UE State Trees

You are an expert in Unreal Engine's State Tree system for building flexible, data-driven state machines.

Context Check

Read .agents/ue-project-context.md to determine:

• Whether StateTreeModule and GameplayStateTreeModule plugins are enabled
• If Mass Entity integration is needed (MassEntity, MassAIBehavior plugins)
• Existing AI frameworks — behavior trees, custom FSMs to migrate from
• Schema types in use and any custom schemas

Information Gathering

Before implementing, clarify:

1. What is the use case? (AI behavior, game logic, UI state, entity processing)
2. What scale? (single actor with UStateTreeComponent vs thousands of Mass entities)
3. How complex? (simple linear FSM vs hierarchical states with linked subtrees)
4. Are there existing behavior trees to migrate from?
5. What external data do tasks need? (actor references, subsystems, world state)

StateTree Architecture

A State Tree is a hierarchical finite state machine authored as a UStateTree data asset:

UStateTree (UDataAsset)
  ├── UStateTreeSchema         ← defines allowed context/external data
  ├── States[]                 ← hierarchical state tree
  │     ├── Tasks[]            ← work performed while state is active
  │     ├── Transitions[]      ← rules for leaving this state
  │     └── Conditions[]       ← gates on transitions
  ├── Evaluators[]             ← global data providers (tick before transitions)
  └── Parameters               ← FInstancedPropertyBag default inputs

Runtime flow per tick: 1) Evaluators tick, 2) Transitions checked from active leaf up to root, 3) If transition fires: ExitState on old tasks then EnterState on new, 4) Active tasks tick.

Key classes:

Build.cs modules: StateTreeModule, GameplayStateTreeModule

The execution context is constructed per-tick from persistent instance data:

FStateTreeInstanceData InstanceData;  // persists across frames
// Each tick:
FStateTreeExecutionContext Context(Owner, *StateTree, InstanceData);
Context.Tick(DeltaTime);

This separates mutable state (FStateTreeInstanceData) from stateless execution logic, making State Trees safe for parallel evaluation in Mass Entity scenarios.

Schema System

Schemas define what context data a State Tree can access, constraining valid tasks and conditions. This prevents authoring errors at edit time rather than runtime.

UStateTreeComponentSchema exposes ContextActorClass (TSubclassOf<AActor>) so the editor knows which components are available for property binding. UStateTreeAIComponentSchema extends it with AIControllerClass (TSubclassOf<AAIController>).

Custom Schemas

Subclass UStateTreeSchema for project-specific trees:

UCLASS()
class UMyGameSchema : public UStateTreeSchema
{
    GENERATED_BODY()
public:
    virtual bool IsStructAllowed(const UScriptStruct* InStruct) const override;
    virtual bool IsExternalItemAllowed(const UStruct& InStruct) const override;
    virtual TConstArrayView<FStateTreeExternalDataDesc> GetContextDataDescs() const override;

#if WITH_EDITOR
    virtual bool AllowEvaluators() const override { return true; }
    virtual bool AllowMultipleTasks() const override { return true; }
    virtual bool AllowGlobalParameters() const override { return true; }
#endif // WITH_EDITOR
};

Override GetContextDataDescs() to declare context objects (actor refs, subsystems). The editor uses this to validate property bindings.

Tasks

Tasks are the primary work units in a state. They are USTRUCTs (not UObjects), making them lightweight and cache-friendly.

FStateTreeTaskBase API

Key virtuals (all const — tasks are immutable at runtime):

Behavioral Flags

Set bShouldCallTick = false for fire-and-forget tasks that only need EnterState/ExitState.

Instance Data Pattern

Tasks are const at runtime — mutable per-instance state lives in a separate struct via the typedef FInstanceDataType pattern:

USTRUCT()
struct FMyTaskInstanceData
{
    GENERATED_BODY()
    float ElapsedTime = 0.f;
};

USTRUCT(meta=(DisplayName="My Custom Task"))
struct FMyTask : public FStateTreeTaskBase
{
    GENERATED_BODY()
    typedef FMyTaskInstanceData FInstanceDataType;  // required — framework allocates storage

    virtual EStateTreeRunStatus EnterState(FStateTreeExecutionContext& Context,
        const FStateTreeTransitionResult& Transition) const override
    {
        FInstanceDataType& Data = Context.GetInstanceData(*this);
        Data.ElapsedTime = 0.f;
        return EStateTreeRunStatus::Running;
    }

    virtual EStateTreeRunStatus Tick(FStateTreeExecutionContext& Context,
        float DeltaTime) const override
    {
        FInstanceDataType& Data = Context.GetInstanceData(*this);
        Data.ElapsedTime += DeltaTime;
        return Data.ElapsedTime >= Duration
            ? EStateTreeRunStatus::Succeeded : EStateTreeRunStatus::Running;
    }

    UPROPERTY(EditAnywhere, Category = "Parameter")
    float Duration = 2.0f;
};

See references/state-tree-patterns.md for complete task, condition, and evaluator templates.

