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.
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