root-ext-cad

CAD-engineering MCP tool server — code-CAD builds (build123d/CadQuery), model inspection, meshing, DFM-lite validation, and mechanical calculators. Built as the mechanical-engineering pack for [ROOT Workspace], but it speaks plain MCP stdio and works with any MCP host.

The workspace client stays domain-free by doctrine; this repo is where the CAD knowledge lives. Tools reach both ROOT runtimes (the brain-governed deployment and the lite Tier-0 harness) through the client's mcp-bridge, and every call is governed — write-class tools gate like any write; network tools park for approval.

The charter (binding)

Source is truth; artifacts are derived. The pack never overwrites a user's file in place: geometry writes land in a caller-named output directory (default derived/ beside the source), and every write tool refuses an output path that collides with its input. tests/test_server.py::test_no_inplace_mutation_tools_exist and the collision-refusal tests enforce this in CI, not just in prose.

Parametric source (a build123d or CadQuery .py script) is the model; STEP/STL/glTF/3MF are exports. model_build executes scripts — it says so plainly, runs them in this pack's own venv as a resource-capped subprocess, and is classed write, never read.

Related MCP server: build123d-mcp

Tools

* Classes derive fail-safe from annotations in the ROOT client: read tools declare readOnlyHint: true, openWorldHint: false; write tools declare readOnlyHint: false, openWorldHint: false (the closed-world write class); network tools (Phase CAD-D: text_to_cad) will declare openWorldHint: true and refuse by name without their key.

Connect it to ROOT Workspace

Settings → Connected tools → the connect form:

• Name: cad-eng

• Command: uv run --project /path/to/root-ext-cad root-ext-cad (a local checkout), or once published: uvx root-ext-cad

Run the row's Check (doctor) after connecting. Keep model files and build scripts inside granted folders so the agent can cross-read them with fs.*. Builds must finish inside the bridge's call ceiling (120 s; 90 s once promoted) — the default subprocess wall cap is 75 s.

Development

uv venv .venv
uv pip install --python .venv/bin/python -e ".[dev]"
.venv/bin/pytest

Layering rule: server.py is wiring only; every capability lives in a plain-Python module (inspection.py, meshing.py, validate.py, buildrun.py, calc.py, geometry.py) with logic separated from I/O so the rules stay unit-testable on synthetic shapes — no fixture model files required.

Roadmap

• Later, each behind its own ADR: FreeCAD GUI-bridge companion, Onshape connector, KCL as a second source dialect, FEA hand-off recipes, PMI/GD&T presence reporting