Fix: Unreal AnimNotifyState Not Ticking Inside Blend Space

You add a footstep AnimNotifyState to a walk-loop Blend Space. Tick fires when running, not walking. The blend weights vary across the X axis; only sequences above the trigger threshold notify.

The Symptom

NotifyTick fires for some blend space coordinates and not others. Or fires once and then silent until you fully transition out. NotifyBegin/End may also misfire.

What Causes This

Blend Space evaluates by sampling participating AnimSequences with weights. Notifies on the Blend Space asset itself don’t exist; notifies live on each sequence. Each notify checks its sequence’s current blend weight against Trigger Weight (default 0.5). Below threshold, no notify.

The Fix

Step 1: Add the notify to each contributing sequence. Open Walk_Forward, Walk_Left, Walk_Right (or your blend axes). Place the same AnimNotifyState at the same time on each. The blend pool now has multiple instances; whichever has weight above the threshold fires.

Step 2: Lower Trigger Weight.

AnimNotify properties:
  Trigger Weight Threshold:  0.1
  Notify Filter Type:        AffectedBoneFilter (optional)

0.1 means even minor blend contributors fire the notify. For footsteps you usually want a higher threshold (0.5) so you don’t double-fire from both feet at once.

Custom CanReceiveNotify

For complex logic (notify only when running but not jumping), override CanReceiveNotify in your AnimNotifyState subclass:

UCLASS()
class UFootstepNotify : public UAnimNotifyState
{
    GENERATED_BODY()
public:
    virtual void NotifyTick(USkeletalMeshComponent* Mesh, UAnimSequenceBase* Anim, float dt, const FAnimNotifyEventReference& Ref) override
    {
        if (UAnimInstance* AI = Mesh->GetAnimInstance())
        {
            if (AI->GetCurveValue("InAir") > 0.5f) return;
        }
        // footstep logic
    }
};

Verifying

Print on NotifyTick. Walk through the blend space coordinates with PIE in slow motion. Tick should fire over the relevant range. If gaps remain, the sequence at that coordinate doesn’t have the notify.

Understanding the issue

Animation systems blend pose data over time. The blend math is straightforward; the timing isn't. State machines, transition curves, layer weights - each compounds with the others.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unreal. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unreal-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Unreal, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Notify per sequence. Trigger Weight tuned. NotifyTick fires across the blend.”

Related Issues

For AnimBP state machine, see AnimBP state. For Control Rig IK foot, see IK foot.

Notify on each sequence. Threshold tuned. Tick fires.