gen-part

name	gen-part
description	Generate a PartCAD part from a natural-language description (and optional reference images/requirements) by authoring a CAD script and validating it with the PartCAD CLI. Use for /pc:gen-part or when the user asks to generate, create, or model a single mechanical part.

pc:gen-part

Generate one PartCAD part. The text after the command ($ARGUMENTS) is the part description — what to build. You author the CAD script yourself and prove it works with the PartCAD CLI; do not use any built-in ai-* part type or generation pipeline. Hard requirement: the finished part passes pc test (its geometry instantiates cleanly).

Steps 4–7 are the reference flow — the same shape the legacy pc add part --ai pipeline used (plan geometry → write script → validate → compare → refine). Treat it as a strong default, not a script: skip, reorder, or replace steps as the part warrants, but always finish by validating.

1. Understand the request

• $ARGUMENTS is the description. Read it and any requirements, and view reference images directly. Ask the user to clarify only genuinely load-bearing ambiguities; otherwise proceed and state your assumptions.
• Work in millimeters and degrees unless told otherwise.

2. Make sure PartCAD is available

Resolve a command as /pc:init does (pc, then partcad, then python -m partcad_cli.click.command). If none is found, stop and run /pc:install executable first.

3. Choose a representation

Pick the script kind that best fits the part — you decide:

Kind	Good for
build123d	general parametric solids (recommended default)
cadquery	fluent / CSG-style modeling
scad (OpenSCAD)	simple constructive geometry, no Python
sdf	organic / implicit shapes

4. (Optional) Plan the geometry

For anything non-trivial, sketch the constructive-solid-geometry plan first — coordinate system, primitives with dimensions/positions/orientations, then unions/differences/intersections and fillets/chamfers. This mirrors the legacy "CSG" step and usually cuts iterations. Skip it for simple parts.

5. Author, register, and mark it

Write the script file first from $ARGUMENTS (plain type — never ai-*), then register it — pc add part requires the file to already exist:

pc add part --desc "<the description>" <kind> <name>.<ext>   # e.g. build123d bracket.py

Language conventions for the script:

• build123d / cadquery / sdf (Python): import everything you use (including math and the library itself); call show_object(<result>) to expose the part; do not export anything. For cadquery, avoid tetrahedron / hexahedron.
• OpenSCAD: a complete script defining all functions and constants; no external modules; no export.

Then mark the part not manufacturable in partcad.yaml — a generated part is not a catalog/supplier item, and this is what lets pc test pass:

parts:
  <name>:
    type: <kind>
    path: <name>.<ext>
    desc: <the description>
    manufacturable: false

If you started the project with pc init, also delete the empty sketches: and assemblies: sections it leaves — a null section crashes pc render on older PartCAD (fixed in partcad/partcad#470).

6. Validate

pc test is the build gate: with manufacturable: false, its CAD check passes only if the geometry instantiates cleanly, and the manufacturability checks are skipped.

pc test <name>

7. Render, compare, and iterate

Produce an image and compare it against the description and any reference images — overall shape and every stated dimension:

mkdir -p /tmp/pc-render                       # the -O directory must already exist
pc render -t png -O /tmp/pc-render <name>     # writes /tmp/pc-render/<name>.png

Fix any error or mismatch, then re-test and re-render. Iterate until it is right — no fixed retry count.

8. Finalize

Summarize what you built — key dimensions and assumptions — and how to view it: pc inspect <name>.