AI agents (MCP)

smlibpy comes with an MCP server. An agent such as Claude writes Python scripts with smlibpy, the server runs them, shows the model in a live 3D viewer and renders pictures the agent uses to check its work. Ask for "a flange with six M8 holes on a 90 mm circle" and you get the part, a picture and a STEP file.

Install

The server is part of the package, behind the mcp extra:

pip install "smlibpy[mcp]"
smlibpy-mcp            # speaks MCP over stdio; your agent host starts it

With uv nothing has to be installed first: uvx --from "smlibpy[mcp]" smlibpy-mcp.

Claude Code

The plugin brings the server and a skill:

claude plugin marketplace add awv-informatik/smlibpy
claude plugin install smlibpy@smlibpy

Or add only the server:

claude mcp add smlibpy --scope user -- uvx --from "smlibpy[mcp]" smlibpy-mcp

Claude desktop app, Cursor, VS Code and others

Every host takes the same command. In the JSON most of them use:

{
  "mcpServers": {
    "smlibpy": { "command": "uvx", "args": ["--from", "smlibpy[mcp]", "smlibpy-mcp"] }
  }
}

That goes to claude_desktop_config.json (Claude desktop app), ~/.cursor/mcp.json (Cursor) or .vscode/mcp.json (VS Code, where the key is servers and each entry has "type": "stdio"). Give the full path to uvx if the host does not find it.

What the agent gets

Tool	What it does
run_script	Runs Python with from smlibpy import *; variables stay between calls
snapshot	A PNG of the model: one view, or front, top, right and iso on one sheet
scene, inspect	What is shown, the variables, measurements of a shape, its faces and edges
list_methods, describe_method, docs	The API index, signatures and the guides of the skill
save, load	STEP, STL, glTF, OBJ, the native format, PNG; STEP and STL in
checkpoint, restore, clear	Go back to an earlier state, or start over
view	The link of the live viewer

show(a, b) in a script chooses what the viewer and the pictures display; without it every part and shell of the session is shown.

The server's instructions tell the agent how to work: plan, build in steps, check volume, bounding box and validity after every step, look at the sheet of views, then deliver a STEP file. The skill holds a guide for every API, written by agents that tested each call against the kernel.

How it runs

Scripts run in a separate process. A script that runs too long is stopped (after 120 s unless the agent asks for more), and a crash of the kernel ends that process, not the server. Either way the session is rebuilt by running the scripts that built it again, so nothing that worked is lost. Checkpoints work the same way.

The viewer is a page on 127.0.0.1 that updates after every script. In the Claude desktop app the agent opens it in its own browser pane; elsewhere it opens in your default browser. Everything runs on your machine; the page loads three.js from a CDN.

Variable	Effect
SMLIBPY_VIEWER	host (the agent opens the link), browser (your browser) or off
SMLIBPY_SNAPSHOT_DIR	Where pictures are kept (default: a folder in the temp directory)

With ClassCAD

If the ClassCAD MCP is installed as well, the agent can hand a part to the ClassCAD app Buerligons: it saves a STEP file here and loads it there, where the part arrives as a solid.