Mob Effect Events

dev.architectury.event.events.common.MobEffectEvent

Fires as mob effects (potion effects) are applied to and removed from living entities. All of these events fire on the server only.

Events

Event	Listener method	Returns
ALLOW_ADD	allowAdd(LivingEntity, MobEffectInstance)	EventResult - interrupt to prevent the effect being applied.
AFTER_ADD	afterAdd(LivingEntity, MobEffectInstance)	void (notification).
ALLOW_REMOVE	allowRemove(LivingEntity, MobEffectInstance)	EventResult - interrupt to prevent the effect being removed.

ALLOW_ADD runs before the effect is applied, so interrupting it with a false result keeps the entity from gaining the effect at all. AFTER_ADD runs once the effect is on the entity and cannot be cancelled.

note

ALLOW_REMOVE covers early removals only - drinking milk, using a totem of undying, or /effect clear. It does not fire when an effect simply runs out on its own.

These mirror NeoForge's MobEffectEvent.Applicable, MobEffectEvent.Added and MobEffectEvent.Remove, and Fabric's ServerMobEffectEvents.ALLOW_ADD, AFTER_ADD and ALLOW_EARLY_REMOVE.

Examples

Make a custom entity immune to poison:

MobEffectEvent.ALLOW_ADD.register((entity, effect) -> {
    if (entity.getType() == MyEntities.MY_MOB.get() && effect.is(MobEffects.POISON)) {
        return EventResult.interruptFalse(); // the effect is not applied
    }
    return EventResult.pass();
});

React once an effect has been applied:

MobEffectEvent.AFTER_ADD.register((entity, effect) -> {
    if (entity instanceof ServerPlayer player) {
        // the player now has `effect`
    }
});

Keep a curse from being cleared early:

MobEffectEvent.ALLOW_REMOVE.register((entity, effect) -> {
    if (effect.is(MyEffects.MY_CURSE)) {
        return EventResult.interruptFalse(); // milk will not clear it
    }
    return EventResult.pass();
});