SDF

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 when the deliverable is an SDFormat document. SDFormat describes simulator and world behavior: models, worlds, frames, poses, links, joints, inertials, visuals, collisions, sensors, lights, physics, plugins, includes, and simulator metadata.

This skill is for SDFormat, not signed-distance-field geometry.

The .sdf file is the source of truth: author and edit the XML directly. There is no gen_sdf() contract.

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:

python -m pip install -r requirements.txt

Rendering additionally needs a browser, which pip cannot supply:

python -m playwright install chromium

Core rules

1. Author .sdf XML directly and validate every created or modified file with cadgen sdf validate before reporting completion.
2. Identify the target consumer before editing: Gazebo/libsdformat version, another simulator, visualization-only tooling, model package, or world handoff.
3. Decide document kind: model-level SDF, world-level SDF, or model-in-world. Prefer model-level SDF for reusable robot/object exports.
4. Use SI units unless the target explicitly requires otherwise: meters, kilograms, seconds, radians.
5. Prefer version="1.12" for new outputs unless the target consumer constrains the version.
6. Establish the design ledger before writing poses, frames, joint axes, mesh scales, inertials, sensors, or plugins, and keep it as a comment block at the top of the .sdf. Use references/design-ledger.md and references/llm-guardrails.md.
7. Write relative_to / expressed_in explicitly on every nontrivial pose and axis. Implicit frame defaults are the top SDF failure mode. See references/frame-semantics.md.
8. Do not infer spatial transforms from visual impression alone. Derive poses, axes, scale, mass, inertia, and frame names from upstream source data, drawings, simulator documentation, measured values, or explicit assumptions. Never freehand computed numbers — use formulas or a throwaway helper script (inertia tensors, unit conversions).
9. When the robot already has a URDF, derive the SDF from it instead of re-authoring geometry; see references/interoperability.md.
10. Regenerate upstream geometry, mesh, robot-description, render, topology, or package assets with their owning workflows before editing SDF that references them.
11. After authoring, run available checks: bundled validation (which runs gz sdf --check itself whenever gz is on PATH), simulator load, joint motion, and plugin/sensor startup.
12. Report assumptions, skipped checks, unresolved resource paths, and target-specific compatibility risks.

Scope

Use this skill for SDFormat outputs. Do not use it for signed-distance-field modeling, raw geometry generation, planning semantics, or to paper over incorrect upstream robot/source data unless the task is explicitly simulator-only.

CAD Viewer Handoff

After completing SDF work that creates or modifies a .sdf, 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. Locate the target .sdf and its consumers.
2. Read or create the design ledger comment block.
3. Read references/frame-semantics.md before editing any <pose>, <frame>, joint axis, relative_to, expressed_in, nested scope, sensor frame, or plugin frame.
4. Author the XML directly, following the worked examples in references/examples.md.
5. Validate the explicit target with cadgen sdf validate; treat bundled validation as a guardrail, not simulator proof.
6. Run target-consumer smoke tests when available (references/smoke-tests.md).
7. Hand the file to $cad-viewer. Static rendering does not execute SDF plugins or read file-authored motion metadata.
8. Report checks run, checks skipped, and assumptions.

Commands

Run cadgen from the Python environment this skill's requirements.txt was installed into (python -m cadgen.cli <verb> with that interpreter is the PATH-independent equivalent). cadgen doctor <skill-dir> verifies the installed cadgen matches this skill's pin — docs drift silently on a mismatched install. Validation itself needs nothing beyond the Python standard library; only snapshots need the browser. Use cadgen <verb> --help for the complete current interface.

cadgen sdf validate path/to/model.sdf
cadgen sdf validate path/to/model.sdf --strict
cadgen sdf validate path/to/model.sdf --json
cadgen sdf snapshot path/to/model.sdf review.png

The validator checks document shape, name scopes, pose/frame graphs, joints, geometry, mesh URIs, inertials, sensors, and plugins, and prints its findings plus a summary. One run validates ONE file: --strict treats warnings as failures and --json prints one line of {"ok", "path", "issues": [{"severity", "code", "message", "element", "hint"}], "summary"}, where element is the XML path. It exits nonzero if the target fails.

External checking is on by default:

cadgen sdf validate path/to/model.sdf --gz-check required
cadgen sdf validate path/to/model.sdf --gz-check never

gz sdf --check is target-consumer validation. --gz-check auto is the default: it runs when gz is on PATH, reporting gz_check_passed or the tool's own output as the error gz_check_failed, and otherwise notes info: gz_check_unavailable and carries on. An absent optional tool says nothing about the file, so it never fails a clean document and --strict does not change that. --gz-check required makes the tool mandatory — a missing gz is then an error — and --gz-check never skips it outright.

Required report shape

When finishing an SDF task, include a compact report:

Validated: path/to/model.sdf
Checks run:
- bundled SDF validation: passed
- gz sdf --check: skipped, gz not installed
- simulator load: skipped, target simulator unavailable
- viewer handoff: `$cad-viewer` link returned
Assumptions:
- Assumed mesh units are meters.
- Assumed lidar frame is coincident with lidar_link.
Risks:
- Camera plugin filename was not verified in the target simulator environment.

Snapshot Tool

cadgen sdf 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.

cadgen sdf snapshot path/to/robot.sdf review.png

It accepts .sdf only (a format door, same TARGET [OUT] grammar as the rest). 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.

A normal snapshot uses deterministic light CAD lighting and hides grid and axis guides. Pass --render light or --render dark (or photographic Render JSON or a file path) for the shared Render scene. An envelope with no studio resolves Light in the CLI. Set its camera inside Render JSON; top-level --camera, --display, and --joint-values control normal snapshots and cannot be combined with Render. Robot link meshes have no CAD-edge or exploded assembly topology, so those display combinations are rejected clearly.

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 sdf snapshot TARGET [OUT] [flags], the same one every format door uses. Use cadgen sdf snapshot --help for the complete current interface — the flags a robot cannot act on are absent from it, not refused by it.

References

• SDF workflow: references/sdf-workflow.md
• Worked examples (golden skeletons): references/examples.md
• LLM guardrails: references/llm-guardrails.md
• Design ledger: references/design-ledger.md
• Frame semantics: references/frame-semantics.md
• Validation scope: references/validation.md
• Smoke tests: references/smoke-tests.md
• Interoperability notes (URDF-derived SDF, meshes, Gazebo): references/interoperability.md