The new Claude Fable 5.1 AI model release from Anthropic brings substantial performance gains alongside a meaningful price cut, positioning it as one of the company’s most significant updates in recent memory. Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, two versions of the same underlying model that differ mainly in their safeguards and access levels.
What the Claude Fable 5.1 AI Model Release Includes
Fable 5.1 is generally available to the public, while Mythos 5.1 is limited to trusted-access programs for vetted cybersecurity and life-sciences users. The new release focuses on stronger coding capabilities, scientific research support, long-running task handling, lower operating costs, better privacy protections and more precise safeguards compared to previous versions.
The two models share the same core model architecture, but differ in availability and permitted use cases. Fable 5.1 allows vulnerability discovery for defensive cybersecurity purposes while restricting exploit development, and keeps advanced life-sciences research capabilities restricted. Mythos 5.1, by contrast, offers reduced safeguards for vetted cyberdefenders and expanded capabilities for verified life-sciences researchers.
Lower Pricing Across the Board
Anthropic has cut the price of cache reads by 75 percent to $0.25 per million tokens, while input and output pricing remains unchanged at $10 per million input tokens and $50 per million output tokens. The company estimates that typical Fable 5.1 workloads will cost around 25 percent less than Fable 5, while highly agentic workloads that make heavy use of caching could see savings of up to 45 percent.
Fable 5.1 defaults to High effort in Claude Code and Medium effort in Claude Cowork and Claude.ai, giving developers and casual users different default performance profiles depending on the platform they’re using.
Performance and Scientific Breakthroughs
Anthropic says Fable 5.1 improves substantially across coding, research, knowledge work, computer use and business automation tasks, and is notably better at solving root causes instead of taking shortcuts. In testing by Millennium, Fable 5.1 reportedly identified the cause of a rare software crash that engineers and other AI models had failed to explain for four to five years.
On the scientific side, Mythos 5.1 designed high-affinity protein binders using open-source protein design and folding tools, achieving binding affinities 10 times higher than the best designs submitted to Adaptyv Bio competitions across three targets, with a hit rate of nearly 50 percent across 12 targets compared to typical protein-design rates of 10 to 15 percent. Fable 5.1 separately trained a neural network to produce a higher-resolution elevation map covering roughly one-third of Venus, improving detail from 10-20 kilometers down to 2-3 kilometers.
Privacy and Safeguard Improvements
Fable 5.1 will support Anthropic’s new Enterprise Frontier Safeguards, providing privacy equivalent to zero data retention while keeping customer data inside infrastructure controlled by the customer, with the system rolling out in phases beginning this fall. Anthropic has also made cybersecurity safeguards more precise, with Claude Code users expected to see around 60 percent fewer safeguard interventions per session, while biology safeguards now trigger 85 percent less often on benign elementary biology and medical questions.
Availability
Claude Fable 5.1 is available now through Anthropic, Amazon Web Services, Google Cloud and Microsoft Azure, with developers able to access it through the API using the model identifier claude-fable-5-1. Claude Mythos 5.1 remains limited to vetted cybersecurity and life-sciences users through Anthropic’s trusted-access programs, including its Cyber Verification Program and Life Sciences Verification Program.
For more details on Anthropic’s model releases, visit the Anthropic website. For more AI news and product coverage, see our technology news section.