cad-mcp

An MCP server that exposes CAD geometry reasoning over STEP files to LLMs. Drop a STEP file, ask geometry questions in natural language, get verifiable answers. Built on build123d (OCCT-backed) and the MCP Python SDK. Runs locally with Claude Code.

Install

Requires Python 3.12, arm64 (Apple Silicon) or x86-64.

git clone https://github.com/your-org/cad-mcp.git
cd cad-mcp
python3.12 -m venv venv
venv/bin/pip install -r requirements.txt

Smoke-test the install:

venv/bin/python smoke_test.py      # loads bracket.step, checks bbox
venv/bin/python verify_server.py   # connects over MCP stdio, calls all tools

Run

The server starts automatically when Claude Code opens this directory (.mcp.json is included). To start it manually:

venv/bin/python server.py

To connect a different STEP file, pass its absolute path in any tool call — no server restart needed.

Tools

Tool	Returns
`list_assembly_tree`	Part/assembly hierarchy as a tree
`get_part_dimensions`	Bounding box (mm) and volume (mm³)
`find_holes`	Cylindrical holes: position, diameter, depth (mm)
`measure_distance`	Euclidean distance between part reference points (mm)
`get_mass_properties`	Mass (kg), centre of mass (mm), moments of inertia (kg·mm²)

Demo

Five questions asked against fixtures/bracket.step (100 × 60 × 10 mm aluminium bracket, four M5 clearance holes) in Claude Code with the MCP server connected.

Is bracket.step a single part or an assembly?

Tool: list_assembly_tree

{
  "root": {
    "type": "part",
    "name": "part_root",
    "part_id": "root",
    "children": []
  }
}

Single part. part_id: "root" is passed to all other tools.

What are the overall dimensions and volume?

Tool: get_part_dimensions

{
  "part_id": "root",
  "bbox_mm": { "x": 100.0, "y": 60.0, "z": 10.0 },
  "volume_mm3": 59117.5266
}

100 × 60 × 10 mm. Volume is net (material only) — gross minus the four holes.

How many holes, what size?

Tool: find_holes

{
  "holes": [
    { "center_x_mm": -42.0, "center_y_mm": -22.0, "center_z_mm": 0.0,
      "diameter_mm": 5.3, "depth_mm": 10.0, "axis": [0.0, 0.0, 1.0] },
    { "center_x_mm":  42.0, "center_y_mm": -22.0, "center_z_mm": 0.0,
      "diameter_mm": 5.3, "depth_mm": 10.0, "axis": [0.0, 0.0, 1.0] },
    { "center_x_mm": -42.0, "center_y_mm":  22.0, "center_z_mm": 0.0,
      "diameter_mm": 5.3, "depth_mm": 10.0, "axis": [0.0, 0.0, 1.0] },
    { "center_x_mm":  42.0, "center_y_mm":  22.0, "center_z_mm": 0.0,
      "diameter_mm": 5.3, "depth_mm": 10.0, "axis": [0.0, 0.0, 1.0] }
  ]
}

Four M5-clearance (5.3 mm) through-holes at the corners, all normal to the XY plane.

What is the space-diagonal distance corner to corner?

Tool: measure_distance with selector_a: "bbox_min", selector_b: "bbox_max"

{
  "distance_mm": 117.047,
  "point_a": { "part_id": "root", "selector": "bbox_min",
               "x_mm": -50.0, "y_mm": -30.0, "z_mm": 0.0 },
  "point_b": { "part_id": "root", "selector": "bbox_max",
               "x_mm":  50.0, "y_mm":  30.0, "z_mm": 10.0 }
}

√(100² + 60² + 10²) = 117.047 mm. Both resolved points are included for traceability.

Mass and centre of mass in aluminium?

Tool: get_mass_properties with density_kg_m3: 2700.0

{
  "mass_kg": 0.159617,
  "volume_mm3": 59117.5266,
  "center_of_mass_mm": { "x_mm": 0.0, "y_mm": 0.0, "z_mm": 5.0 },
  "moments_of_inertia_kg_mm2": {
    "Ixx": 48.772745,
    "Iyy": 132.122917,
    "Izz": 178.235373
  }
}

≈ 160 g. Centre of mass at geometric centroid (0, 0, 5) — expected for a symmetric bracket with symmetric holes.

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