y coord minimum getMinBuildHeight() getMinBuildHeight() getMinY() getMinY()
Spawn egg item def Not needed Not needed Not needed items/*.json with tints

Note: 1.21.3 is a hotfix release that shares modules with 1.21.2. No separate code changes are needed.

Common Issues

CRITICAL: The most common mistakes are missing Fabric client registrations. Always verify both model layer AND renderer are registered in ChronoDawnClientFabric.java.

Issue Cause Solution
Entity not spawning Missing attribute registration Add to Fabric/NeoForge attribute events
Model not rendering Missing model layer registration Add EntityModelLayerRegistry.registerModelLayer() in ChronoDawnClientFabric.java
Crash on entity spawn Wrong API for version Check version differences table above
NullPointerException on entityRenderer Missing Fabric renderer registration Add EntityRendererRegistry.register() in ChronoDawnClientFabric.java
Entity invisible Missing renderer registration Add to EntityRendererRegistry in BOTH Fabric and NeoForge client classes
Spawn egg crash Missing spawn egg color handler See custom-mob-spawn-egg skill
Loot table error (1.21.2) Using old looting_enchant function Use enchanted_count_increase with "enchantment": "minecraft:looting"

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