[BUG] Windows/MSIX 1.28929.0: cross-session messages land in the target's composer but are never submitted — session never responds #86069

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?

On Windows (MSIX/Store install), send_message between Claude Code sessions reports success and the message visibly appears in the target session's input composer — but it is never submitted. The target never produces a turn. Observed sitting unprocessed for 39+ minutes.

Typing the identical text into that same session by hand works instantly, so the target session is healthy. Only the delivered message fails to execute.

Two additional findings:

1. Queued messages OVERWRITE each other. Sending a second message to a session with one still unprocessed replaces it — the first is lost silently with no notice. Messages can disappear entirely.
2. In the target's transcript JSONL the message is written as a last-prompt record, never as a type: "user" turn. It is stored as a pending prompt rather than enqueued.

This worked reliably until 2026-08-11 ~18:48 ET and has failed 100% since.

What Should Happen?

The target session should process the delivered message as a conversation turn and respond, as it did prior to 2026-08-11.

Error Messages/Logs

No error is produced anywhere. The tool returns:

  Message sent to session local_<id> ("Flow HQ").

The message renders correctly in the target session UI. Nothing fails loudly — it simply never executes.

Delivery counts across six long-running sessions, cross-session messages processed as real turns vs parked:

  Session A: 45 processed, 0 parked
  Session B: 30 processed, 0 parked
  Session C: 23 processed, 0 parked
  Session D: 21 processed, 0 parked
  Session E:  6 processed, 0 parked
  Session F:  3 processed, 0 parked

128 successful deliveries, zero failures, through 2026-08-11 18:48 ET.
Since 2026-08-12 07:54 ET: 17 sends, 0 processed. Every one returned success.

Steps to Reproduce

1. Open two Claude Code sessions, A and B, on Windows (MSIX/Store install).
2. From session A, call send_message targeting session B.
3. Tool returns: Message sent to session ("B").
4. Open session B — the message is visibly present as a "Message from A" block.
5. Session B never processes it. No assistant turn is ever generated.
6. Type any text into B by hand — it responds immediately.

Ruled out during diagnosis:

• Privacy/telemetry env vars (CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC, DISABLE_TELEMETRY, DO_NOT_TRACK, DISABLE_GROWTHBOOK) — all unset at process, user and machine scope
• Windows Firewall — all Claude inbound/outbound rules present, enabled, action Allow
• Stale process state — reproduces after a full quit (all processes killed and verified) and relaunch
• Orphaned pre-existing sessions — reproduces in a session created minutes ago, after the failure began

Change window: four changes landed between the last success (8/11 18:48) and the first failure (8/12 07:54):

• Claude 1.28929.0.0 installed 8/11 21:10 — its release notes mention fixing MSIX installs "failing to save chat history, settings, and scheduled tasks," so this build touched MSIX persistence
• KB5123304 (8/11), KB5121003 and KB5120708 (8/12)

I cannot isolate which. Flagging the MSIX persistence change as most suspicious given the install type and the symptom: a prompt that is stored but never enqueued.

Possibly related: the 2026-08-08 fix for "cross-session messages staying parked without a notice or expiry" — same class of parking, reappearing after 8/11.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

unknown — the build immediately prior to 1.28929.0.0 (MSIX update removed the old package folder). Last confirmed working 2026-08-11 18:48 ET.

Claude Code Version

1.28929.0.0

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

No response