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name: cad description: Create, modify, inspect, and validate STEP-first build123d/Python CAD parts and assemblies. Use for natural-language CAD specs, STEP/STP generation, build123d source, build123d source-level joints, @cad references, geometry facts, measurements, mating deltas, CAD Explorer handoffs, and secondary DXF/STL/3MF outputs.

CAD generation, inspection, and validation

Purpose

Create or modify parametric CAD models from natural-language requirements, generate validated STEP/STP artifacts, inspect geometry references, and return checked outputs. Treat STEP as the primary CAD artifact. Treat DXF, STL, 3MF, and native GLB as secondary workflows that branch from, or accompany, a STEP-first process. For assemblies, prefer source-level build123d joints and named mating datums when the parts have functional assembly relationships.

Use this skill when

Use this skill when the user asks for CAD files, STEP/STP files, build123d source, @cad[...] references, mechanical parts, assemblies, enclosures, brackets, fixtures, holes, counterbores, countersinks, slots, pockets, bosses, standoffs, ribs, fillets, chamfers, shells, source-level joints, mating, or measurements.

Also use it when the user asks for DXF, STL, 3MF, or native GLB output from CAD geometry. Keep those workflows secondary and load dxf.md or supported-exports.md for details.

Do not use this skill for render-only concept art, CAM toolpaths, engineering certification, FEA conclusions, architectural BIM, or freehand illustration unless the user also needs CAD geometry.

Default assumptions

Use these defaults unless the user specifies otherwise:

• Units: millimeters.
• Origin: center of the main part or assembly unless a mating interface or fixed root component suggests a better origin.
• Base plane: XY.
• Up/extrusion axis: positive Z.
• Output geometry: closed, positive-volume solids unless the user requests surfaces or construction geometry.
• STEP structure: one valid solid, a compound of solids, or a labeled assembly compound.
• Assembly structure: fixed root part, part-local frames, named mating datums, build123d joints where applicable, and explicit generated placements.
• Small plastic enclosure wall: 2.0-3.0 mm when unspecified.
• Cosmetic fillet: 1.0-3.0 mm when safe for local geometry.
• M3/M4/M5 normal clearance holes: 3.4/4.5/5.5 mm unless another standard is requested.

Ask one focused clarification question only when missing information makes the model impossible, fit-critical, safety-critical, or compliance-bound. Otherwise proceed with explicit assumptions.

Natural-language specs only

Do not ask the user to provide a JSON specification and do not make JSON the user-facing workflow. Convert the user's prose into an internal CAD brief with dimensions, features, assumptions, output paths, and validation criteria. Use references/natural-language-specs.md for brief-writing patterns.

Root model

Keep these roots separate:

• CAD skill directory: this folder. Tool launchers live here as scripts/step, scripts/inspect, and scripts/dxf.
• Tool process cwd: relative CAD targets are resolved from the command's current working directory. Use absolute target paths when running from the skill directory, or run from the workspace root and invoke the launchers with a path to this skill directory.
• Render: this skill does not own Explorer startup. After creating or modifying supported artifacts, hand off explicit paths to $render when that skill is available; $render checks/reuses a live viewer and returns links.

Short command examples in this skill use launcher paths relative to the CAD skill directory. Adapt the launcher path or target path so project CAD files resolve from the intended workspace, not accidentally under the skill directory.

Prefer keeping a STEP output and its Python generator in the same directory so the source stays easy to discover. Unless the user explicitly requests otherwise, keep the STEP basename and generator basename the same even when they cannot live side by side.

Available tools

From the CAD skill directory, the launcher shape is:

python scripts/step ...
python scripts/inspect ...
python scripts/dxf ...

Use the active project Python interpreter. If only the repo-local virtualenv is available, invoke that interpreter while keeping the root model above explicit.

Use python scripts/<tool> --help for the complete current command interface; reference docs show recommended workflows, not every flag.

