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name: ue-mcp-animation description: Use when creating, modifying, retargeting, rigging, constraining, or validating skeletal animation through UE-MCP. Covers native UE 5.8 IK Rig and Retargeter authoring, the Control Rig begin/read/apply/bake loop, generic contact locks, per-rig anatomical and mirrored-axis discovery, quaternion keying, deterministic bone analysis, and exact fixed-frame Unreal capture.

UE-MCP native animation workflow

Create quality animation from measured pose constraints, not guessed control angles. Control names, local axes, palm axes, handedness, and scale are properties of the selected rig. Discover them again for every unfamiliar rig; never paste values from another rig or negate a left-side Euler pose to make a right-side pose.

Focused skeletal mesh weighting

Use the existing skeleton-edit actions for bone creation, removal, reparenting, and reference-pose changes. For a small, explicit skin-weight correction:

1. Call asset(action="list_skeleton_bones", assetPath=<mesh>, includeTransforms=false) and use the returned names exactly.
2. Call asset(action="read_skeletal_mesh_skin_weights", assetPath=<mesh>, lodIndex=0, vertexIndices=[...]). Keep each read to the selected source vertex IDs; the action caps a call at 256 vertices.
3. Call asset(action="set_skeletal_mesh_skin_weights", assetPath=<mesh>, lodIndex=0, edits=[{vertexIndex, influences:[{boneName, weight}]}]) with the complete replacement influence set for every selected vertex. Inspect the returned before and after: Unreal normalizes and prunes the list, and stores source weights as uint16 values. The render buffer may quantize again to 8-bit unless the LOD enables high-precision skin weights.
4. Open the rebuilt mesh with editor(action="open_asset", assetPath=<mesh>) and capture the asset editor at the reference pose and useful preview poses. Re-read the same selected vertices before moving on.

The weight setter edits one existing source LOD and profile, saves only when a selected value changed, and leaves all unlisted vertices, LODs, and profiles alone. It does not infer bones, fit a mesh, or auto-rig a character.

To size a physics body, socket or hit volume from the mesh, call asset(action="read_skeletal_mesh_bone_vertex_bounds", assetPath=<mesh>). Each bone's boneSpace box is its dominant vertices in the bone's own bind-pose frame, the frame a physics body on that bone uses; componentSpace is the same set in the mesh's bind pose. Pass boneNames to limit the report, or minWeight to count a vertex for every bone weighing at least that much.

Required loop

1. Call project(action="get_status"); verify the intended project and editor.
2. Establish the character's authoring baseline before editing clips. Search for a Control Rig already bound to the target mesh/skeleton and inspect it with read_control_rig_hierarchy and read_control_rig_graph. It must have the controls, Forward Solve, and Backward Solve required for the intended edits. If the project or character has no suitable rig, create one first with the bundled Epic 5.8 controlrig actions (epic_create, epic_import_bones_from_asset, epic_add_control, deliberate Forward Solve nodes/links, and epic_add_backward_solve_graph), then save the exact rig with asset(epic_save_assets). epic_create alone creates no imported bones, authored controls, or solver wiring. Run an unchanged source-to-controls-to-bones round trip on the exact production mesh. Do not begin production work on an unverified or merely name-compatible rig.
3. Resolve the source AnimSequence, mesh, skeleton, verified Control Rig, and frame rate. When IK/retarget assets are part of the job, use read_ik_rig and read_ik_retargeter first. On UE 5.8 use configure_ik_rig for validated roots, ancestry-valid chains, concrete goals, solver connections and effectors; use configure_ik_retargeter for the default op stack, source and target rig assignment to every op, auto/manual mappings, preview meshes, and a named retarget pose. Read both assets back after saving. A chain's goal string is not proof that a goal or solver exists.
4. Create a versioned session with begin_control_rig_edit. Use a unique LevelSequence path and bindingTag, an end-exclusive frame range, and onConflict="error". Use rigMode="asset" for a project rig or "fk" only when generated FK controls are intentional. The source must be non-additive and mesh-compatible; flatten an additive clip against its intended base first. layered controls the session layer, not an additive source base. A finite non-zero source RateScale is compensated in Sequencer so the raw timeline maps once without modifying the source asset; zero is rejected.
5. Call read_control_rig_edit at rest, transitions, extrema, and end in both local and global space. Here global is rig/global (normally mesh component) space, not actor world space.
6. Inspect every control's controlType, animatable, and enum metadata. Write scalars with the matching set_bool, set_float, or set_int; enum values must come from enumOptions. Never write animatable=false controls.
7. Define anatomical component-space targets, then solve proximal to distal: shoulder/upper arm, elbow pole and bend, forearm direction, wrist, palm normal, then secondary motion. For a wave, the forearm must rise, the wrist must sit above the elbow/near the shoulder region, and the probed palm normal must face the intended viewer before wrist oscillation is added.
8. When an axis is uncertain, make an immutable probe session. Apply a small positive and negative rotation to one local axis at one fixed frame, bake, and inspect the resulting component-space shoulder/forearm/hand landmarks with analyze_animation. Probe the right side separately; mirrored parents or negative scale can reverse anatomical meanings. Preserve the full scale read from the control.
9. Apply absolute set_keys transforms with finite normalized quaternions, complete translation/rotationQuaternion/scale payloads, and strictly increasing frames. Preserve translation and scale unless intentionally editing them. If the source bake has dense keys, key every affected frame; sparse keys will not replace the intervening source motion. Apply related controls and scalar switches in one transaction. For a fixed contact, use contact_lock in the same operation batch. Supply a keyable translatable driver, optional driven bone/socket, inclusive frame range, component-space target, optional pole/stabilizer controls, and position/rotation tolerances. The session must contain one source animation section. The bridge samples it per frame, writes dense smooth-edged keys, and transactionally reads back the driver and stabilizers. A driven bone/socket returns verification=bake_and_analyze_required; bake, analyze every constrained frame, and reject the output if its residual misses the motion's acceptance tolerance. To follow a moving source bone/socket, set targetReference; omit target to preserve the first-frame relative transform, or provide target as an explicit transform in the reference's space. This is the generic two-hand/hand-to-prop relationship primitive. FK contacts whose translation is ignored by skeleton retargeting use a local rotation-chain solve and report solver=fk_rotation_chain; that path requires a driven bone and does not accept stabilizers. Position-only locks leave the driven control orientation unkeyed. The bridge does not guess the driver, pole, foot roll, friction, joint limits, or pelvis compensation.
10. Read back the edited frames in local and global space. Reject elbow flips, discontinuities, wrong forearm direction, wrong palm normal, or unexpected changes outside the edited chain before baking.
11. Bake to a new versioned AnimSequence with bake_control_rig_edit, reduceKeys=false, and onConflict="error". Never overwrite source, another iteration's session, or prior approved output assets.

