Anthropic upgrades Claude with new Fable 5.1 model, details here

In June, Anthropic released Fable 5, its Mythos-class AI model for Claude. Three months later, they enhanced Claude with its new Fable 5.1 model upgrade. Fable is the most capable model in Anthropic's lineup, with Haiku, Sonnet, and Opus following below it. According to Anthropic, Claude Fable 5.1 sets a new standard for coding, knowledge work, and long-running problem-solving tasks.

The new model outperforms Fable 5 and achieves similar or better results at a much lower cost, especially when set to low or medium effort. It avoids shortcuts that lead to poorer-quality work and can fix the root causes of software issues. An investment firm's testing found Fable 5.1 successfully identified the cause of a rare crash on internal systems that no engineers or other models had solved after years of attempts.

Anthropic is also releasing Mythos 5.1 for users with access, noting it's the same model as Fable 5.1 but with different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is restricted to Anthropic's trusted access programs, designed for cybersecurity and life sciences work. The new model will cost about 25% less than Fable 5 for typical workloads, with even larger savings—up to 45%—for highly agentic tasks.

Anthropic has reduced the price of Fable 5.1's cache reads, where the model reuses processed and stored inputs, leading to a significant decrease in overall model costs. Further details can be found in Anthropic's announcement post.