build123d-mcp
An MCP (Model Context Protocol) server that exposes build123d CAD operations as tools, enabling AI assistants to build, inspect, and iterate on 3D geometry interactively.
Why
When using an AI to write build123d scripts, the AI writes blind — it cannot see the geometry it produces. This server closes the feedback loop: the AI can create geometry, render views, query dimensions, and catch errors incrementally rather than writing complete scripts and hoping they are correct.
Evidence it works: on the public CADGenBench leaderboard (June 2026), driving GPT‑5.5 through build123d‑mcp scores 0.457 — the top result on the board — versus 0.360 for the same model writing build123d scripts blind: a +27% gain that also raises CAD validity from 88% to 100%, and beats even the larger GPT‑5.5 Pro baseline. The feedback loop measurably improves what an LLM can model.
Tools
Core
execute— run build123d Python code in a persistent session; useshow(shape, name)to register named partsreset— clear session back to empty state (namespace, shapes, snapshots)
