[BUG] Idle compaction was enabled remotely by feature flag with no notice or opt-in

Preflight Checklist

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

What's Wrong?

My idle sessions started being compacted. No local setting changed and I installed nothing to get this. The behavior is gated by the server-side flag tengu_sunny_locket, whose default mode is "off" and which was switched on remotely. No in-session notice explained the new behavior or how to opt out. Compaction discards conversation detail, so a silent remote flip destroyed context in long design sessions I had not finished. The opt-out key (idleCompaction: false) exists only in the settings schema; nothing in the UI surfaced it, and per #101616 it did not exist when the behavior first shipped.

What Should Happen?

A behavior change that discards user context should be opt-in, or at minimum show a one-time in-session notice naming the behavior and its opt-out before it first fires. Any remote flag that alters or discards user context should require that notice.

Error Messages/Logs

> Compacted after an hour idle, using a summary written while you were away

Steps to Reproduce

1. Use an account on the 1-hour prompt cache, with no idleCompaction key in settings.
2. Run a long session (past roughly 200K tokens).
3. Leave it idle for over an hour.
4. Return. The session has been compacted with the message above. No prior notice,

no prompt, no indication of how to turn it off.

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.300 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

Observed by reading strings in the 2.1.300 binary: flag tengu_sunny_locket with modes log / compact / compact_on_return and default "off"; the idle path checks autoCompactEnabled then idleCompaction before firing; a second SleepCompactor path checks the same two settings. Related: #101616, #101070