---
name: urdf
description: URDF robot description authoring and validation. Use when creating, editing, inspecting, validating, or debugging `.urdf` files, robot links, joints, limits, inertials, visual/collision geometry, mesh references, frame conventions, or robot-description artifacts. Use the SRDF skill for MoveIt2 semantic groups and IK/path-planning semantics; use the cad-viewer skill for local MoveIt2 server controls; use the CAD skill for STEP/STL/3MF/DXF/GLB outputs.
---

# URDF

Provenance: maintained in [earthtojake/text-to-cad](https://github.com/earthtojake/text-to-cad).
Use the installed local skill files as the runtime source of truth; the
repository link is only for provenance and release review.

Use this skill for URDF robot-description outputs. Treat URDF work as constrained kinematic modeling, not just XML writing. The main correctness risks are frame placement, joint-axis semantics, unit consistency, mesh scale, and inertial data.

## Core Rules

1. The `.urdf` file is the source of truth. Author and edit URDF XML directly; do not build a Python generation pipeline for it. There is no `gen_urdf()` contract.
2. Before writing or changing URDF XML, establish the robot's frame, joint, geometry, unit, and assumption ledger and embed it as a comment block at the top of the `.urdf` file. See `references/design-ledger.md`.
3. Use URDF frame semantics exactly. Joint origins, link frames, joint axes, and visual/collision/inertial origins use different reference frames. See `references/frame-semantics.md`.
4. Do not infer spatial transforms, mesh units, handedness, axes, or joint signs from vague prose. Use CAD transforms, dimensioned drawings, measured values, existing source data, or explicit documented assumptions.
5. Never freehand numeric values that are the result of computation — inertia tensors, centers of mass, unit conversions across many links, mirrored transforms. Compute them: closed-form formulas for primitives, or a throwaway helper script for mesh-derived values. See `references/inertials.md`.
6. For physical links, model `inertial`, `visual`, and `collision` separately when the target consumer needs them. Frame-only links may intentionally omit mass and geometry.
7. Validate every created or modified `.urdf` with `scripts/validate` before reporting completion. See `references/validation.md`.
8. Helper scripts are allowed and encouraged for computation, but they are scaffolding, not the artifact's source of truth. For complex or genuinely parametric models it is reasonable to keep a model-local helper script on disk next to related source code (for example STEP generator sources) and note it in the ledger; this is optional, and the checked-in `.urdf` remains canonical.

## CAD Viewer Handoff

After completing URDF work that creates or modifies a `.urdf`, you must ALWAYS hand the explicit file path to `$cad-viewer` when that skill is installed. `$cad-viewer` must start CAD Viewer if it is not already running and return link(s) to the relevant created or updated file(s); if `$cad-viewer` is unavailable or startup fails, report that instead of silently omitting the handoff.

## Workflow

1. Identify the target `.urdf` file and its consumers: RViz, robot_state_publisher, Gazebo/Ignition, MoveIt, a real robot driver, or another simulator.
2. Read or create the design ledger before editing frames, origins, axes, mesh scale, limits, or inertials. Keep the ledger as a comment block in the `.urdf` itself.
3. Prepare mesh assets first when links reference meshes: one mesh per link, exported in that link's frame by the owning CAD/mesh workflow. See `references/meshes.md`.
4. Author or edit the URDF XML directly, following `references/authoring-contract.md` for structure, ordering, and naming.
5. Compute — never guess — inertials and other derived numbers. See `references/inertials.md`.
6. Validate with `scripts/validate`; fix findings and re-validate until clean.
7. Run the verification recipe in `references/validation.md`: external tools when available (`check_urdf`), then a viewer review sweeping every joint.
8. Report remaining assumptions, unchecked spatial data, and validation gaps.

## Commands

Run with the Python environment for the project or workspace. Treat `python` in examples as an interpreter placeholder; if bare `python` is unavailable, substitute `python3`, a project virtualenv interpreter, or the configured interpreter path. The validator uses only the Python standard library.

From this skill directory, the validator shape is:

```bash
python scripts/validate path/to/robot.urdf
python scripts/validate path/to/a.urdf path/to/b.urdf
python scripts/validate path/to/robot.urdf --strict
python scripts/validate path/to/robot.urdf --format json
python scripts/validate path/to/robot.urdf --package robot_description=/path/to/pkg
```

The validator collects all findings in one pass (severity, code, XML path) across XML structure, tree topology, joint semantics (limits, mimic, dynamics), geometry, mesh references, materials, inertial physics, and misspelled elements, and prints a per-file summary. `--strict` treats warnings as failures; `--format json` emits a machine-readable findings document; `--package NAME=PATH` resolves `package://` mesh URIs. It exits nonzero if any target fails. Relative targets resolve from the current working directory; when running from outside this skill directory, prefix the launcher path so target files still resolve from the intended workspace.

Validation is a guardrail, not spatial proof: a URDF can pass every structural check while placing a joint in the wrong spot. The ledger and viewer sweep exist for that reason.

## Snapshot Tool

`scripts/snapshot` renders the robot to a PNG still or an orbit GIF, using the same shared
CLI and headless browser runtime every rendering skill uses — so a snapshot matches what
the CAD Viewer shows.

```bash
python scripts/snapshot --input path/to/robot.urdf --output review.png
python scripts/snapshot --input path/to/robot.urdf --output turntable.gif --mode orbit
```

It accepts `.urdf` only. Pose the robot with the job field `"jointValues"` (joint name to
degrees, defaulting to the rest pose) rather than `--params`, which is STEP-only; robots
are authored in metres and are framed on the robot scene scale automatically.

Theme settings live under one `--theme`, mirroring the viewer's Theme tab. The default
theme is `snapshot` — Workbench Light with the ground grid, origin axis and shadows
removed, because in a still image those read as geometry. There is no `--display`: display
settings (mode, clip, exploded, edges) are CAD topology settings, and a robot carries none.

Link meshes are resolved relative to the description, so they must be present: an
unhydrated Git LFS pointer fails as "No link mesh loaded for robot". Run
`git lfs checkout <mesh dir>` first.

Use `python scripts/snapshot --help` for the complete current command interface.

## References

- Authoring contract (structure, ordering, golden skeleton): `references/authoring-contract.md`
- Design ledger: `references/design-ledger.md`
- Frame semantics: `references/frame-semantics.md`
- Mesh preparation and references: `references/meshes.md`
- Inertials (formulas, scripts, sanity gates): `references/inertials.md`
- URDF edit workflow: `references/urdf-workflow.md`
- Validation and verification recipe: `references/validation.md`