Where it sits in the range
Claude Fable 5.1 is the newest addition to Anthropic's line of artificial intelligence models, sitting at the top of the company's range above Haiku, Sonnet and Opus.
Anthropic, the AI company behind the Claude chatbot and developer platform, released the model on 1 September, alongside a sister version called Mythos 5.1.
The two are built on the same underlying system but carry different safety layers: Fable 5.1 is available to the general public, while Mythos 5.1 is restricted to vetted organisations working in cybersecurity and life sciences.
Built for coding and long tasks
Anthropic pitches Fable 5.1 primarily at coding, knowledge work and long-running problem-solving tasks, the kind of jobs that require a model to work through many steps without losing track of the goal.
The model can handle a context window of one million tokens, roughly the length of several long novels, and can produce up to 128,000 tokens of output in a single response.
Anthropic says the model avoids shortcuts that lead to poor-quality fixes, and is instead built to track down the root causes of software problems.
The company points to an internal test by the investment firm Millennium, in which Fable 5.1 found the cause of a rare system crash that had gone unexplained by human engineers for several years.
Cheaper to run
On pricing, Anthropic has cut the cost of cache reads, where the model processes input it has already seen, from a dollar to 25 cents per million tokens.
That amounts to roughly 25% lower costs on typical workloads and up to 45% on heavily automated tasks, while base pricing remains at $10 per million input tokens and $50 per million output tokens.
Availability
Fable 5.1 is available now to Pro, Max, Team and Enterprise subscribers, as well as to developers through the Claude Platform, Amazon Web Services, Google Cloud and Microsoft Foundry.
The release follows a difficult few months for Anthropic's top-tier model line, after Fable 5 was briefly withdrawn in June under US export controls before being restored, and after Anthropic's own Opus 5 undercut it on price in July.