Required workflow

1. Classify the task. Identify whether this is a new part, new assembly, source modification, direct STEP/STP inspection, reference selection, measurement/mating check, render review, or secondary output request.
2. Load only the needed references. Use the triggers below instead of reading the whole reference set.
3. Create a natural-language CAD brief. Extract dimensions, units, coordinate convention, feature intent, output paths, assumptions, and validation targets.
4. Plan before coding. Define parameters, labels, source paths, expected bounding boxes, and any mating/positioning datums before editing.
5. Edit source, not generated artifacts. Prefer build123d Python with gen_step() for STEP generation.
6. Generate explicit targets. Use scripts/step for STEP/STP generation and sidecars. Use --kind part or --kind assembly only for direct STEP/STP imports. Only ever use --skip-explorer when the user explicitly asks to skip Explorer, GLB/topology, or renderable topology output. Do not run directory-wide generation.
7. Validate geometrically. Use scripts/inspect refs --facts --planes --positioning, then targeted measure, mate, frame, or diff when needed.
8. Hand off to render. Always pass created or modified .step, .stp, .stl, .3mf, .dxf, or native .glb paths to $render for live viewer links when that skill is available.
9. Tier visual review. For generation feedback, prefer the render skill's snapshot CLI over opening the viewer manually or using Playwright. Use snapshots when still image files are needed for multimodal critique, section/wireframe review, user-facing snapshots, or risk-based semantic validation. For non-trivial parts and assemblies, prefer one small diagnostic still-image packet after geometric validation. Generate GIFs only for STEP-module parameter animation review; otherwise use still snapshots, not GIFs. Do not run repeated snapshots unless a source repair changed visible geometry or a specific visual finding needs confirmation.
10. Repair and rerun. If a check fails, change the smallest responsible source section, regenerate, and rerun the failed validation.

Non-negotiables

• Treat generated STEP/STP, STL, 3MF, GLB/topology, DXF outputs, and Explorer sidecars as derived artifacts.
• Keep STEP as the primary validated CAD artifact; DXF/STL/3MF are secondary unless the user explicitly says otherwise.
• When a Python generator exists, run scripts/step on the generator. Use a direct STEP/STP target only when the generator is unavailable or the user explicitly identifies that STEP/STP file as the target.
• Use named parameters, closed solids, explicit labels, and source-controlled geometry intent.
• Author assembly positioning in source with part-local datums, explicit Location transforms, or build123d joints. Treat CLI inspect mate as read-only validation, not as a source-editing API.
• Do not use git status, git diff, or file-size churn as CAD comparison for large exported STEP/STP, GLB/topology, STL, 3MF, or DXF artifacts. Compare source changes, scripts/inspect summaries, CAD Explorer renders, or CAD Explorer output instead; use path-limited git status only for bookkeeping.
• Always hand off supported created or modified artifacts to $render for live viewer links when that skill is available; report if $render is unavailable or the viewer check fails.
• Report only checks that actually ran or are directly supported by tool output.
• If $render is unavailable or fails, say so and rely on CLI inspection for validation.

Progressive references

Load these files only when their trigger applies:

• references/natural-language-specs.md — converting prose requirements into a CAD brief without requiring user JSON.
• references/parameters.md — parameter, control, and animation design best practices.
• references/step-generation.md — STEP generation, direct STEP/STP targets, part-vs-assembly behavior, and post-generation inspection.
• references/inspection-and-validation.md — validation gates, @cad[...] refs, facts, planes, topology, measurements, mating, diff, frame, and final validation reporting.
• references/render-review.md — risk-based render triggers, small render packets, targeted visual views, multimodal critique, and converting visual findings into geometry checks.
• references/positioning.md — part-local datums, assembly transforms, build123d joints, CLI mate validation, and positioning reports.
• references/dxf.md — secondary DXF workflow.
• references/supported-exports.md — secondary STL/3MF/native GLB sidecar workflows.
• references/build123d-modeling.md — build123d modeling patterns, topology, selectors, features, assemblies.
• references/repair-loop.md — diagnosis and repair procedures.

Final responses should include generated files, CAD Explorer links when $render is available, validation actually run, assumptions, and caveats. Use references/inspection-and-validation.md for report structure.