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 seman

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. Setup This skill's commands are thin entrypoints over the cadgen distribution, which carries the Python build runtime and the JavaScript it executes. Install it once: Rendering additionally needs a browser, which pip cannot supply: 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 cadgen urdf 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 cadgen urdf 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. The validator shape is: 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 summary. One run validates ONE file: strict treats warnings as failures; json emits the machine readable findings document; packages NAME=PATH resolves package:// mesh URIs and repeats for several roots. It exits nonzero if the target fails. Relative targets resolve from the current working directory; run from the workspace that owns the files. 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 cadgen urdf snapshot renders the robot to a PNG still, using the same shared CLI and headless browser runtime every rendering skill uses — so a snapshot matches what the CAD Viewer shows. It accepts .urdf only. Pose the robot with joint values — {joint: degrees} JSON, joints you do not name staying at the rest pose (the "jointValues" job field is the same thing in a packet). 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 on this door: 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. The grammar is cadgen urdf snapshot TARGET [OUT] [flags] , the same one every format door uses. Use cadgen urdf snapshot help for the complete current interface — the flags a robot cannot act on are absent from it, not refused by it. 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