Anthropic published Claude Fable 5.1 and Mythos 5.1 — same model, different safeguards

Anthropic published Claude Fable 5.1 and Claude Mythos 5.1 and says they are the same model with different safeguards.

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

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 — roughly a quarter less spent on tokens, the billed chunks of text — 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.