/ue-animation-system

name: ue-animation-system
description: "Use this skill when working with Unreal Engine animation: AnimInstance, montage playback, blend space, state machine, anim notify, IK, AnimGraph, skeletal mesh, or linked anim graphs. See references/anim-notify-reference.md for notify patterns and references/locomotion-setup.md for locomotion setup. For montage integration with GAS, see ue-gameplay-abilities."
metadata:
  version: 1.0.0

UE Animation System

You are an expert in Unreal Engine's animation system.

Context Check

Read `.agents/ue-project-context.md` first. Note which plugins are enabled (Control Rig, Motion Matching, Full Body IK), whether GAS is in use, and the skeleton/character hierarchy.

Information to Gather

1. What animation need? (locomotion, ability, cinematic, IK, procedural) 2. C++ only, Blueprint only, or mixed? 3. Does the character use `ACharacter` with an existing `USkeletalMeshComponent`? 4. Is GAS active? (affects montage replication) 5. Multiplayer? (determines replication strategy) 6. Does the project use modular linked anim layers?

---

Architecture

ACharacter / AActor
  └── USkeletalMeshComponent
        └── UAnimInstance subclass
              ├── NativeInitializeAnimation()           [setup, game thread]
              ├── NativeUpdateAnimation(float dt)       [game thread]
              ├── NativeThreadSafeUpdateAnimation(dt)   [worker thread]
              ├── FAnimInstanceProxy                    [worker thread eval]
              └── Montage API / Linked Layers

Animation updates run in two phases. Game thread: `NativeUpdateAnimation` — safe to read gameplay state. Worker thread: blend tree evaluation. Write all shared state as `UPROPERTY() Transient` members in `NativeUpdateAnimation`; read those cached values in `NativeThreadSafeUpdateAnimation`.

---

AnimInstance

Subclass Pattern

// MyAnimInstance.h
UCLASS()
class MYGAME_API UMyAnimInstance : public UAnimInstance
{
    GENERATED_BODY()

    virtual void NativeInitializeAnimation() override;
    virtual void NativeUpdateAnimation(float DeltaSeconds) override;
    virtual void NativeThreadSafeUpdateAnimation(float DeltaSeconds) override;

protected:
    UPROPERTY(Transient) TObjectPtr<ACharacter> OwningCharacter;
    UPROPERTY(Transient) TObjectPtr<UCharacterMovementComponent> MovementComp;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    float Speed = 0.f;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    float Direction = 0.f;

    UPROPERTY(Transient, BlueprintReadOnly, Category="Locomotion")
    bool bIsInAir = false;
};

// MyAnimInstance.cpp
void UMyAnimInstance::NativeInitializeAnimation()
{
    Super::NativeInitializeAnimation(); // ALWAYS call super
    OwningCharacter = Cast<ACharacter>(TryGetPawnOwner());
    if (OwningCharacter)
        MovementComp = OwningCharacter->GetCharacterMovement();
}

void UMyAnimInstance::NativeUpdateAnimation(float DeltaSeconds)
{
    Super::NativeUpdateAnimation(DeltaSeconds);
    if (!OwningCharacter || !MovementComp) return;

    const FVector Velocity = MovementComp->Velocity;
    Speed    = Velocity.Size2D();
    bIsInAir = MovementComp->IsFalling();

    if (Speed > 3.f)
    {
        const FRotator ActorRot    = OwningCharacter->GetActorRotation();
        const FRotator VelocityRot = Velocity.ToOrientationRotator();
        Direction = UKismetMathLibrary::NormalizedDeltaRotator(
            VelocityRot, ActorRot).Yaw;
    }
}

void UMyAnimInstance::NativeThreadSafeUpdateAnimation(float DeltaSeconds)
{
    Super::NativeThreadSafeUpdateAnimation(DeltaSeconds);
    // Only read UPROPERTY members written in NativeUpdateAnimation above.
    // Do NOT call any UObject functions not marked BlueprintThreadSafe.
}

FAnimInstanceProxy — Thread-Safe Access

Heavy animation logic can run on worker threads via `NativeThreadSafeUpdateAnimation`. Access shared data through the proxy:

void UMyAnimInstance::NativeThreadSafeUpdateAnimation(float DeltaSeconds)
{
    FMyAnimInstanceProxy& Proxy = GetProxyOnAnyThread<FMyAnimInstanceProxy>();
    Proxy.Speed = Proxy.Velocity.Size();
    Proxy.bIsFalling = Proxy.MovementMode == EMovementMode::MOVE_Falling;
}

// FAnimInstanceProxy declaration — worker thread data container
USTRUCT()
struct FMyAnimInstanceProxy : public FAnimInstanceProxy
{
    GENERATED_BODY()
    FMyAnimInstanceProxy() = default;
    explicit FMyAnimInstanceProxy(UAnimInstance* Instance) : FAnimInstanceProxy(Instance) {}

    float Speed = 0.f;
    FVector Velocity = FVector::ZeroVector;
    TEnumAsByte<EMovementMode> MovementMode = MOVE_None;

    virtual void PreUpdate(UAnimInstance* AnimInstance, float DeltaSeconds) override;
    virtual void Update(float DeltaSeconds) override;
};

// In UMyAnimInstance: override CreateAnimInstanceProxy to return your proxy
virtual FAnimInstanceProxy* CreateAnimInstanceProxy() override
{ return new FMyAnimInstanceProxy(this); }

The engine copies data between game thread and worker thread at safe sync points.

---

Montages

Source: `AnimMontage.h`, `AnimInstance.h`

Key API (`UAnimInstance`):

float Montage_Play(UAnimMontage*, float PlayRate=1.f,
    EMontagePlayReturnType=MontageLength, float StartAt=0.f, bool bStopAll=true);
void  Montage_Stop(float BlendOut, const UAnimMontage* Montage=nullptr);
void  Montage_Pause(const UAnimMontage* Montage=nullptr);
void  Montage_Resume(const UAnimMontage* Montage);
void  Montage_JumpToSection(FName Section, const UAnimMontage* Montage=nullptr);
void  Montage_SetNextSection(FName From, FName To, const UAnimMontage* Montage=nullptr);
bool  Montage_IsActive(const UAnimMontage*) const;
bool  Montage_IsPlaying(const UAnimMontage*) const;
FName Montage_GetCurrentSection(const UAnimMontage* Montage=nullptr) const;
float Montage_GetPosition(const UAnimMontage*) const;

Playing + Delegate Pattern

void UMyComponent::PlayAttackMontage(UAnim