App Builders

Start a kernel

rat serve py --http --port 8717 --cwd ~/project --venv ~/project/.venv

rat serve r --http --port 8718

rat serve py

Stdio mode (default) for local integrations.

Connect from your app

const py = new McpClient("http://localhost:8717/mcp");
await py.callTool("run", { code: "x = 42" });
await py.callTool("look");
await py.callTool("look", { at: "df" });
await py.callTool("ctl", { op: "reset" });

Claude Desktop / Cursor config

Start a kernel, then point your client at it:

rat start py

rat status -v

Add the URL to your client config (e.g. ~/Library/Application Support/Claude/claude_desktop_config.json):

{
  "mcpServers": {
    "python": { "url": "http://127.0.0.1:8717/mcp" }
  }
}

The port is auto-assigned starting from 8717. Check rat status -v for the actual URL.