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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

AITopTools Editorial TeamSeptember 1, 2026

What changed

Anthropic released Claude Fable 5.1 for general use and Claude Mythos 5.1 for vetted organizations in its Project Glasswing program. Fable 5.1 scored 52.6% on a science and terminal-use test, up from 24.7% for Fable 5, supports up to one million units of text at once, and cuts cache-read prices by 75% while keeping base prices at $10 and $50.

What this means for you

Fable 5.1 is available through Anthropic’s API, AWS, Google Cloud, and Microsoft Foundry, giving organizations a more capable option with cheaper repeated-input reads. Developers should check compatibility before upgrading because three API changes are breaking, including removal of forced tool use.

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