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Anthropic has updated its safety-layered models with Fable 5.1, delivering a significant jump in scientific reasoning capabilities while reducing inference costs through optimized cache reads. This release highlights the trade-off between rigorous safeguard interventions and raw performance metrics, offering developers a more cost-effective option for agentic workflows on major cloud platforms.

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Anthropic Releases Claude Fable 5.1 and Claude Mythos 5.1: 52.6% on Terminal-Bench-Science and 75% Cheaper Cache Reads

MarkTechPostSeptember 1, 20264 min read

Anthropic has released Claude Fable 5.1 and Claude Mythos 5.1, the same model behind two different safeguard layers. Fable 5.1 is generally available on the Claude API, AWS, Google Cloud, and Microsoft Foundry; Mythos 5.1 remains restricted to vetted organizations under Project Glasswing. Fable 5.1 scores 52.6% on Terminal-Bench-Science 0.1 against 24.7% for Fable 5, ships a 1M token context window, and drops cache reads 75% to $0.25 per million tokens while base pricing holds at $10 and $50. Three API changes are breaking, including the removal of forced tool use. The post Anthropic Releases Claude Fable 5.1 and Claude Mythos 5.1: 52.6% on Terminal-Bench-Science and 75% Cheaper Cache Reads appeared first on MarkTechPost.

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…

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