Anthropic has released Claude Fable 5.1, an upgrade to its Fable 5 model, claiming improved performance on coding, knowledge work, and long-running tasks. The company emphasizes that Fable 5.1 avoids shortcuts that lead to poorer-quality work and can fix root causes of software issues. In testing by investment firm Millennium, Fable 5.1 identified the cause of a rare system crash that engineers and other models had failed to explain for years.

Alongside Fable 5.1, Anthropic introduced Mythos 5.1 for users with Mythos access. Both models are the same, but Mythos 5.1 includes different safeguards tailored for cybersecurity and life sciences work. Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programs.

Pricing for Fable 5.1 is reduced by about 25% compared to Fable 5 for typical workloads, with cache reads priced lower. For highly agentic work, savings can reach up to 45%. Anthropic states that the price cut on cache reads leads to a substantial reduction in overall model costs, while other pricing remains unchanged.

The announcement was made via a post on Anthropic's website, with further details available there.