OpenCode

OpenCode is an open-source terminal coding agent. Its project MCP config lives in ./opencode.json at the repo root.

Connect (one-click)

Open cairn's web UI, go to the sidebar Connect, and click Connect next to OpenCode. 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:opencode, then verifies the connection.

Manual setup

Paste this into ./opencode.json:

{
  "$schema": "https://opencode.ai/config.json",
  "mcp": {
    "cairn": {
      "type": "local",
      "command": ["/absolute/path/to/bin/cairn", "serve", "--actor", "agent:opencode", "--repo", "/absolute/path/to/project"],
      "enabled": true
    }
  }
}

The first element of 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

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

Gotchas

• OpenCode uses a different shape from the others: a top-level mcp object, type: "local", and command is a single array (binary + args), not separate command/args.
• cairn seeds $schema for fresh files.
• OpenCode also reads a global ~/.config/opencode/opencode.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:opencode (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.