Claude Fable 5.1 thinking blocks only replay to the same or newer models
Tech · Releases · Tue, September 1, 2026
Thinking blocks from Claude Fable 5.1 and Mythos 5.1 can't be read by earlier models and are dropped if replayed to them. Fable 5.1 also checks that nothing before a replayed block has changed, which matters for agents that edit conversatio
Every year
Also on Sep 1, 2026
| Date | Event | When |
|---|---|---|
| Tue, Sep 1, 2026 | Bear season opens (MN) | Now |
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- 2026-09-25: added to 2026-09-01