attach-end-effector

---

name: attach-end-effector description: Mount an end-effector (parallel-jaw gripper, dexterous hand, suction tool, etc.) on a robot arm by attaching its MJCF to the arm's attachment_site via MuJoCo's MjSpec API. Use when combining an arm MJCF and an end-effector MJCF into a single combined model. Handles known gotchas the underlying script doesn't: mesh collisions, wrong mount poses, empty-prefix failures, skipped meshes, and tendon-driven hands.

attach-end-effector

Wrap attach_arm_end_effector.py. The script loads two MJCFs as MjSpec, attaches the end-effector at the arm's <site name="attachment_site"/> with a name prefix, compiles, and writes a combined <output>.xml + <output>.mjb + scene.xml + README.md under robots/<output>/.

The script gets the kinematic attach right but does not handle:

1. Mesh-filename collisions between arm and end-effector mesh dirs — when both ship a same-named STL/OBJ (e.g. base_link.stl), the second copy is silently dropped and mesh refs resolve to the wrong geometry.
2. End-effector mounts at the wrong pos / orientation: the standalone palm quat composes with the arm's attachment_site quat in unpredictable ways, the palm's origin may sit on the front face of the palm (Allegro), and the script's natural pos is in wrist body frame (not site frame), so a naive offset translates the palm in the wrong direction when the site has a non-identity quat. Use compute_wrist_mount.py (preflight) to get the right pos+quat.
3. Empty-prefix failures: entity-name collisions (repeated name 'X') AND bare nested <default> blocks in the eef source (empty class name). Internal namespacing only protects against the first.
4. End-effectors whose <compiler meshdir> isn't assets/ — the script hard-codes the eef mesh source, silently copies zero meshes, and emits no warning. Compile fails on missing mesh files.
5. Tendon-driven hands — MjSpec.attach can drop most tendon actuators/sensors when the eef exposes controls as <position tendon="...">.

These are addressed in references/gotchas.md and the post-script checklist in references/workflow.md.

When to use this skill

• Combining any arm + end-effector pair where both ship a standalone MJCF and the arm exposes <site name="attachment_site"/> at its wrist flange.
• The output goes into robots/<arm>_<eef>/ as a combined MJCF model (with merged assets/).

When NOT to use this skill

• The arm has no attachment_site site (add one first, in arm-only).
• You want a URDF combined model (this skill is MJCF-only).
• You want to attach two arms (no attachment_site convention there).

Workflow

1. Verify the arm MJCF has <site name="attachment_site" .../> on the wrist link. If not, add it first in the arm-only file.
2. Choose a prefix. Default to a non-empty prefix (e.g. barrett_, right_). Empty prefix has two failure modes — entity-name collisions AND bare nested <default> blocks — and internal namespacing (e.g. barrett's bh_*) only protects against the first. See references/gotchas.md §3 for the two grep checks that justify empty if you really want it.
3. Preflight: compute the mount pos + quat with the bundled helper. Pass BOTH the arm and the eef so it reads the attachment_site quat and outputs values in wrist body frame:

   ./.venv/bin/python .claude/skills/attach-end-effector/scripts/compute_wrist_mount.py \
       robots/<eef>/<eef>.xml --arm robots/<arm>/<arm>.xml
   

   It prints a single pos="X Y Z" quat="W X Y Z" line to apply on the prefixed root body after the script runs (step 7 below). If the palm visually mounts rotated, re-run with --twist 90/180/270 and pick the one that looks right. See references/gotchas.md §2.
4. Run the script (see Commands).
5. Verify end-effector meshes were actually copied. The script hard-codes <eef>/assets/ as the source; if the end-effector's <compiler meshdir> is anything else (e.g. meshes, unset, etc.), zero eef meshes get copied and the script emits no warning. See references/gotchas.md §4 for the parse-meshdir + cp recipe.
6. Check for mesh-filename collisions in the merged assets/. See references/gotchas.md §1 — same-named STL/OBJ files between arm and eef silently overwrite each other.
7. Apply the helper's pos + quat to the prefixed root body in the combined XML. Both must be applied together — using one without the other reproduces the original problem; see references/gotchas.md §2.
8. Open the combined scene.xml in mujoco.viewer and verify the palm mounts flush at the flange with no finger/wrist clipping. If the orientation is off, dial in --twist (step 3) and re-apply.
9. Verify all combined actuators/tendons/sensors survived. Expected nu is usually arm.nu + eef.nu; tendon-driven hands can also require ntendon and <sensor> checks. See references/gotchas.md §5 for Aero-style tendon hands.
10. Verify all combined actuators map to the intended joints or tendons (sliders in viewer should move the right things on both arm and eef).

Commands

Run from the bundle root with the bundle-local venv. The wrapped script ships in this skill at .claude/skills/attach-end-effector/scripts/attach_arm_end_effector.py. The script resolves relative --arm / --end-effector / --output paths against its own directory, so pass absolute paths via $(pwd)/ to land outputs under the bundle's robots/.

cd robot-assets-skills/
./.venv/bin/python .claude/skills/attach-end-effector/scripts/attach_arm_end_effector.py \
    --arm          "$(pwd)/robots/<arm>/<arm>.xml" \
    --end-effector "$(pwd)/robots/<eef>/<eef>.xml" \
    --output       "$(pwd)/robots/<arm>_<eef>" \
    --prefix       "<prefix>_" \
    --no-viewer

Verify the model compiles and loads:

./.venv/bin/python -c "
import mujoco
m = mujoco.MjModel.from_xml_path('robots/<arm>_<eef>/<arm>_<eef>.xml')
print('nq=', m.nq, 'nu=', m.nu, 'nbody=', m.nbody)
"

View interactively (set $DISPLAY first if headless — see AGENTS.md):

./.venv/bin/python -m mujoco.viewer \
    --mjcf robots/<arm>_<eef>/scene.xml

If .venv/ doesn't exist or is missing packages, bootstrap per AGENTS.md's "Python environment" section. If robots/ is empty, bootstrap per README.md's "First-time setup" (copy your arm + eef folders in).

References

• Step-by-step procedure: references/workflow.md
• Known gotchas with concrete fixes: references/gotchas.md