Multiple Tasks Per State

When AllowMultipleTasks() is true, a state runs several tasks simultaneously. Any task returning Failed fails the state immediately; all must return Succeeded for the state to succeed.

Conditions

Conditions gate transitions — evaluated to determine whether a transition should fire.

USTRUCT(meta=(Hidden))
struct FStateTreeConditionBase : public FStateTreeNodeBase
{
    virtual bool TestCondition(FStateTreeExecutionContext& Context) const;  // default: false
    EStateTreeExpressionOperand Operand = EStateTreeExpressionOperand::And;
    int8 DeltaIndent = 0;  // indent level for logical grouping
    EStateTreeConditionEvaluationMode EvaluationMode = EStateTreeConditionEvaluationMode::Evaluated;
};

Operands: And (both must be true), Or (either), Copy (hidden/internal). DeltaIndent creates logical grouping — conditions at the same indent level are evaluated together, enabling (A AND B) OR (C AND D) without nesting.

Built-in conditions: FStateTreeCompareIntCondition, FStateTreeCompareFloatCondition, FStateTreeCompareEnumCondition, FGameplayTagMatchCondition, FStateTreeObjectIsValidCondition, FStateTreeCompareDistanceCondition. Bind inputs via property bindings.

Evaluators

Evaluators run globally (not per-state) and execute before transitions and task ticks each frame. They inject external world data into the tree via property bindings, decoupling tasks from direct world queries.

USTRUCT(meta=(Hidden))
struct FStateTreeEvaluatorBase : public FStateTreeNodeBase
{
    virtual void TreeStart(FStateTreeExecutionContext& Context) const;
    virtual void TreeStop(FStateTreeExecutionContext& Context) const;
    virtual void Tick(FStateTreeExecutionContext& Context, float DeltaTime) const;
    // Note: DeltaTime is 0 during preselection
};

Evaluators use the same FInstanceDataType typedef pattern as tasks. Their instance data properties can be bound to task/condition inputs in the editor: Evaluator populates data, tasks/conditions read it.

When to use evaluators vs external data: Evaluators for data that changes every frame (nearest enemy, world time). External data handles for stable references (owning actor, subsystem).

Transitions

Transitions define how and when states change. Each state has an ordered list evaluated top-to-bottom — the first matching transition fires.

Trigger Types

EStateTreeTransitionTrigger is a bitmask (ENUM_CLASS_FLAGS, supports bitwise OR):

Priorities and Properties

EStateTreeTransitionPriority: Low, Normal, Medium, High, Critical. Higher-priority transitions on child states evaluate before lower-priority ones on parents.

Targets: GotoState (specific named state), NextState (next sibling), Succeeded/Failed (complete parent with that status), or tree-root Succeeded/Failed to complete the entire tree.

State Types and Selection

State Types

LinkedAsset is useful for sharing common behavior patterns (patrol, investigate, flee) across AI archetypes.

Selection Behavior

EStateTreeStateSelectionBehavior controls how child states are chosen on entry:

FStateTreeActiveStates::MaxStates = 8 — maximum depth of active state hierarchy. Stay within this limit.

Events

State Trees use a GameplayTag-based event system for decoupled communication.

FStateTreeEvent contains: FGameplayTag Tag, FInstancedStruct Payload (optional typed data), FName Origin (optional sender name for debugging).

// From outside via UStateTreeComponent
TreeComp->SendStateTreeEvent(FGameplayTag::RequestGameplayTag("AI.Alert"));

// With payload
FMyAlertData AlertData;
AlertData.ThreatLevel = 5;
TreeComp->SendStateTreeEvent(
    FGameplayTag::RequestGameplayTag("AI.Alert"),
    FConstStructView::Make(AlertData), TEXT("PerceptionSystem"));

// From inside a task via execution context
Context.SendEvent(Tag, FConstStructView::Make(ResultData), TEXT("MyTask"));

FStateTreeEventQueue holds up to MaxActiveEvents = 64 events per tick. Events are processed during transition evaluation. Use bConsumeEventOnSelect = true (default) to prevent one event triggering multiple transitions.

External Data

External data provides typed references to objects outside the tree (actors, components, subsystems) without going through evaluators.

// Declare handles in your task/condition/evaluator struct
TStateTreeExternalDataHandle<FMyActorContext, EStateTreeExternalDataRequirement::Required> ActorHandle;
TStateTreeExternalDataHandle<FMySubsystemContext, EStateTreeExternalDataRequirement::Optional> SubsystemHandle;

// Link in the Link override
virtual bool Link(FStateTreeLinker& Linker) override
{
    Linker.LinkExternalData(ActorHandle);
    Linker.LinkExternalData(SubsystemHandle);
    return true;
}

// Access at runtime
auto& ActorCtx = Context.GetExternalData(ActorHandle);       // Required — reference
auto* SubsystemCtx = Context.GetExternalDataPtr(SubsystemHandle);  // Optional — pointer

The schema's CollectExternalData populates these handles at tree start, validating at link time rather than runtime.

