How is a client supposed to resume an existing session with the agent?

[gsabran]

Claude Code / Codex provide some sort of --resume parameter that allows to continue an existing "agent session".

It doesn't seem that the session id provided by the ACP layer when initializing the "ACP session" matches the id of the "agent session"

When an app is restarted, there is no memory persistence (and I don't think the implementation of the Claude Code / Codex ACP maintain their own persisted storage). How then can the app allow the user to resume a session with the agent?

sequenceDiagram
    autonumber
    participant App
    participant ACP
    participant Agent as Agent (Claude Code / Codex)

    App->>ACP: initialize_session()
    ACP-->>App: { session_id: "S-1234" }

    App->>ACP: prompt("Hello", session_id="S-1234")
    ACP->>Agent: prompt("Hello")
    Agent-->>ACP: { session_id: "A-9876", output: "Hi!" }
    ACP-->>App: { output: "Hi!" }
    Note over ACP: agent's session_id is not returned to the App

    Note over App,Agent: App restarts — memory lost

    App->>ACP: prompt("--resume", session_id="S-1234")
    ACP->>Agent: prompt("--resume")  <!-- no agent's session_id -->
    Agent-->>ACP: { session_id: "A-5555", output: "(new session)" }
    ACP-->>App: { output: "(new session)" }

Loading

I've seen the load session part of the protocol, but I don't think this is it, and I don't know how well supported it is.

[am-will]

There's a history button, but it doesn't work as far as I can tell. CTRL + SHIFT + H

[gsabran]

I don't think the history button is part of the ACP protocol. I was looking in the protocol for a place that describes how to access the history that is kept by most agents themselves.

[gsabran]

This is great to hear.

Yes I think for CC the ACP adapter generates a random id.

[benbrandt]

Yeah that doesn't seem necessary, open to changing that for sure.

[gsabran]

I made an attempt at changing that, which didn't work (here in case you want to have a look).

I think the issue is that Claude Code is not giving a session id until prompted, and ACP mandates that we give a session id during newSession, before prompting.

[benbrandt]

Ah yes, that rings a bell. There are a few things that I have bugged them about that would be nice to be able to access prior to a prompt

[josevalim]

I wrote a RFD about this here: #234