[Bug] Fable 5.1 excessive token consumption on low-effort tasks

**Bug Description**
fable 5.1 has eaten through my session quota incredibly quickly, even on loweffort, last night it was fine (both have 3-4 fable 5.1 on low inparallel) but last night was fine, today seems to have gone through in less than 2 hours

**Environment Info**
- Platform: darwin
- Terminal: Apple_Terminal
- Version: 2.1.263
- Feedback ID: 28b18b4a-9cc3-4e19-99a4-81f2cf46273e

**Errors**
```json
[]
```

No files, tests, or code entry points are named. Start by reproducing the report with Claude Code 2.1.263 on darwin/Apple_Terminal and use the Feedback ID to compare token or quota behavior for Fable 5.1 low-effort sessions. Done means there is a confirmed cause for the excessive token consumption and a verified fix or regression test.

索引モデルが issue の本文から書いたものです。