Custom Mob Spawn Egg Implementation Guide

Purpose: Ensure all required steps are completed when adding a spawn egg for a custom mob. Spawn eggs require multiple registration points across Fabric, NeoForge, and multi-version resource files. Missing any step causes visual bugs (grayscale, invisible texture).

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Spawn Egg Implementation Checklist

When adding a spawn egg for a new custom mob, all of the following steps must be completed:

1. Item Registration (ModItems.java)

Register the spawn egg item in ModItems.java for each version.

1.21.2 (requires .setId()):

public static final RegistrySupplier<Item> MOB_NAME_SPAWN_EGG = ITEMS.register(
    "mob_name_spawn_egg",
    () -> new com.chronodawn.items.DeferredSpawnEggItem(
        ModEntities.MOB_NAME,
        0xAABBCC, // Background color
        0xDDEEFF, // Spots color
        new Item.Properties()
                .setId(ResourceKey.create(Registries.ITEM,
                    ResourceLocation.fromNamespaceAndPath(ChronoDawn.MOD_ID, "mob_name_spawn_egg")))
    )
);

1.21.1 / 1.20.1 (simpler, no .setId()):

public static final RegistrySupplier<Item> MOB_NAME_SPAWN_EGG = ITEMS.register(
    "mob_name_spawn_egg",
    () -> new com.chronodawn.items.DeferredSpawnEggItem(
        ModEntities.MOB_NAME,
        0xAABBCC, // Background color
        0xDDEEFF, // Spots color
        new Item.Properties()
    )
);

2. Initialize Spawn Egg (ModItems.java)

Add the initializeSpawnEgg() call inside the initializeSpawnEggs() method:

if (MOB_NAME_SPAWN_EGG.get() instanceof com.chronodawn.items.DeferredSpawnEggItem) {
    ((com.chronodawn.items.DeferredSpawnEggItem) MOB_NAME_SPAWN_EGG.get()).initializeSpawnEgg();
}

3. Creative Tab Entry (ModItems.java)

Add to the populateCreativeTab() method:

if (MOB_NAME_SPAWN_EGG.isPresent()) {
    output.accept(MOB_NAME_SPAWN_EGG.get());
}

4. Item Model File (CRITICAL)

Without this file, the spawn egg will be completely invisible in the inventory.

Create models/item/mob_name_spawn_egg.json in all version resource directories:

• common-1.20.1/src/main/resources/assets/chronodawn/models/item/
• common-1.21.1/src/main/resources/assets/chronodawn/models/item/
• common-1.21.2/src/main/resources/assets/chronodawn/models/item/

Content (identical for all versions):

{
  "parent": "item/template_spawn_egg"
}

No custom texture file is needed - Minecraft generates the spawn egg texture procedurally from the two color values.

5. NeoForge Color Handler Registration (CRITICAL)

Without this, the spawn egg will appear grayscale on NeoForge. Fabric handles this automatically via vanilla mechanisms, but NeoForge requires explicit registration.

File: neoforge-base/src/main/java/com/chronodawn/neoforge/client/ChronoDawnClientNeoForge.java

Add ModItems.MOB_NAME_SPAWN_EGG.get() to the item list in the onRegisterItemColors method:

@SubscribeEvent
public static void onRegisterItemColors(RegisterColorHandlersEvent.Item event) {
    event.register(
        (stack, tintIndex) -> {
            if (stack.getItem() instanceof com.chronodawn.items.DeferredSpawnEggItem egg) {
                int color = egg.getColor(tintIndex);
                return 0xFF000000 | color;
            }
            return 0xFFFFFFFF;
        },
        // ... existing spawn eggs ...
        ModItems.MOB_NAME_SPAWN_EGG.get()  // <-- Add here
    );
}

6. Language Files

Add entries to both en_us.json and ja_jp.json for all 3 versions:

"item.chronodawn.mob_name_spawn_egg": "Mob Name Spawn Egg"

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Common Issues

Symptom	Cause	Fix
Spawn egg invisible, or black square (dark purple on hover)	Missing item model JSON	Add models/item/xxx_spawn_egg.json with "parent": "item/template_spawn_egg"
Grayscale on NeoForge only	Missing NeoForge color handler	Add to onRegisterItemColors in ChronoDawnClientNeoForge.java
Correct on NeoForge, broken on Fabric	Missing initializeSpawnEgg() call	Add to initializeSpawnEggs()
No name displayed	Missing lang entry	Add to en_us.json / ja_jp.json
Not in creative tab	Missing creative tab entry	Add to populateCreativeTab()

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Why NeoForge Needs Explicit Color Registration

Minecraft spawn eggs use tintIndex (0 = background, 1 = spots) to colorize the template texture at runtime. Fabric integrates with vanilla's ColorProviderRegistry automatically through DeferredSpawnEggItem, but NeoForge uses its own event system (RegisterColorHandlersEvent.Item) which requires manual registration. The alpha channel (0xFF000000 |) must also be added explicitly on NeoForge.