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, 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.