ACP: Name new chats "Untitled Chat N" and rename to the ACP session title on start (wire session_info_update)

[gnoviawan]

Will you contribute an implementation?

Yes — I plan to open a pull request.

Problem or Opportunity

When a new ACP agent chat is created, the chat shows a placeholder name like Agent 5f3a9c1b — the literal string Agent plus a slice of the random agent/session id. This comes from the deriveTitle() fallback:

// src/renderer/lib/acp-history-persistence.ts:49-55
export function deriveTitle(messages: ChatMessage[], agentId: string): string {
  const firstUser = messages.find((m) => m.role === 'user')
  if (firstUser) {
    const text = firstUser.blocks
      .map((b) => (b.type === 'text' ? (b.text ?? '') : ''))
      .join(' ')
      .trim()
    if (text.length > 0) return text.length > 40 ? `${text.slice(0, 40)}…` : text
  }
  return `Agent ${agentId.slice(0, 8)}` // ← random-looking placeholder
}

A freshly created session has title: null and no messages yet (src/renderer/stores/acp-store.ts:103, :798), so the Chats history list renders Agent <random>. This is noisy and meaningless to the user — it looks like a bug, and multiple new chats are hard to tell apart.

Separately, ACP already defines a first-class mechanism for agent-generated session titles, but Termul currently throws those titles away. The session_info_update notification (agent → client) is dropped in the Rust client:

// src-tauri/src/acp/client.rs:296-303
// SessionInfoUpdate and any future (non_exhaustive) variants have no
// dedicated P0 event; ignore them — but log so a silently-dropped
// update can be diagnosed instead of vanishing.
ref other => {
    log::debug!("[acp] agent {agent_id} sent an unhandled session/update variant: {other:?}");
}

So even when an ACP agent auto-generates a nice session title after the first prompt (e.g. "Implement user authentication"), the renderer never sees it and the chat keeps its placeholder / first-message-slice name.

Proposed Solution

Two coordinated changes:

1. Friendly placeholder for not-yet-started chats

Replace the Agent <random> fallback with an incrementing Untitled Chat N label (e.g. Untitled Chat 1, Untitled Chat 2, …). The number comes from a monotonic counter so concurrent new chats stay distinguishable.

2. Rename on chat start using the ACP session title

Wire the ACP session_info_update notification end-to-end so a started chat is renamed to the agent-provided session name, with a graceful fallback chain:

agent-pushed title (session_info_update) → first user message (existing deriveTitle behavior) → Untitled Chat N

Per the ACP spec, SessionInfo.title is "Human-readable title for the session. May be auto-generated from the first prompt," and agents send session_info_update "after the first meaningful exchange to auto-generate a title" — exactly the "rename when the chat starts" behavior requested here.

Implementation outline:

• Rust (src-tauri/src/acp/)
  • Add EVENT_SESSION_INFO_UPDATE (acp:session_info_update) in events.rs and a SessionInfoUpdateEvent { agent_id, session_id, title } struct.
  • In client.rs::emit_session_update, handle SessionUpdate::SessionInfoUpdate and emit the new event instead of letting it fall into the debug-log catch-all.
• Renderer API (src/renderer/lib/acp-api.ts)
  • Add sessionInfoUpdate: 'acp:session_info_update' to ACP_EVENTS (~line 312) and export a SessionInfoUpdateEvent type.
• Store (src/renderer/stores/acp-store.ts)
  • Add _onSessionInfoUpdate reducer that sets session.title from the agent title and re-persists the session index entry; subscribe it in initAcpEventListeners.
  • Introduce the Untitled Chat N counter and use it as the placeholder.
• Persistence (src/renderer/lib/acp-history-persistence.ts)
  • Update deriveTitle() fallback to Untitled Chat N.

Alternatives Considered

• Keep deriving from the first user message only — already the current behavior once a message exists, but it does nothing for the pre-prompt placeholder and ignores the richer agent-generated titles ACP already provides.
• session/setTitle (client → agent), spec PR #1199 — enables user-initiated rename pushed back to the agent. It's not in Termul's vendored SDK yet (only session_info_update is stabilized — see src-tauri/vendor/agent-client-protocol/CHANGELOG.md:102), so it's out of scope here but a natural follow-up for manual rename.

Success Criteria

• A newly created ACP chat shows Untitled Chat N (not Agent <random>) in the Chats history list and tab.
• After the chat starts, if the agent sends session_info_update, the chat is renamed to the ACP session title in real time and the new title persists across app restarts.
• If the agent never pushes a title, the chat still falls back to the first-user-message derivation, then to Untitled Chat N.
• session_info_update notifications are no longer silently dropped (resolves item fix: resolve test failures, typecheck errors, and improve Settings UI #2 of ACP: Cleanup dead code, dropped protocol messages, and stale comments #363).
• Unit tests cover deriveTitle fallback, the _onSessionInfoUpdate reducer, and the Rust event emission.

Additional Context

ACP references

• Session List (SessionInfo.title + session_info_update notification): https://agentclientprotocol.com/protocol/session-list
• Session Info Update RFD: https://agentclientprotocol.com/rfds/session-info-update
• Schema: https://agentclientprotocol.com/protocol/schema
• session/setTitle (future, client→agent): Add session/setTitle method agentclientprotocol/agent-client-protocol#1199

Code references

• src/renderer/lib/acp-history-persistence.ts:49-55 — deriveTitle() placeholder
• src-tauri/src/acp/client.rs:296-303 — dropped SessionInfoUpdate
• src/renderer/stores/acp-store.ts:103, :476, :798 — session title lifecycle
• src/renderer/lib/acp-api.ts:312 — ACP_EVENTS
• src-tauri/src/acp/events.rs:20-47 — acp:* event constants
• src/renderer/components/chat/AgentBadge.tsx:33, src/renderer/components/workspace/WorkspaceTabBar.tsx:649 — label surfaces

Related: Complements #363 (item #2 calls out the dropped SessionInfoUpdate); this issue is the "implement it properly" counterpart.