fusion-cad-mcp

An MCP server that gives Autodesk Fusion a named, typed tool surface: create_sketch, extrude, add_hole, create_joint, export, and 70 more.

Fusion ships its own MCP server, but it exposes four broad tools that take raw Python. That works, and it means every call is an opportunity to get the Fusion API wrong. This sits in front of it and turns the common operations into validated tools with structured errors, so an agent can do CAD without carrying a large prose skill in context.

Status: beta (0.2.x). 75 tools, 467 tests, and a full pass of live verification against Fusion 2704.1.23 that found and fixed 17 bugs. Used in production for parametric part design.

Requirements

• Autodesk Fusion, running, with a design open

• Preferences > General > API > Fusion MCP Server enabled

• Python 3.11+

Related MCP server: fusion-mcp

Install

Not a developer? Start with AI-SETUP-PROMPT.md. Paste one prompt into Claude and it walks you through the whole thing, install to first extrude, and verifies it worked.

pip install "fusion-cad-mcp @ git+https://github.com/Mfrostbutter/fusion-cad-mcp.git"

Then point your MCP client at it. For Claude Code or Claude Desktop:

{
  "mcpServers": {
    "fusion": {
      "command": "fusion-cad-mcp"
    }
  }
}

See CONNECT.md for per-client details and troubleshooting.

How it works

MCP client (Claude, Cursor, Cline, ...)
  -> fusion-cad-mcp            this package, stdio MCP server
    -> httpx
      -> 127.0.0.1:27182/mcp   Fusion's own MCP
        -> adsk.fusion / adsk.core

Each tool generates a Python script and runs it through Fusion's execute. It is a peer of Autodesk's bridge, not a replacement, so anything you can do in the Fusion API you can still do here via the execute escape hatch.

The tools

75 registered, 74 usable. Full reference with signatures, enums, return keys, and error codes is in tools.md, or search it in place with the find_tool tool.

rib is registered but always returns rib_not_scriptable: Fusion exposes RibFeatures as a read-only collection, so ribs cannot be created through the API at all. Model one as a thin join-extrude instead.

Every tool returns the same envelope

{
  "ok": bool,
  "message": str,          # stdout from the generated script
  "result": dict | None,   # the payload
  "image": dict | None,    # base64 PNG, for screenshots
  "error": str | None,     # error code when ok is False
  "traceback": str | None
}

ok: true does not always mean the thing happened. Fusion has several operations that decline silently, and the tools surface that rather than hiding it:

• drive_joint returns applied: false when a drive past a joint limit was ignored

• move_component returns moved: false when a joint solver overrode the move

• export returns bytes_written: 0 when nothing was written

• doc_state returns active_design: false when no design is open

Read result, not just ok.

The knowledge tools

Four tools answer questions without touching Fusion, so they work with it closed:

The first three ship with the package. find_api needs a local copy of Autodesk's help, which is their content and is not redistributed, so you build your own once:

pip install "fusion-cad-mcp[corpus] @ git+https://github.com/Mfrostbutter/fusion-cad-mcp.git"
fusion-cad-mcp corpus build --i-accept-autodesk-terms

That crawls at 1 request/second into ~/.fusion-cad/corpus/, which is where the server looks first. Set FUSION_CAD_CORPUS_DIR to keep it elsewhere. Everything else works without it.

Development

pip install -e ".[dev]"
pytest              # 467 tests, no Fusion required

The tests cover script generation, envelope parsing, argument validation, and packaging integrity. They deliberately do not need Fusion running, which is also their limit: passing tests prove the generated Python is valid, not that Fusion accepts it. Several of the bugs found in this project passed every unit test and only surfaced against a live document, so verify behavioral changes in Fusion and check entity counts rather than trusting a success envelope.

One test guards a bug class worth knowing about: json.dumps(None) emits null, which is not valid Python, and ast.parse cannot catch it because null is a legal identifier. Generated code must embed Python values with repr().

License

MIT. See LICENSE.