Entity Spawn Rules and Vanilla Usage Patterns

Every spawn rule lives in the BP/spawn_rules/ directory of your behavior pack. While the file name itself does not strictly have to match the entity identifier, naming it after your mob (such as custom_boar.json) keeps your project maintainable. If you want to override a vanilla mob's spawning behavior, use the exact file name and identifier from the base game.

A spawn rule file is wrapped in the minecraft:spawn_rules object and requires a format_version. Inside, it splits into two sections: a description block that links the rule to an entity and a population cap, and a conditions array that lists every valid scenario where the mob is allowed to appear.

{
  "format_version": "1.8.0",
  "minecraft:spawn_rules": {
    "description": {
      "identifier": "my_pack:forest_boar",
      "population_control": "animal"
    },
    "conditions": [
      {
        "minecraft:spawns_on_surface": {},
        "minecraft:brightness_filter": {
          "min": 7,
          "max": 15,
          "adjust_for_weather": false
        },
        "minecraft:weight": {
          "default": 10
        },
        "minecraft:herd": {
          "min_size": 2,
          "max_size": 4
        },
        "minecraft:biome_filter": {
          "test": "has_biome_tag",
          "operator": "==",
          "value": "forest"
        }
      }
    ]
  }
}

The population_control field in the description block prevents your entity from flooding the world and causing severe lag. Bedrock Edition divides natural spawns into distinct mob caps. Before the game attempts to run your conditions, it checks if the chosen pool currently has room for more entities in the active spawning area around the player.

Choosing the wrong pool is a common reason custom mobs appear too rarely or too often. Passive creatures that stay in the world permanently usually belong in the animal pool, while hostile threats that despawn when the player walks away belong in the monster pool.

Each object inside the conditions array represents a separate spawning rule. If an entity can spawn in two wildly different ways—such as on the surface in swamps and deep underground in lush caves—you create two separate objects in the conditions array. For a single condition object to succeed, every component inside that specific object must pass its check simultaneously.

Vanilla spawn rules rely on a core set of condition components to check the environment, light level, difficulty, and group sizes.

The minecraft:biome_filter component is the primary way vanilla rules keep polar bears in frozen areas and husks in deserts. Rather than checking for exact biome names, vanilla Bedrock rules almost always test against biome tags using the has_biome_tag filter test. This makes your custom mob automatically compatible with new vanilla biomes or even custom biomes from other add-ons that use standard tags.

You can combine multiple filter tests using all_of, any_of, and none_of logic groups. This is standard practice in vanilla files: a common hostile monster will require the monster biome tag while excluding mushroom islands and the deep dark.

{
  "format_version": "1.8.0",
  "minecraft:spawn_rules": {
    "description": {
      "identifier": "my_pack:cursed_skeleton",
      "population_control": "monster"
    },
    "conditions": [
      {
        "minecraft:spawns_on_surface": {},
        "minecraft:spawns_underground": {},
        "minecraft:brightness_filter": {
          "min": 0,
          "max": 7,
          "adjust_for_weather": true
        },
        "minecraft:difficulty_filter": {
          "min": "easy",
          "max": "hard"
        },
        "minecraft:weight": {
          "default": 40
        },
        "minecraft:herd": {
          "min_size": 1,
          "max_size": 2
        },
        "minecraft:biome_filter": {
          "all_of": [
            {
              "test": "has_biome_tag",
              "operator": "==",
              "value": "monster"
            },
            {
              "test": "has_biome_tag",
              "operator": "!=",
              "value": "mooshroom_island"
            },
            {
              "test": "has_biome_tag",
              "operator": "!=",
              "value": "deep_dark"
            }
          ]
        }
      }
    ]
  }
}

When designing a custom entity's spawn behavior, the safest workflow is to copy the pattern of the closest vanilla equivalent. For a standard overworld animal (like cows or sheep), use the animal population pool, set minecraft:spawns_on_surface, restrict the brightness_filter from 7 to 15, add a minecraft:spawns_on_block_filter for minecraft:grass_block, and filter for the animal biome tag with a weight between 8 and 12.

For aquatic entities, combine minecraft:spawns_underwater with the water_animal pool and test for the ocean or river biome tags. If your mob spawns in the Nether, remember that ambient light in the Nether is higher than in overworld caves; vanilla Nether mobs like piglins and magma cubes either raise their maximum brightness_filter or omit the brightness filter entirely depending on their design.

Format version 1.8.0 is the standard used across almost all vanilla spawn rules in current Bedrock releases (1.20 and 1.21+) and requires no experimental toggles. Newer format versions up to 1.11.0 also exist in some vanilla files and are stable.

Read out of the game itself, so the ids match the version you are building for.