URDF

Provenance: maintained in 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.