Anthropic released the Claude Fable 5 model from the Mythos class

Anthropic released the Claude Fable 5 model from the Mythos class

On June 9, the Anthropic Claude Fable 5 model became available. According to the company, it is a cutting-edge model in nearly every respect. Initially, it was offered free to Pro, Max, Team, and Enterprise subscribers until June 23.

Anthropic classifies the model as part of the Mythos line and applies extra safety measures for several sensitive domains. For example, when users query topics related to cybersecurity, biology, chemistry, or model distillation, responses are handled by the Opus 4.8 model (with a prior warning displayed to users).

According to multiple benchmarks, Claude Fable 5/Mythos 5 delivers top-tier results in programming, scientific research, and data handling. The company reports that Mythos 5 maintains task focus across millions of tokens by keeping its own notes, performs on par with Mythos Preview, and surpasses it in some areas.

Anthropic still believes the widespread use of Mythos-class models could pose risks, so significant attention was given to securing agents that use these models. The internal Anthropic security team and external researchers through the bug bounty program collectively spent over 1,000 hours hunting for generalized jailbreak methods; the model consistently refused to execute malicious prompts.

Those users who previously received access to Mythos Preview were able to upgrade to Mythos 5. Anthropic also expanded access to Mythos 5 to a number of companies engaged in biological and pharmaceutical research.

To support the release, the company also published prompting examples for more effective model use.

However, on June 12 Anthropic issued a notice on access suspension for both Fable 5 and Mythos 5 models for all users. Anthropic says the US government cited a way to circumvent the model's limitations, but the company has not received details of that method. In Anthropic's assessment, the issue is limited in scope and does not grant unique capabilities compared to other current models. No timeline for restoring access has been announced.