[BUG] SendMessage to a cloud session reports success and renders in the cloud UI, but the cloud agent has no session-inbox event and no FetchInboxMessage tool

What's wrong

A cross-session message sent from a local Claude Desktop session to a cloud Claude Code session reaches the cloud session's UI but never reaches the cloud agent. The sender gets success: true, the recipient's transcript shows a "Received message from another session" row, and the agent in that session has no way to read it.

Repro

1. Local session (Claude Desktop, Windows 11, bundled Claude Code 2.1.280, bypassPermissions). ListAgents lists the target as Cloud session test [eb4bc2] · cloud · idle.
2. SendMessage({to: "Cloud session test", message: "Test ping …"}) returns success: true, msg_id ce7a0f5a-b6a1-45f3-927f-669b13344fde, with the note that a cloud session is one-way and "its response appears in its own transcript".
3. In the cloud session's UI a collapsed "Received message from another session ›" row appears. Nothing on it asks for approval.
4. Asked "can you see the message from local session?", the cloud agent says no.
5. Asked to look for a source="session-inbox" event and to call FetchInboxMessage with its file_id, the cloud agent (container Claude Code 2.1.281) reports:

• no session-inbox event (and no <cross-session-message> wrapper) anywhere in its context;
• no FetchInboxMessage tool, loaded or deferred; a tool search for FetchInboxMessage inbox returns only SendMessage.

Expected

Either the message reaches the cloud agent (a session-inbox notification plus FetchInboxMessage, as the local side has), or the send fails with a reason instead of reporting success.

Why it matters

It fails silently in both directions. The sender is told the message arrived, and the operator sees it arrive in the UI, so nothing tells either of them that the agent never got it. We only found out by asking the cloud agent directly.

Related (none covers a cloud recipient)

• #86671 / #86012 — local → local: message rendered in the target's UI but never enqueued (same visible symptom, different surface; closed as fixed).
• #94624 — SendMessage reports success, message never arrives (local → local, macOS).
• #86608 — send reports success when the receiving side has no working inbox.
• #92540 — feature request for cloud ↔ cloud SendMessage.

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Filed at operator direction 2026-09-24