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Sep 2026

Anthropic published Claude Fable 5.1 and Mythos 5.1

In September 2026, Anthropic published Claude Fable 5.1 and Claude Mythos 5.1. It says they are the same model with different safeguards.

Fable 5.1 is listed as generally available on Anthropic’s own platforms plus AWS, Google Cloud, and Azure as claude-fable-5-1. Anthropic estimates about 25% lower token cost than Fable 5 for typical workloads, mostly from cheaper cache reads.

Mythos 5.1 is not a general product. The page puts it in trusted-access programs for cybersecurity and life sciences.

Enterprise Frontier Safeguards — customer-controlled storage — is described as arriving in phases later this fall. Until then, eligible customers can use Fable 5.1 with zero data retention, per Anthropic. The page gives a month, not a day.

The announcement is on anthropic.com. The bench table and the 25% cost line are Anthropic’s numbers, not an independent audit.

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