Anthropic's Claude Fable 5.1 More Than Doubles Its Predecessor on an Agentic Science Benchmark

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, three months after Fable 5. The headline result is on Terminal-Bench-Science 0.1, a benchmark that scores a model running scientific research tasks in a terminal. Anthropic reports Fable 5.1 at 52.6%, against 24.7% for Fable 5, 29.0% for Claude Opus 5 and 22.4% for OpenAI's GPT-5.6 Sol.

The company also published the standard error, between roughly 3.5 and 4.5 points per model. That is an unusual disclosure, and it matters for the conclusion: a 28-point gap over Fable 5 survives that uncertainty comfortably, while several of the narrower comparisons Anthropic draws elsewhere do not. On Terminal-Bench 4.0 the numbers are 55.8% for Fable 5.1 and 60.9% for Mythos 5.1, and on Humanity's Last Exam, Fable 5.1 reaches 60.9% without tools and 65.0% with them, per MarkTechPost's summary of the release.

Mythos 5.1 is the more consequential product decision. Anthropic describes it as the same underlying model with lighter safeguards, available only to organizations that clear its Cyber Verification Program or Life Sciences Verification Program, and only in the United States at launch. The five-point Terminal-Bench gap between the two variants is a direct measurement of how much capability the safety layer costs.

Anthropic also cut cache read pricing by 75%, which matters more for agentic workloads than the benchmark deltas do. Long-running agents re-read the same context repeatedly, and cache economics decide whether a multi-hour research run is affordable.