Anthropic has announced Claude Fable 5.1 and Claude Mythos 5.1, its latest AI models. The company described the new models as “the world’s most advanced models for coding and knowledge work.” Anthropic says the models can also help with scientific research and could play a bigger role in scientific discovery. Fable 5.1 is available to everyone, while Mythos 5.1 is being offered only to approved users and organisations working in areas such as cybersecurity and life sciences. Keep reading for all the details you need to know about Claude Fable 5.1 and Claude Mythos 5.1.
According to Anthropic, Fable 5.1 will be around 25 per cent cheaper than Fable 5 for typical workloads when users are charged by token. For highly agentic tasks, the savings could be even larger, with Anthropic estimating reductions of up to 45 per cent. The company has achieved this mainly by lowering the cost of cache reads, where the model processes information that has already been stored.
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We’re introducing Claude Fable 5.1 and Claude Mythos 5.1.
— Claude (@claudeai) September 1, 2026
They're the world’s most advanced models for coding and knowledge work. pic.twitter.com/8P9PSrWPi3
Anthropic also claims that Fable 5.1 delivers a major performance boost over its predecessor. It says the model can handle long-running tasks better and is less likely to take shortcuts that affect the quality of its work.
The company highlighted testing at investment firm Millennium. According to Anthropic, Fable 5.1 identified the cause of a rare crash in Millennium’s internal systems. Engineers and other AI models had been unable to explain the issue for years.
Claude Mythos 5.1 is the same model as Fable 5.1, but with different levels of safeguards. Mythos 5.1 will be available through two trusted access programmes. The Cyber Verification Program and Life Sciences Verification Program.
“Claude Mythos 5.1 is available to vetted cyberdefenders and life scientists. Currently, it is only available to a set of US organizations, though we’re coordinating with the US government to expand access to a broader set of domestic and international partners as quickly as possible,” the company said.
Claude Fable 5.1 is available across Amazon Web Services, Google Cloud and Microsoft Azure. Developers can also access the model through the Claude API.