Anthropic on September 1 released Claude Fable 5.1 and Claude Mythos 5.1, its first update to the Mythos-class model line since Fable 5 launched on June 9. The company describes the new AI models as the world's "most advanced for coding and knowledge work."
Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5's 24.7% and ahead of Opus 5's 29.0%. On Terminal-Bench 4.0, which tests agentic coding in command-line sessions, it reached 55.8%, up from Fable 5's 42.0% and above Opus 5's 52.3%. Mythos 5.1 scored 60.9% on the same test with lighter cybersecurity filters. On Humanity's Last Exam, Fable 5.1 achieved 60.9% without external tools and 65.0% with them.
Pricing remains unchanged at $10 per million input tokens and $50 per million output tokens, but cache reads now cost 75% less, cutting typical workload costs by roughly 25% and highly agentic workloads by up to 45%. Fable 5.1 is not included in standard Pro, Team, or Enterprise subscriptions; those users draw from pay-as-you-go credits. Max and premium Team/Enterprise seats include it for up to 50% of weekly usage.
Anthropic also introduced expanded enterprise privacy options, including a zero-data-retention mode that allows clients to run models on their own infrastructure without data outflows. Enterprise Frontier Safeguards will roll out in June. The system card rates Mythos 5.1 as low-risk for automated AI development, but notes a slight regression in overall misaligned behavior compared to Opus 5, while improving over Mythos 5 and Claude Sonnet 5. Mythos 5.1 remains restricted to vetted cybersecurity and life-sciences professionals.
Fable 5.1 is available now via the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry under the model ID "claude-fable-5-1."
