[BUG] Desktop app: turns triggered by cross-session messages hang at 0 tokens (manual prompts work) #86386

Preflight Checklist

• I have searched existing issues and this hasn't been reported yet
• This is a single bug report (please file separate reports for different bugs)
• I am using the latest version of Claude Code

What's Wrong?

Environment: Claude Code desktop app on Windows 11 Pro (build 26200), Claude Code
version 2.1.212, multiple local sessions in the same project folder (D:\Vexalia),
subscription plan (Max).

When one local session sends a message to another via the built-in cross-session
messaging (send_message), the receiving session's turn starts automatically
(spinner shows, session state = running) but hangs forever: 5+ minutes observed,
zero tokens spent, nothing appended to the receiving session's transcript.

Typing a manual prompt into the same receiving session works normally, and it then
also processes the queued cross-session message correctly.

This worked before the desktop app auto-update on 2026-08-12: sessions replied to
each other's messages automatically with no human intervention.

What Should Happen?

A turn triggered by an incoming cross-session message should run exactly like a
manually typed prompt: call the model, spend tokens, and produce a reply.

Error Messages/Logs

No error is shown anywhere. The spinner just runs indefinitely at zero tokens.
Nothing is appended to the receiving session's transcript file while stuck.

Steps to Reproduce

I run multiple local Claude Code sessions in the same project folder (a PM session
coordinating specialist sessions). Sessions message each other via the built-in
cross-session messaging (send_message).

Steps:

1. Session A sends a message to Session B via send_message. Session B's process is
   awake (its chat is open and it had just completed a manual turn successfully).
2. Session B's turn starts automatically (spinner shows, session state = running).
3. The turn hangs forever: 5+ minutes observed, ZERO tokens spent, nothing written
   to the session's transcript (.jsonl).
4. Stopping the spinner and typing any manual prompt into Session B works normally,
   and it then also processes the queued cross-session message correctly.

This worked before the desktop app update on 2026-08-12: sessions replied to each
other's messages automatically with no human intervention.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown exact number — whatever version was current until 2026-08-12 (auto-updated that day; the hang started right after).

Claude Code Version

2.1.212 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

No response