RFD: Forking of existing sessions

[josevalim]

No description provided.

[SteffenDE]

For context: we opened zed-industries/claude-code-acp#145 to explore how this could be implemented. The PR also suggested a session/resume functionality, but that would be a separate draft.

For Claude Code, the SDK already provides the blocks we'd need to implement this: forkSession and resume. I'm imagining the API could look like this:

{
  "jsonrpc": "2.0",
  "method": "session/fork",
  "params": {
    "sessionId": "EXISTING-ACP-SESS-ID"
  }
}

Optionally, we could require to also pass any other parameters from session/new, like cwd and mcpServers for the forked session, but that might be different depending on the agent.

The agent would then reply with the same format as session/new, creating a new ACP sessionId for the forked session.