Anthropic Releases Claude Fable 5.1 and Claude Mythos 5.1: 52.6% on Terminal-Bench-Science and 75% Cheaper Cache Reads
Anthropic has released Claude Fable 5.1 and Claude Mythos 5.1, three months after the Fable 5 line shipped in June 2026. The two are the same underlying model behind different safeguard layers. Fable 5.1 is generally available as claude-fable-5-1; Mythos 5.1 stays restricted to vetted organizations. Both carry a 1M token context window and 128K max output tokens, with adaptive thinking always on. The headline capability number is 52.6% on Terminal-Bench-Science 0.1, against 24.7% for Fable 5 an
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Anthropic has released two versions of new AI models, Fable 5.1 and Mythos 5.1, which are the same underlying model with different safety restrictions. Fable 5.1 is available to the general public on multiple platforms, while Mythos 5.1 is restricted to vetted U.S. organizations. The models show significant performance improvements, scoring 52.6% on Terminal-Bench-Science 0.1 compared to 24.7% for the previous Fable 5 model, and offer a 75% reduction in pricing for cached reads, bringing that cost from $1.00 to $0.25 per million tokens. The release includes three breaking changes for developers using earlier models, including changes to how forced tool use and thinking blocks function.
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