prompt() hangs when Claude Code binary doesn't support CLAUDE_CODE_EMIT_SESSION_STATE_EVENTS

[vincentye38]

Bug Description

Commit 7f1c6c2 ("Use idle session state as end of turn (#463)") and its follow-up 23b3073 ("Cleanup based on new idle state #463 (#480)") moved the prompt loop's exit from the result message handler to session_state_changed(idle). This relies on the Claude Code binary supporting the CLAUDE_CODE_EMIT_SESSION_STATE_EVENTS env var.

When the Claude Code binary does not support this feature (e.g., older versions), session_state_changed(idle) is never emitted. The prompt loop calls session.query.next() after processing the result message and blocks forever, because the process is still alive but sends no further messages.

This causes the JSON-RPC response for session/prompt to never be sent, and the client's conn.prompt() future never resolves — deadlocking the client.

Steps to Reproduce

1. Use claude-agent-acp v0.23.0+ (which includes commit 7f1c6c2)
2. Connect with a Claude Code binary that does not support CLAUDE_CODE_EMIT_SESSION_STATE_EVENTS (e.g., Claude Code 2.1.77 installed by npm, set env CLAUDE_CODE_EXECUTABLE to point to it)
3. Send a session/prompt request
4. The agent processes the turn, sends streaming notifications, receives result(success) — but never returns the PromptResponse
5. Client hangs indefinitely

Root Cause

In the prompt loop (acp-agent.ts:prompt()), all result subtypes (success, error_during_execution, error_max_*) now set stopReason and break instead of return. The only return path is via session_state_changed(idle), which requires the env var to be supported by the binary:

// Result handler — no longer returns
case "success": {
    // ... error checks ...
    break;  // was: return { stopReason: "end_turn", usage }
}

// Only exit path
case "session_state_changed": {
    if (message.state === "idle") {
        return { stopReason, usage: sessionUsage(session) };  // never reached
    }
}

After break, the loop calls await session.query.next() which blocks forever since the process sends no more messages.

Expected Behavior

The prompt loop should return a PromptResponse even when session_state_changed is not available.

Environment

• claude-agent-acp: v0.23.0+ (commits 7f1c6c2, 23b3073)
• Claude Code binary: any version that doesn't support CLAUDE_CODE_EMIT_SESSION_STATE_EVENTS

[vincentye38]

This issue causes jupyter-ai-acp-client issue jupyter-ai-contrib/jupyter-ai-acp-client#77