Anthropic Releases Claude Fable 5.1 and Mythos 5.1

Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1, with Fable 5.1 available now on all platforms, the company announced on its website. The company says the two models are the same model with different safeguard levels, with Fable 5.1 generally available and Mythos 5.1 limited to trusted access programs.

Anthropic says Fable 5.1 costs about 25% less than Fable 5 for typical workloads where usage is billed by token. For complex agentic tasks, the company says savings can reach about 45%, due to a 75% cut in cache read pricing to $0.25 per million tokens. Other pricing remains $10 per million input tokens and $50 per million output tokens.

The release also includes Enterprise Frontier Safeguards, a system that stores customer data in cloud infrastructure controlled by the customer rather than Anthropic. The company says the feature will roll out to enterprise customers in phases starting this fall. Eligible customers can use Fable 5.1 with zero data retention until then.

Anthropic says Fable 5.1 can now be used to identify software vulnerabilities, while some cybersecurity tasks, including penetration testing and exploit generation, will still be redirected to Opus models. Claude Mythos 5.1 is available to vetted cyberdefenders and life scientists through trusted access programs, and Claude Security is now powered by Mythos 5.1.