Claude Fable 5.1 launches alongside Claude Mythos 5.1 with 1M token context
Tech · Releases · Tue, September 1, 2026
Claude Fable 5.1 succeeds Claude Fable 5 for long-running agentic coding, knowledge work and research, with Claude Mythos 5.1 for Project Glasswing participants. Both have a 1M context, 128k output and adaptive thinking at $10 / $50 per MTo
Every year
Also on Sep 1, 2026
| Date | Event | When |
|---|---|---|
| Tue, Sep 1, 2026 | Bear season opens (MN) | Now |
Source, method and history
- Source
- anthropic
- How we know
- From the Releases API (releases.fru.dev), which reads each vendor's changelog.
- Who
- anthropic
- Recurs
- one-off
- More on fru.dev
- releases.fru.dev
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- Changes
- 2026-09-25: added to 2026-09-01