In June, Anthropic first released Fable 5, its Mythos-class AI model for Claude. Three months later, Anthropic is enhancing Claude with its new Fable 5.1 model upgrade.
Claude Fable 5.1 and Mythos 5.1 arrive today
In Anthropic’s world, Fable is Claude’s most capable model. Haiku, Sonnet, and Opus are the small, medium, and large models below Fable.
“Claude Fable 5.1 sets a new standard on coding, knowledge work, and long-running problem-solving tasks,” Anthropic says. The company says the new model has a higher level of performance compared to Fable 5.
Anthropic also says Fable 5.1 “achieves similar or better results than Fable 5 at a much lower cost” when set to low or medium effort.
More from Anthropic below:
Fable 5.1 avoids shortcuts that result in poorer-quality work, and it’s smart enough to fix the root causes of software issues. For example, in testing by the investment firm Millennium, Fable 5.1 found the cause of a rare crash on their internal systems that none of their engineers (or any other model) had been able to explain after several years of trying.
In addition to Fable 5.1, Anthropic is also releasing Mythos 5.1 for users with Mythos access.
“Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards,” Anthropic explains. “Fable 5.1 is generally available, while Mythos 5.1 is available only through our trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences.”
Here’s pricing data for Fable 5.1:
Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token. This is because we’re reducing our pricing on cache reads (where the model reads inputs that have already been processed and stored). For highly agentic work, the savings will often be much larger—up to approximately 45%.
In terms of pricing, Anthropic says it has “reduced the price of Fable 5.1’s cache reads (where the model reuses context it has already processed) wherever usage is billed by token, such as on our API.” The company says this “leads to a substantial reduction in the overall cost of running the model.” Pricing is otherwise unchanged from Fable 5.
You can learn much more in the announcement post here.
We’re introducing Claude Fable 5.1 and Claude Mythos 5.1.They're the world’s most advanced models for coding and knowledge work. pic.twitter.com/8P9PSrWPi3
— Claude (@claudeai) September 1, 2026