Fix: Unity IK Rig Constraint Not Affecting Bone

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:

• Root: the upper arm (shoulder or upper arm bone)
• Mid: the elbow
• Tip: the wrist/hand
• Source Objects > Target: where to move the tip toward
• Hint: where the elbow should point (optional but recommended)

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.

“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.