What Claude Fable 5.1 / Mythos 5.1 is
Claude Fable 5.1 and Claude Mythos 5.1 are Anthropic's flagship frontier models as of September 2026, succeeding the Claude Opus 4.5 / Opus 5 line. Anthropic markets the Fable/Mythos 5.1 pair as 'the world's most advanced models for coding and knowledge work,' alongside the more recently dated Claude Opus 5 (July 2026) and Sonnet 5 (June 2026) in the broader model family.
Key features
- Claude Mythos 5.1 and Fable 5.1 billed as 'the world's most advanced models for coding and knowledge work'
- 200K+ token context window
- Agentic Claude Code, Cowork, and Claude Code for Enterprise
- Claude Sonnet 5 (June 2026) and Claude Opus 5 (July 2026) in the same family
Where it lives
Alternatives to consider
Full comparisonRelated reading
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Guides and articles
32
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Prompt collections
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Sources & media
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