Why don't my Mass entity traits apply on spawn?

The MassEntityConfig must be assigned on the spawn point (MassSpawner or MassSpawnerLocation). Traits register fragments and tags; without the config the spawned entity has none. Verify the asset is set in the spawner's Entities array.

A modular package of fragments, tags, and processors that compose into an entity archetype. Stack traits in an EntityConfig: e.g. a Fly trait + Swarm trait + Hostile trait builds an enemy archetype.

Where do processors run?

Project Settings → Mass → Processor Settings. Processors must be enabled and ordered. A trait that requires a processor not enabled in this list silently does nothing.

Fix: Unreal Mass Entity Config Asset Not Applied on Spawn

Quick answer: Assign the MassEntityConfig in your MassSpawner’s Entities array. Each trait in the config registers fragments. Verify the required processors are enabled in Project Settings → Mass → Processor Settings.

Mass spawner spawns 1000 entities. They appear but don’t move, don’t collide, don’t do anything. The EntityConfig is missing or its traits aren’t producing fragments.

The Symptom

Mass entities visible in debug viewer but inert. Traits look configured but no behavior runs. Mass logs may show no processors picked up the archetype.

The Fix

Step 1: MassSpawner with EntityConfig.

MassSpawner Actor:
  Entities:
    Entity Type:    BP_FlyingEnemy_Config (UMassEntityConfigAsset)
    Number To Spawn: 1000
  Spawn Locations:
    LocationGenerator: MassEQSGenerator (or volume)

Step 2: EntityConfig with traits.

BP_FlyingEnemy_Config:
  Traits:
    + UMassMovementTrait (provides FMassVelocityFragment)
    + UMassFlyMovementTrait
    + UMassRepresentationTrait (provides ISMs)
    + UMassDistanceLODTrait
    + UMyEnemyAITrait

Each trait adds fragments and tags. Stack to compose the archetype.

Step 3: Enable processors. Project Settings → Mass. The processor list defines what runs. Disable processors you don’t need; enable any new ones your traits require.

Verifying

Console: mass.debug.printentitiesinrange. Lists each spawned entity and its fragments. Confirms the archetype matches what the EntityConfig should produce.

Or use the Mass Debugger window for a visual graph of processors and archetypes.

“Config on spawner. Traits stacked. Processors enabled. Entities behave.”

Related Issues

For Niagara skeletal stale, see Niagara skel. For UObject GC, see UObject GC.

Config + traits + processors. Mass moves.