Validation and visual review

Run analyze_animation on source and output using the same mesh, explicit frames, and bones. Include root, pelvis, the complete edited chain, feet, opposite side, and any controls/bones expected to remain unchanged. Write its native manifest.json and samples.ndjson beneath Saved/Codex/AnimationQA.

Check numeric integrity, invalid transforms, selected-bone bounds, root displacement/speed, and loop seam metrics. Derive gesture-specific checks from component transforms: shoulder-relative wrist height, elbow-to-wrist vector, elbow angle/plane stability, probed palm-normal alignment, speed/acceleration, direction changes, and drift in untouched bones. Numeric samples are the source of truth; screenshots are the human visual gate.

Use the analyzer's rateScale, effectiveDurationSeconds, and per-notify rawTriggerTimeSeconds / effectiveTriggerTimeSeconds fields when validating gameplay release timing. Effective times use the asset-rate magnitude and are null when the asset rate is zero.

For an exact native frame capture, without Computer Use or Python:

1. editor(action="open_asset", assetPath=<baked_anim>).
2. editor(action="find_object") for className="AnimSingleNodeInstance", nameContains="AnimPreviewInstance", world="any"; select the match under the current AnimationEditorPreviewActor.
3. In one editor(action="invoke_object_functions") call, invoke SetPlaying with bIsPlaying=false, then SetPosition with InPosition=frame*rateDenominator/rateNumerator and bFireNotifies=false on that object path.
4. Open the asset again to focus its window and call editor(action="capture_screenshot", target="window").
5. Capture start, entry, both extrema, exit, and end with a consistent view.

Keep versioned V&V fixtures for a from-scratch gesture, full-body IK authoring, a retarget between known skeletons, a copied animation modified through IK plus its pole target, a bone/socket contact with an unrelated simultaneous edit, and edge cases covering mirrored/negative scale, dense keys, additive-source rejection/flattening, layered sessions, scalar enums, root motion, loops, and short clips.

The loop generalizes beyond humanoid arms. Re-discover the rig mapping, then express legs as hip/knee-pole/foot/contact constraints; spine and head as arc/twist/aim constraints; tails, tentacles, and ropes as length-preserving chain curves with delayed phase; and props or mechanisms as pivot, attachment, contact, and clearance constraints. Only control/bone names, axes/signs, mirrored scale, limits, and motion constraints are rig-specific.

Compatibility and endpoint rule

The four Control Rig session actions, configure_ik_rig, configure_ik_retargeter, and contact_lock are UE 5.8 only and return unsupported_engine_version on older engines; do not invent a reflected or raw-bone fallback. Legacy IK create/read behavior is unchanged. analyze_animation is cross-version through the native APIs in the compiled engine. See the public Native Control Rig Animation guide for full call shapes and fixture guidance.

Edit ranges are [startFrame, endFrameExclusive). The bridge keeps an internal support frame for Unreal's exact-duration export sample without making the exclusive end authorable. Validate the last visible frame and exact-duration sample separately. A non-looping endpoint must hold the intended final pose without an adjacent-frame teleport or rotation jump; a looping endpoint must pass the requested seam check. A large endpoint discontinuity fails the bake.