For context: we opened agentclientprotocol/claude-agent-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.

Thank you!!! 🥳 🍾

And the agent would respond with optional data such as config options, the same as session/load.

I think the session/fork specification should be stricter on how it should mimic session/load. Currently the session/fork proposal does not specify that an agent MUST stream all previous session/update notifications on fork, but session/load require that.

A real world problem with this is that both OpenCode and goose implement experimental forking, but only the former stream all pre-fork notifications to the client, goose does not replay any previous messages at all. It would be great if the specification could make this consistent and unambiguous.