Quick answer: Verify RigBuilder is on the Animator root, the Rig layer is non-null and enabled, the constraint weight is > 0, and all Source/Target references point to valid transforms. Animation Rigging fails silently on invalid references.

Here is how to fix Unity IK Rig constraint not affecting bone. You add an Animation Rigging Two Bone IK Constraint to your character’s arm. You set the target to a grip marker on a sword. In play mode, the arm ignores the target entirely — it plays the animator’s arm animation as if the constraint did not exist. Animation Rigging layers constraints on top of animator output, and every layer has prerequisites that must all be met for the constraint to take effect.

The Symptom

An Animation Rigging constraint configured in the Inspector does not visibly affect the bone it should drive. Play mode shows the animator’s original motion without constraint influence. No errors in the console, or warnings about missing references that you assumed were fine.

Variant: constraint works on one character but not another with identical setup — typically a hierarchy or bone reference difference.

What Causes This

Missing RigBuilder. The RigBuilder component must exist on the same GameObject as the Animator. Without it, Rig layers do nothing. Easy to miss if you added constraints to a child but forgot to add RigBuilder to the root.

Rig layer not assigned. RigBuilder has a Rig Layers list. Each Rig layer references a Rig component in the hierarchy. If the list is empty or a reference is null, no constraints evaluate.

Weight at zero. Every Rig and every constraint has a weight property. Weight = 0 means “no effect.” The default is 1, but can get set to 0 via animation, code, or Inspector edit.

Invalid target or source reference. The constraint references specific transforms for Source and Target. If either is null or points to a deleted GameObject, the constraint does nothing silently.

Bone chain broken. Two Bone IK needs three bones: Root, Mid, Tip. If they are not in a parent-child chain (e.g. Root -> Mid -> Tip), or the hierarchy has non-bone nodes between them, the IK solver fails.

The Fix

Step 1: Verify RigBuilder placement. Select the root of your character (the GameObject with the Animator component). Confirm RigBuilder is on the same GameObject. If not, add it: Component > Animation Rigging > Rig Builder.

Step 2: Configure the Rig layer. In RigBuilder, the Rig Layers list should have at least one entry. Each entry’s Rig field should reference a Rig component (usually on a child GameObject). The Rig’s children contain the actual constraints.

// Expected hierarchy
Character (Animator, RigBuilder)
  |- Armature
  |    |- Hips
  |    |    |- Spine -> ... -> LeftHand, RightHand, etc.
  |- RigLayer (Rig component)
  |    |- LeftHandIK (TwoBoneIKConstraintData)
  |    |- RightHandIK (TwoBoneIKConstraintData)

RigBuilder references RigLayer. Constraints under RigLayer reference specific arm bones.

Step 3: Check weights. Select the constraint. The Weight slider at the top of the Inspector should be > 0 for the constraint to affect the bone. Also check Rig weight on the parent Rig — both multiply.

using UnityEngine.Animations.Rigging;

public class ToggleIK : MonoBehaviour
{
    [SerializeField] private TwoBoneIKConstraint armIK;

    public void SetIKActive(bool active)
    {
        armIK.weight = active ? 1f : 0f;
    }
}

Step 4: Verify all references. On the Two Bone IK Constraint, the Data section has:

All must be non-null. If any shows “None (Transform),” assign the correct bone or target.

Debugging with Rig Builder Preview

Select RigBuilder in the Inspector. Click the tree gizmo icons to preview constraints in Scene view. If an IK constraint is active, you see the target position and a line to the tip bone. No preview means the constraint is not firing.

Toggle Show Preview and Show Preview In Game per-rig for tracking real-time constraint behavior.

Execution Order

Rig layers execute top to bottom. If you have Arm IK followed by Spine IK, Spine IK may override the arm position. Reorder layers in RigBuilder (drag to reorder). Typical order: body > limbs > fingers.

Understanding the issue

AI bugs are emergent. The code is correct in isolation; the behavior emerges from interaction with other systems. Reproducing means controlling the interaction; fixing means deciding which interaction was wrong.

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

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Unity. 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

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

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

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

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

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For Unity-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

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Unity, 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

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

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

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

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.

“Animation Rigging is a layer cake. Every layer needs its own builder, weight, and references. Miss one and the stack collapses silently.”

Related Issues

For root motion issues, see Unity Animator Root Motion Not Applied. For animation constraint bugs, Animation Rigging Constraint Not Working covers related scenarios.

RigBuilder + Rig + non-zero weights + valid bone chain. Four things that must all be true.