Anthropic has upgraded its Claude AI platform with the new Fable 5.1 model, arriving three months after the company first released Fable 5 in June. The refresh targets coding, knowledge work, and long-running problem-solving tasks, and it launches alongside a companion model called Mythos 5.1.
In Anthropic’s lineup, Fable represents Claude’s most capable tier. Below it sit Haiku, Sonnet, and Opus, positioned as the small, medium, and large models respectively.
What Fable 5.1 Improves
According to Anthropic, Claude Fable 5.1 “sets a new standard on coding, knowledge work, and long-running problem-solving tasks,” delivering a higher level of performance than the prior Fable 5 release. The company adds that the model “achieves similar or better results than Fable 5 at a much lower cost” when configured to low or medium effort.
Anthropic says the model avoids shortcuts that lead to lower-quality output and is capable of addressing the root causes of software problems. In one example cited by the company, investment firm Millennium used Fable 5.1 to identify the cause of a rare crash on its internal systems, an issue that its engineers and other models had reportedly failed to explain over several years.
Mythos 5.1 and Access Tiers
Alongside the main release, Anthropic is rolling out Mythos 5.1 for users who have Mythos access. The two are the same underlying model but carry different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is offered only through the company’s trusted access programs. Its safeguards are designed specifically to support work in cybersecurity and the life sciences.
Pricing and Cost Savings
Anthropic estimates that Fable 5.1 will cost roughly 25% less than Fable 5 for typical workloads billed by token. The savings come from reduced pricing on cache reads, where the model reuses inputs it has already processed and stored, such as through the company’s API. For highly agentic tasks, the company says the savings can climb to approximately 45%.
Aside from the cache read reduction, pricing is otherwise unchanged from Fable 5. Anthropic describes the change as a “substantial reduction in the overall cost of running the model.” Both Claude Fable 5.1 and Claude Mythos 5.1 are available starting today.
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