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

MarkTechPostSeptember 1, 2026en

MarkTechPost

AI Global Wire

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

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