UStateTreeComponent

UStateTreeComponent extends UBrainComponent and manages the full tree lifecycle on an actor.

void SetStateTree(UStateTree* NewStateTree);
void SetStateTreeReference(FStateTreeReference NewStateTreeRef);
void SendStateTreeEvent(const FStateTreeEvent& Event);
void SendStateTreeEvent(FGameplayTag Tag, FConstStructView Payload, FName Origin);
EStateTreeRunStatus GetStateTreeRunStatus() const;
// Delegate — fires when run status changes
FStateTreeRunStatusChanged OnStateTreeRunStatusChanged;  // BlueprintAssignable
bool bStartLogicAutomatically = true;  // EditAnywhere

FStateTreeReference

FStateTreeReference wraps a UStateTree* with parameter overrides:

FStateTreeReference TreeRef;
TreeRef.SetStateTree(MyStateTreeAsset);
TreeRef.SyncParameters();
TreeRef.GetParameters().SetValueFloat(TEXT("AggroRange"), 1500.f);

FStateTreeReferenceOverrides swaps tree references at runtime by tag:

Overrides.AddOverride(Tag_CombatVariant, CombatTreeRef);
Overrides.RemoveOverride(Tag_CombatVariant);

Subclass UStateTreeComponent to customize context via SetContextRequirements(FStateTreeExecutionContext&, bool bLogErrors) and CollectExternalData(...).

AI Integration

UStateTreeAIComponentSchema adds AIControllerClass to the component schema, making the AI controller available as context data for property bindings.

AMyAIController::AMyAIController()
{
    StateTreeComp = CreateDefaultSubobject<UStateTreeComponent>(TEXT("StateTree"));
    StateTreeComp->bStartLogicAutomatically = true;
}

Assign the UStateTree asset in the controller defaults. Set the schema's ContextActorClass to your Pawn class so the editor can bind to its components.

State Tree vs Behavior Tree

Prefer State Trees for new AI needing Mass Entity scaling or data-driven transitions. Keep Behavior Trees for deeply nested decision logic with complex abort/decorator patterns.

Mass Entity Integration

State Trees integrate natively with Mass Entity for processing thousands of entities. See references/state-tree-mass-integration.md for complete setup.

Key concepts:

• UMassStateTreeSchema constrains trees to Mass-compatible node types (FMassStateTreeTaskBase, etc.)
• UMassStateTreeSubsystem manages pooled instance data (AllocateInstanceData/FreeInstanceData)
• UMassStateTreeProcessor evaluates trees per entity each frame
• FMassStateTreeExecutionContext wraps execution context with SetEntity/GetEntity
• Mass-specific tasks override GetDependencies(UE::MassBehavior::FStateTreeDependencyBuilder&) to declare fragment read/write requirements

For Mass Entity architecture details, see ue-mass-entity.

Common Mistakes

Persisting FStateTreeExecutionContext across frames:

// WRONG — context is per-tick, NOT persistent
FStateTreeExecutionContext* SavedContext;  // dangling after tick

// RIGHT — reconstruct each tick from persistent instance data
FStateTreeExecutionContext Context(Owner, *StateTree, InstanceData);
Context.Tick(DeltaTime);

Mutating task struct directly instead of using instance data:

// WRONG — task structs are const at runtime
virtual EStateTreeRunStatus Tick(...) const override {
    Timer += DeltaTime;  // compile error — 'this' is const
}

// RIGHT — use FInstanceDataType typedef
typedef FMyInstanceData FInstanceDataType;
virtual EStateTreeRunStatus Tick(...) const override {
    auto& Data = Context.GetInstanceData(*this);
    Data.Timer += DeltaTime;
}

Conditions with side effects: TestCondition may be called multiple times per frame during transition evaluation. Never modify state in conditions — they must be pure functions.

Evaluators doing heavy work every tick: Evaluators run every frame before transitions. Cache results in instance data and only refresh when inputs change.

Exceeding MaxStates depth: FStateTreeActiveStates::MaxStates = 8. Deeply nested hierarchies silently fail. Flatten with linked states or subtrees.

Forgetting to link external data: Unlinked Required handles assert at runtime; Optional handles silently return nullptr. Always link all declared handles in Link().

Not consuming events: With bConsumeEventOnSelect = false, the same event can trigger multiple transitions in one frame. Leave default true unless broadcast behavior is intended.

Related Skills

• ue-ai-navigation — Behavior tree patterns, AI controller setup, perception system
• ue-mass-entity — Mass Entity architecture, processors, fragments, traits
• ue-gameplay-abilities — Ability-driven state transitions, GAS integration
• ue-gameplay-framework — Game state machines, controller/pawn lifecycle
• ue-actor-component-architecture — Component setup for UStateTreeComponent
• ue-cpp-foundations — USTRUCT patterns, delegates, subsystem access