UE Animation System
Target engine: UE 5.8. APIs below are verified against the 5.8 headers; older forms are listed under "Deprecated — do not use".
Covers the runtime animation path in the Engine module — UAnimInstance, FAnimInstanceProxy, UAnimMontage, UAnimNotify/UAnimNotifyState, curves, blend spaces, anim state machines, linked anim layers and root motion — plus the animation plugins shipped in 5.8 (PoseSearch, MotionWarping, Chooser, IKRig, ControlRig, BlendStack, OptimusCore, UAF). Build.cs: Engine for everything under Runtime/Engine/Classes/Animation, AnimGraphRuntime for the helper anim nodes such as FAnimNode_LayeredBoneBlend; plugin module names are listed per row below.
Context
Read .agents/ue-project-context.md if it exists (module names, conventions, enabled plugins, GAS/networking setup). Do not stop if it is missing.
Identify the area from the request and the codebase. Ask only when two plausible readings would produce different code.
AnimInstance and Proxy
UAnimInstance (Animation/AnimInstance.h) runs in two phases. Game thread: NativeUpdateAnimation — safe to read gameplay state. Worker thread: NativeThreadSafeUpdateAnimation and blend-tree evaluation. Cache gameplay reads on the game thread; read only those cached values on the worker thread.
Verbatim virtuals (AnimInstance.h:1436-1457 public, :1716-1719 protected):
FAnimInstanceProxy (Public/Animation/AnimInstanceProxy.h) is the worker-thread data container. Its override points are protected (:603-669):
Reach the proxy with the protected templates GetProxyOnGameThread<T>() (:1746) and GetProxyOnAnyThread<T>() (:1769).
Query pending-update state with NeedsUpdate() (AnimInstance.h:512); the bNeedsUpdate member is deprecated.
Montages
Source: Animation/AnimInstance.h, Animation/AnimMontage.h. Verbatim signatures (AnimInstance.h:626-796):
EMontagePlayReturnType (AnimInstance.h:70) is MontageLength or Duration. Once blend-out starts, Montage_IsPlaying and Montage_IsActive return false and Montage_GetIsStopped returns true, even though the pose is still blending (Stop() removes the instance from ActiveMontagesMap, AnimInstance.cpp:3709). Use OnMontageBlendingOut / Montage_SetBlendingOutDelegate for blend-out start and OnMontageEnded for the end. Delegate types (AnimInstance.h:79-80) are single-cast:
For broadcast, use the dynamic multicast members OnMontageStarted, OnMontageEnded and OnMontageBlendingOut (:758-770).
USkeletalMeshComponent (Components/SkeletalMeshComponent.h):
PlayAnimation puts the component in single-node mode; SetAnimInstanceClass puts it in Animation Blueprint mode. Mixing both on one component silently drops one of them.
Multiplayer: with GAS, drive montages through UAbilityTask_PlayMontageAndWait::CreatePlayMontageAndWaitProxy (GameplayAbilities/Public/Abilities/Tasks/AbilityTask_PlayMontageAndWait.h:66) — see ue-gameplay-abilities. Without GAS, replicate the montage choice yourself and call Montage_Play from the replication callback; never call it independently per net role.
Anim Notifies
Source: Animation/AnimNotifies/AnimNotify.h, AnimNotifyState.h. Override these exact signatures:
Received_Notify (AnimNotify.h:62) and Received_NotifyBegin/Received_NotifyTick/Received_NotifyEnd (AnimNotifyState.h:45-51) are BlueprintImplementableEvent entry points — implement them in Blueprint, never override them in C++.
Montage notify timing is per event: FAnimNotifyEvent::MontageTickType (Public/Animation/AnimTypes.h:318) is EMontageNotifyTickType::Queued (fired at the end of the evaluation phase — the header states it is not suitable for changing sections or montage position) or BranchingPoint (fired as encountered, suitable for section changes). A native notify forces the branching-point path by setting bIsNativeBranchingPoint = true in its constructor (AnimNotify.h:123, AnimNotifyState.h:108).
Bind named montage notifies from outside the AnimInstance with OnPlayMontageNotifyBegin / OnPlayMontageNotifyEnd (AnimInstance.h:1820-1823, type FPlayMontageAnimNotifyDelegate).
See references/anim-notify-reference.md [blocked] for the built-in notify catalog, complete custom notify and notify-state examples, FAnimNotifyEvent fields and linked-instance propagation.
Animation Curves
Curves are addressed by FName in 5.8 — no smart names, no curve UIDs, no mapping lookups.
Per-curve flags (does the curve drive a material or a morph target) live in CurveMetaData on the skeleton (Animation/Skeleton.h:387-467): GetCurveMetaData(FName), AddCurveMetaData(FName CurveName, bool bTransact = true), GetCurveMetaDataNames(TArray<FName>& OutNames), SetCurveMetaDataMorphTarget(FName CurveName, bool bOverrideMorphTarget).
Blend Spaces and State Machines
Blend spaces are assets sampled by AnimGraph nodes; drive them from UPROPERTY members the AnimGraph reads.
