Pi

Pi is an MCP-capable coding agent. Its preferred project MCP config lives in ./.mcp.json at the repo root.

Connect (one-click)

Open cairn's web UI, go to the sidebar Connect, and click Connect next to Pi. cairn writes the config shown below, launching the cairn binary over stdio with the absolute binary path and --repo set to this project, under the identity agent:pi, then verifies the connection.

Manual setup

Paste this into ./.mcp.json at the repo root:

{
  "mcpServers": {
    "cairn": {
      "command": "/absolute/path/to/bin/cairn",
      "args": ["serve", "--actor", "agent:pi", "--repo", "/absolute/path/to/project"]
    }
  }
}

command is the absolute path to your built cairn binary (the Connect page fills this in automatically; if writing by hand, use your bin/cairn absolute path), and --repo is this project's absolute path.

Identity

Pi connects as agent:pi by default. Edit the identity on the card to run multiple instances (e.g. agent:pi-2). See Sessions for how identity is enforced.

Gotchas

• Pi's preferred project config is ./.mcp.json, the same file Claude Code uses (see /agents/claude), so connecting both Pi and Claude writes one shared cairn entry, and whichever you connect last sets the --actor.
• Pi also reads ~/.pi/agent/mcp.json and ~/.config/mcp/mcp.json.

Alternative: HTTP transport

While the cairn app is running it also exposes an MCP endpoint at http://127.0.0.1:7777/mcp?repo=<project>&actor=agent:pi (the port may differ; cairn prints CAIRN_WEB_URL= on startup). URL-based MCP clients can point at that instead of the stdio binary. The tradeoff: the app must be running for the endpoint to respond.