[BUG] Prompt cache expires during active session, causing massive token spikes on next prompt

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

2 days i see new message: Idle xxx minutes. The prompt cache has likely expired, so your next message will re-cache about 629k tokens.

### What Should Happen?

No idle count or cache expired as before!!
I don't wanna waste tokens on a bugged software working 2 minutes and then hit a 5h limit!

Pro-subscription, Opus 5

### Error Messages/Logs

```shell

```

### Steps to Reproduce

In not even long session just continue to work

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

10 September 2026

### Claude Code Version

2.1.270

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

VS Code integrated terminal

### Additional Information

_No response_

No repository file or test is named. Reproduce on Windows in the VS Code integrated terminal with Claude Code 2.1.270 by continuing a session until the idle/cache-expired message appears, then compare with the 10 September 2026 version. Done means active sessions no longer report cache expiry or trigger the reported token recache.

索引モデルが issue の本文から書いたものです。