Axis smoothing is FInterpolationParameter (BlendSpace.h:80-110): InterpolationTime, DampingRatio, MaxSpeed, and InterpolationType of type TEnumAsByte<EFilterInterpolationType> — values BSIT_Average, BSIT_Linear, BSIT_Cubic, BSIT_EaseInOut, BSIT_ExponentialDecay, BSIT_SpringDamper (Engine/EngineTypes.h:1356).
Bind native C++ to a compiled state machine from NativeInitializeAnimation (AnimInstance.h:1461-1473):
Query at runtime with GetStateMachineIndex(FName MachineName) (:1202), GetStateMachineInstanceFromName(FName MachineName) (:1191) and GetInstanceStateWeight(int32 MachineIndex, int32 StateIndex) (:1116).
See references/locomotion-setup.md [blocked] for a full blend space, state machine, layered-blend-per-bone and aim-offset setup.
Linked Anim Graphs and Layers
Source: Animation/AnimNode_LinkedAnimGraph.h, Animation/AnimNode_LinkedAnimLayer.h.
LinkAnimClassLayers(nullptr) restores the default layer implementations. Notify flow across linked instances is set by SetReceiveNotifiesFromLinkedInstances(bool bSet) and SetPropagateNotifiesToLinkedInstances(bool bSet) (:523-531), or per node by bReceiveNotifiesFromLinkedInstances / bPropagateNotifiesToLinkedInstances (AnimNode_LinkedAnimGraph.h:76-80). SetUseMainInstanceMontageEvaluationData(bool bSet) (:537) makes linked layers share the main instance's montage evaluation.
Blend the swap with PendingBlendInDuration / PendingBlendOutDuration on the linked node (AnimNode_LinkedAnimGraph.h:60-67), or request one from code:
Root Motion
ERootMotionMode::Type (Animation/AnimEnums.h:27-42): NoRootMotionExtraction, IgnoreRootMotion, RootMotionFromEverything, RootMotionFromMontagesOnly. The header marks RootMotionFromEverything as unsuitable for networked multiplayer and RootMotionFromMontagesOnly as the networked choice.
Montage root motion and RootMotionSource are separate systems: the montage path feeds the pose-derived delta into the movement component, while RootMotionSource applies scripted movement with no animation attached. Networked prediction, correction and bAllowPhysicsRotationDuringAnimRootMotion (GameFramework/CharacterMovementComponent.h:1083) belong to ue-character-movement.
5.8 Animation Plugins
Each is disabled by default in the .uproject unless noted; add the module to Build.cs when you use its types.
Deprecated — do not use
Common Mistakes
Touching the owning actor in NativeThreadSafeUpdateAnimation: that function runs on a worker thread, so TryGetPawnOwner(), component queries and traces are unsafe there. Cache what you need in NativeUpdateAnimation or in FAnimInstanceProxy::PreUpdate, and read only those members on the worker thread.
Binding montage delegates before Montage_Play: Montage_SetEndDelegate and Montage_SetBlendingOutDelegate resolve the active montage instance, which does not exist yet. Call Montage_Play first, check the return value is greater than zero, then bind.
Overriding Received_Notify in C++: it is a BlueprintImplementableEvent. Override Notify(USkeletalMeshComponent*, UAnimSequenceBase*, const FAnimNotifyEventReference&) instead.
Jumping sections from a queued notify: queued notifies fire after the evaluation phase, so Montage_JumpToSection from one lands a frame late. Set bIsNativeBranchingPoint = true and override BranchingPointNotify for section control.
Looking curves up by smart name or UID: FSmartName, FSmartNameMapping and AnimCurveUID are deprecated. Pass FName to GetCurveValue and keep per-curve flags in CurveMetaData.
Declaring a proxy override public: Initialize, PreUpdate, Update, Evaluate and PostUpdate are protected on FAnimInstanceProxy, and so are CreateAnimInstanceProxy / DestroyAnimInstanceProxy on UAnimInstance. Keep the access level or the override will not compile.
Skipping the base call: every Native* override calls its Super:: form, and every proxy override calls FAnimInstanceProxy::<Name>; the base implementations drive montage ticking, notify queues and proxy bookkeeping.
Related Skills
ue-character-movement—UCharacterMovementComponent,RootMotionSource, networked root motion prediction and correction.ue-gameplay-abilities—UAbilityTask_PlayMontageAndWait, GAS montage replication, ability-driven animation.ue-actor-component-architecture—USkeletalMeshComponentsetup, attachment, component tick ordering.ue-sequencer-cinematics— Level Sequence animation tracks and cinematic playback on skeletal meshes.ue-mass-entity— crowd-scale animation through Mass representation instead of per-actor AnimInstances.ue-cpp-foundations— delegate binding,UPROPERTY/UFUNCTIONspecifiers,TObjectPtr.ue-niagara-effects— Niagara systems spawned from notifies and skeletal mesh data interfaces.ue-audio-system— UAudioComponent, MetaSounds, submixes, attenuation and concurrencyue-mover— the Mover plugin: movement modes, layered moves and rollback networking


