We’re introducing Claude Fable 5.1 and Claude Mythos 5.1. They're...
Claude@claudeai
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Sep 02, 2026
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We’re introducing Claude Fable 5.1 and Claude Mythos 5.1.
They're the world’s most advanced models for coding and knowledge work.
They're the world’s most advanced models for coding and knowledge work.
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Fable 5.1 excels at complex, long-running tasks. And its research capabilities offer an early glimpse of how AI models will contribute to scientific progress.
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Across our benchmarks, the model sets a new standard.
It scores 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5. On Terminal-Bench 4.0, it scores 55.8% against 42.0% for Fable 5.
It scores 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5. On Terminal-Bench 4.0, it scores 55.8% against 42.0% for Fable 5.
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As well as being capable of much higher performance than Fable 5, it can also achieve similar or better results at a much lower cost when set to lower effort levels.
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Cache reads with Fable 5.1 cost 75% less than Fable 5’s.
This reduces the cost of the model in practice by around 25% for typical workloads, and up to 45% for highly agentic ones.
This reduces the cost of the model in practice by around 25% for typical workloads, and up to 45% for highly agentic ones.
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We’re also introducing Enterprise Frontier Safeguards (EFS), which give enterprise customers complete privacy (the same as zero data retention), while still being state-of-the-art at preventing adversarial use. EFS rolls out in phases, starting this fall. anthropic.com/news/enterpris…
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Finally, we’ve improved our safeguards. Our cybersecurity safeguards now flag benign requests about 60% less often.
On basic biology and medical questions, we’ve recently reduced the fallback rate by around 85%.
On basic biology and medical questions, we’ve recently reduced the fallback rate by around 85%.
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Claude Fable 5.1 is available everywhere today. Claude Mythos 5.1, our model for cyberdefenders and life scientists, is available through trusted access programs.
Read more: anthropic.com/claude-fable-a…
Read more: anthropic.com/claude-fable-a…