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.

Tools

Tool	Class*	What it does
model_inspect	read	STEP/BREP/STL → bounding box, volume/area, mass (given density), topology counts, per-solid summary
model_validate	read	DFM-lite lint: open (non-watertight) shells, invalid B-rep, tiny edges/faces, pairwise interference & clearance — every finding carries its evidence and threshold
model_build	write	Run a build123d/CadQuery parametric script (sandboxed subprocess, CPU/memory/wall caps) → STEP/STL/glTF/3MF into the output dir + a build report
model_mesh	write	STEP/BREP → tessellated STL/glTF/3MF with linear + angular tolerance controls
calc_fits	read	ISO 286 limits & fits (hole-basis subset), clearance/interference verdicts
calc_fastener	read	Metric coarse threads: pitch, stress area, tap & clearance drills, preload/torque first-cut
calc_beam	read	Beam bending first-cut: cantilever / simply-supported, point / UDL, rect / round / tube sections
calc_mass_rollup	read	Assembly mass + 3D center of gravity from a component list, envelope check
model_diff	read	Geometric diff of two revisions: volume/area/bbox deltas, bodies unchanged/moved/added/removed by shape signature — geometry, not a feature tree, and the report says so
drawing_project	write	Named-view (front/top/…/iso) SVG or DXF projection: visible edges + dashed hidden layer — a silhouette for hand-off, not a dimensioned drawing
text_to_cad	external	Starter model from a text prompt via Zoo's ML-ephant API (+ its editable KCL source when returned); needs ZOO_API_KEY, refuses by name without it

* 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