b123d-modeling

Build 3D Geometry with build123d

Use this skill when asked to model, build, or modify a 3D part or assembly with build123d — from a text description, a technical drawing (image or PDF), dimensions in a spec, or an existing STEP/STL file.

Use the build123d-mcp MCP server as the execution and verification authority. Build incrementally with execute() by default and checkpoint the first valid model. If matching-view evidence then shows a body-family/global-form mismatch, keep the replacement in one canonical model.py and run it with execute_file(); then use measure() → validate() → render_view() before exporting it. execute_file() runs in a clean namespace and preserves the previous valid model if the rebuild fails, so a necessary global correction does not sacrifice the verified checkpoint. Do not globally rebuild a checkpoint merely because a canonical source file is available.

Step 0 — Start the session

1. reset(), then execute("from build123d import *").
2. Read the build123d://quickref resource before writing build code — it has accurate API syntax for the current build123d version.
3. If the part uses fasteners, bearings, or threads, read build123d://bd_warehouse and probe catalogue sizes before scripting.

Step 1 — Extract the spec before modeling

Do not model while reading the input. First convert it into a parameter block.

From a technical drawing (image or PDF):

• Identify the views — front / plan (top) / side — and the projection convention; the title block states first- or third-angle, the SCALE (e.g. 2:1), and the units/tolerance standard.
• Printed dimension callouts are real part dimensions. Never replace one with a pixel-derived value, and never multiply a printed dimension by drawing scale.
• Hidden (dashed) lines are internal features — holes, pockets, bores. Centrelines (long-short chain) mark hole centres and symmetry axes. Section hatching shows solid material in a cut view.
• Symbols: Ø diameter (not radius), R radius, M6/M8… threads, ⌴ counterbore, ⌵ countersink, typical hole note 4× Ø6.6 THRU.
• Cross-check every feature in at least two views before trusting it — a circle in plan with no matching hidden lines in front is probably a boss, not a hole.