[Bug]: Mobile sends skip "When you return" idle compaction #27911

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/mobile

Steps to reproduce

1. Keep idle compaction at "When you return" (the default).
2. Use a Claude thread above 200K tokens and leave it idle past its prompt cache TTL (70 minutes when the provider reports no TTL).
3. Send a message to it from the iOS or Android app.

Expected behavior

The same as the web composer: the send compacts first ("Compact and send", a /compact turn with idleCompaction: "onReturn"), and the message runs after it.

Actual behavior

The message starts at once with no compaction, and Claude re-reads the whole context uncached.

Observed on 2026-10-31: an Android client returned to a 293,611-token Claude thread after 8h40m idle. The first request after the return reported cache_read 14,250 and cache_creation 282,954, the session's JSONL has no compact_boundary, and the run that carried the message has creationSource: "mobile".

"When you return" lives only in the web composer: shouldOfferResumeCompaction (apps/web/src/components/chat/ContextWindowMeter.logic.ts) feeds compactBeforeSend in planComposerTurn (ChatView.logic.ts), and useComposerSend sends /compact before the message (#16631). The server's IdleCompactionService handles only "Before the cache expires", and nothing in apps/mobile sends the compaction, so the setting does nothing for mobile sends.

Impact

Minor bug or occasional failure

Version or commit

main @ d2d1e4f

Environment

Android app (release build, Android 16), server on Linux and on the macOS desktop app, Claude Agent provider

Workaround

Send /compact by hand before the message, or switch idle compaction to "Before the cache expires".