Developer Yuki Yoshioka (@yoppy0123) notes that from Claude Code v2.1.260, Fable 5.1 keeps the prompt cache valid when effort is changed mid-conversation, cutting repeated prefill cost from effort switches. The post warns Opus 5 is not listed in the notes and likely lacks the same fix yet, so treat models differently after upgrading.

Key Takeaways

  • ✓Claude Code v2.1.260: Fable 5.1 mid-chat effort changes no longer invalidate the prompt cache.
  • ✓Helps adjust reasoning effort without re-paying large context prefill.
  • ✓Opus 5 not confirmed in notes — do not assume all models behave the same.
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