GoKawiilLooking ahead: Anthropic has released Claude Fable 5.1, making its newest flagship model available through its API, cloud platforms and the Claude desktop app. It also released Mythos 5.1, a version of the same underlying model reserved for approved cybersecurity and life-sciences organizations. These updates make Anthropic's most advanced models easier for companies to use on complex work that takes time, involves sensitive data and needs clear safeguards.
Fable 5.1 is the broadly available version, with safeguards that block some high-risk work in cybersecurity and scientific research. Mythos 5.1 has fewer of those restrictions, but access is limited to Anthropic's trusted-access program.
The update targets customers using AI for assignments larger than a single prompt or code request. Anthropic said Fable 5.1 is better at handling software projects, reviewing code across an application, troubleshooting problems in external libraries and running multistep scientific work. It can also help design experiments, model possible outcomes and interpret detailed tables and diagrams.
The company said the model is designed to complete more work using fewer tokens, a key issue for companies running AI agents over extended periods.
Anthropic reported substantial gains on several internal and third-party-style evaluations. Fable 5.1 scored 55.8% on Terminal-Bench 4.0, a benchmark for command-line coding work, compared with 42% for Fable 5. Mythos 5.1 scored 60.9% under its more permissive safety settings.
On Terminal-Bench-Science 0.1, which measures agentic scientific research, Anthropic reported a 52.6% score for Fable 5.1, more than double Fable 5's 24.7%. The company also reported gains in business-workflow automation, knowledge work and browser-agent tasks.
Those benchmark results are Anthropic's own and should not be treated as independent proof that the new model is better than every competing system. Still, the results line up with the company's broader claim that Fable 5.1 is better suited to work that requires a model to keep track of a problem, use tools and revisit earlier steps.
Some early customers described that kind of use. Millennium said Fable 5.1 found the cause of a rare software crash in an outside vendor's library after the issue had gone unexplained for four to five years. Ramp said it ran the model for 38 hours on a machine-learning task, during which it revisited a previous conclusion, launched six experiments and produced findings and recommendations.
Anthropic did not cut Fable 5.1's standard API rates. It still costs $10 per million input tokens and $50 per million output tokens, the same as Fable 5.
Instead, the company cut the price of cache reads by 75%. Reading cached input now costs $0.25 per million tokens, down from $1 per million for Fable 5. Cache writes remain $12.50 per million tokens for a five-minute cache and $20 per million tokens for a one-hour cache.
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