Build123d MCP Server

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.

Tools

• execute — run build123d Python code in a persistent session; use show(shape, name) to register named parts
• render_view — render one or more shapes as PNG or SVG; supports assembly compositing, high-quality tessellation, and cross-section clip planes
• measure — query bounding box, volume, surface area, topology, minimum wall thickness, or clearance between two named bodies
• export — export as STEP, STL, or both in one call; targets a named object or the current shape
• session_state — full JSON snapshot of active shapes, named objects, and snapshot names
• health_check — verify VTK/SVG/STEP/STL dependencies work end-to-end before starting work
• save_snapshot / restore_snapshot / diff_snapshot — checkpoint, recover, and compare geometric state
• interference — check intersection volume between two named shapes
• list_objects — list all named shapes with geometry stats
• version — return the server version
• reset — clear the session back to empty state

See llms.md for full tool reference and usage patterns.

Requirements

• uv
• An MCP-compatible client (Claude Code, Claude Desktop, Cursor, etc.)

All Python dependencies (build123d, vtk, etc.) are installed automatically by uv.

Installation

No clone needed. Install directly from PyPI:

pip install build123d-mcp

Or just use uv tool run — it fetches and runs the package in one step with no prior install required (see below).

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Adding to MCP clients

The server runs over stdio — the client launches it as a subprocess using uv tool run build123d-mcp.

Note on Python version. All examples below pass --python 3.12. VTK and cadquery-ocp do not yet ship wheels for Python 3.13+, so pinning to 3.12 is required. uv will auto-download a managed Python 3.12 if you don't already have one.

Claude Code

Add to your project's .mcp.json (or ~/.claude/mcp.json for global use):

{
  "mcpServers": {
    "build123d-mcp": {
      "command": "uv",
      "args": ["tool", "run", "--python", "3.12", "build123d-mcp"]
    }
  }
}

Restart Claude Code after editing. The tools appear automatically once connected.

Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "build123d-mcp": {
      "command": "uv",
      "args": ["tool", "run", "--python", "3.12", "build123d-mcp"]
    }
  }
}

Restart Claude Desktop after saving.

Cursor

Open Settings → MCP and add a new server entry, or edit ~/.cursor/mcp.json:

{
  "mcpServers": {
    "build123d-mcp": {
      "command": "uv",
      "args": ["tool", "run", "--python", "3.12", "build123d-mcp"]
    }
  }
}

VS Code (GitHub Copilot / Continue)

For Continue extension, add to .continue/config.json:

{
  "mcpServers": [
    {
      "name": "build123d-mcp",
      "command": "uv",
      "args": ["tool", "run", "--python", "3.12", "build123d-mcp"]
    }
  ]
}

For GitHub Copilot with MCP support, add to .vscode/mcp.json in your workspace:

{
  "servers": {
    "build123d-mcp": {
      "type": "stdio",
      "command": "uv",
      "args": ["tool", "run", "--python", "3.12", "build123d-mcp"]
    }
  }
}

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System prompt

For best results, paste the contents of default_prompt.md as a system prompt in your AI client. This tells the assistant to work incrementally, verify geometry after each step, and use the tools in the right order.

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Status

Active development (v0.1.0).