Anthropic ships Claude Fable 5.1, more than doubling Fable 5 benchmark

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on Tuesday. The releases were the first update to the Mythos-class model line since Fable 5 launched on June 9. Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1. Fable 5 scored 24.7% on that benchmark. Opus 5 scored 29.0%. Terminal-Bench-Science 0.1 measures whether an AI agent can perform scientific research tasks in a command-line environment. Fable 5.1 scored 55.8% on Terminal-Bench 4.0. Fable 5 scored 42.0% on that test. Opus 5 scored 52.3%. Mythos 5.1 scored 60.9% on Terminal-Bench 4.0 with lighter cybersecurity filters. Anthropic said its safeguards intercepted and rerouted some tasks to Opus 4.8. Fable 5.1 scored 60.9% on Humanity's Last Exam without external tools. Fable 5.1 scored 65.0% with external tools. Anthropic says the model is its most advanced model for academic purposes. Anthropic claims the new models are the world's most advanced models for coding and knowledge work. Fable 5.1 and Mythos 5.1 use the same underlying model with different safety filters, according to Anthropic. Fable 5.1 is generally available to anyone with a Claude account. Mythos 5.1 is restricted to vetted cybersecurity and life-sciences professionals through Anthropic's Cyber Verification Program and Life Sciences Verification Program. Those programs replaced the Project Glasswing access track that restricted Mythos 5. Fable 5.1 draws from pay-as-you-go usage credits on Pro plans and standard Team or Enterprise seats. Fable 5.1 is not included in those plans' weekly limits. Max plans and premium Team or Enterprise seats include Fable 5.1 for up to 50% of weekly usage. Fable 5.1 costs $10 per input token and $50 per output token. Opus 5 costs $5 per input token and $25 per output token. Anthropic says low or medium reasoning effort on Fable 5.1 can match or exceed Fable 5's previous results at lower cost. Fable 5.1 is available through the Claude API, Amazon Bedrock, Google Cloud, and Microsoft Foundry. Its model ID is claude-fable-5-1.