Mob Definitions

Declaring a mob

mod.content().entity("golem_friend", EntityDefinition.builder(ResourceId.of("my_mod", "textures/entity/golem_friend.png"))
        .maxHealth(20)
        .movementSpeed(0.25)
        .panicWhenHurt()
        .temptedBy(SPARK)                  // follows you when you hold this item
        .spawnEgg(CreativeTab.SPAWN_EGGS)
        .behavior(new EntityBehavior() {   // onServerTick, onInteract
            @Override
            public InteractionResult onInteract(EntityInteraction interaction) {
                if (!interaction.world().isClientSide()) {
                    interaction.entity().addEffect(GLOWING_HEART, 100, 0);
                }
                return InteractionResult.SUCCESS;
            }
        })
        .build());

• You pick standard AI goals (wandering, looking at players, fleeing when hurt, attacking, being tempted by an item) for a game pathfinding mob. Custom goals are in AI goals and riding.
• By default, the mob is drawn with the player model wearing a 64×64 skin texture. To define a box model yourself, see Mob models.
• Create it with /summon, a spawn egg, or world.spawnEntity.

Builder method	Meaning
size(width, height)	Collision box size
category(MobCategory)	CREATURE, MONSTER, AMBIENT, WATER_CREATURE
maxHealth, movementSpeed	Attributes
noWandering(), panicWhenHurt(), attackPlayers(damage), temptedBy(item)	Pick standard AI goals
goal(priority, factory), targetGoal(priority, factory)	Custom AI goals
rideable(steerable)	Can be ridden (steerable if true)
noGravity(), notPushable()	No gravity; cannot be pushed
spawnEgg(CreativeTab)	Spawn egg item (<id>_spawn_egg)
spawnsNaturally(biomes, weight, minGroup, maxGroup)	Natural spawning
model(EntityModelDefinition)	Box model
flying(flyingSpeed), swimming()	Flying mob, swimming mob. See "Movement" below
behavior(EntityBehavior)	onServerTick, onInteract callbacks

No data fixer registered for <id>

Minecraft 26.2 has no data conversion schema for mod entities, so it writes this error log line at startup. It does not affect behavior.

Movement

By default, a mob walks and runs, and floats up to the surface in water (Movement.WALK). Changing the movement style changes pathfinding, movement control, and the default wandering goal together.

Builder	Behavior
flying(flyingSpeed)	Finds paths through the air, and hovers in place when it stops (like a bee). No fall damage. Air speed is flyingSpeed (parrot 0.4)
swimming()	Finds paths only through water, and swims in any direction (like a dolphin). Breathes in water. Crawls slowly out of water. To spawn in water, use category(MobCategory.WATER_CREATURE)

mod.content().entity("minnow", EntityDefinition.builder(ResourceId.of("my_mod", "textures/entity/minnow.png"))
        .size(0.5f, 0.4f)
        .category(MobCategory.WATER_CREATURE)
        .movementSpeed(1.0)
        .swimming()
        .build());

MobView.navigateTo(x, y, z, speed) also finds paths in the mob's movement style. A flying mob goes to a point in the air, and a swimming mob goes to a point in the water.

Natural spawning

EntityDefinition.builder(texture)
        .spawnsNaturally(BiomeSelector.ids(ResourceId.minecraft("plains")), 5, 1, 2);

• When the server creates the world, the mob is added to the spawn list of the selected biomes (log line [Poly Mod Loader] Added N natural spawn(s)).
• Spawn location rules follow game mobs of the same category. Creatures spawn on bright #minecraft:animals_spawnable_on blocks, monsters in the dark, and water creatures in water.
• Check with WorldView.naturalSpawns(pos, category).
• For code biomes, add it with BiomeDefinition.builder().spawn(...). See World generation.

Projectiles

mod.content().projectile("bolt", ProjectileDefinition.builder(SPARK)   // item look drawn on the client
        .damage(2)
        .behavior(new ProjectileBehavior() {
            @Override
            public void onHitEntity(ProjectileHit hit) {
                if (hit.entity().orElseThrow() instanceof LivingEntityView living) {
                    living.addEffect(ResourceId.minecraft("glowing"), 100, 0);
                }
            }
        })
        .build());

player.launchProjectile(ResourceId.of("my_mod", "bolt"), 1.5f, 1.0f);   // speed, inaccuracy

• It flies like a snowball. At the first thing it hits, it deals damage, calls the behavior callback (onHitEntity, onHitBlock), and then disappears.
• You can set size, gravity, and damage.
• launchProjectile also fires game projectiles such as minecraft:snowball.

Entity views

Type	Main methods
EntityView	uuid, type, name, x/y/z, blockPos, world, isAlive, remove, teleport, changeDimension, data(key)
LivingEntityView	health, maxHealth, setHealth, hurt(amount), hurt(damageType, amount), addEffect, hasEffect, launchProjectile
MobView	Pathfinding, gaze, attack target. See AI goals and riding
PlayerView	sendMessage, give, countItem, mainHandItem, mainHandStack, inventory, openMenu, advancements, hasPermission

EntityView golem = world.spawnEntity(GOLEM, x, y, z);
List<EntityView> near = world.entitiesNear(x, y, z, 5);
entity.changeDimension(ResourceId.minecraft("the_nether"), x, y, z);