Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, two versions of the same underlying model with different safeguards and access levels.

Fable 5.1 is generally available, while Mythos 5.1 is limited to trusted-access programs for vetted cybersecurity and life-sciences users.

The new release focuses on stronger coding, scientific research, long-running tasks, lower costs, better privacy, and more precise safeguards.

Anthropic has cut the price of cache reads by 75% to $0.25 per million tokens.

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% less than Fable 5, while highly agentic workloads could save up to 45%.

Fable 5.1 defaults to High effort in Claude Code and Medium effort in Claude Cowork and Claude.ai.

Anthropic says Fable 5.1 improves substantially across coding, research, knowledge work, computer use, and business automation.

Anthropic says the model is also better at solving root causes instead of taking shortcuts. In testing by Millennium, Fable 5.1 identified the cause of a rare software crash that engineers and other models had failed to explain for four to five years.

Anthropic tested both models across scientific research tasks.

Mythos 5.1 designed high-affinity protein binders using open-source protein design and folding tools. Across three targets, its binding affinities were 10 times higher than the best designs submitted to Adaptyv Bio competitions.

Its hit rate reached nearly 50% across 12 targets, compared with typical protein-design rates of 10% to 15%.

Mythos 5.1 also optimized seven open-source biology models, improving inference speeds by 1.4x to 2.5x on Nvidia H100 GPUs while keeping outputs identical. Anthropic estimates the changes could reduce GPU costs for genome-wide analyses by 30% to 60%.

Fable 5.1 separately trained a neural network to produce a higher-resolution elevation map covering around one-third of Venus. The map improves detail from roughly 10–20 kilometers to 2–3 kilometers and produces height estimates up to 25% more accurately.

Fable 5.1 will support Anthropic’s new Enterprise Frontier Safeguards, which provide privacy equivalent to zero data retention while keeping customer data inside infrastructure controlled by the customer.

The system will roll out in phases beginning this fall. Until then, eligible customers can use Fable 5.1 with zero data retention.

Anthropic has also made cybersecurity safeguards more precise. Claude Code users should see around 60% fewer safeguard interventions per session, while Fable 5.1 can now identify software vulnerabilities for defensive purposes.

Penetration testing, exploit generation, and binary-based vulnerability scanning remain restricted.

Biology safeguards now trigger 85% less often on benign elementary biology and medical questions than the protections that originally launched with Fable 5.

Mythos 5.1 will be available through Anthropic’s Cyber Verification Program and Life Sciences Verification Program.

The life-sciences program was developed with the US government and has already enrolled its first participants.

Anthropic says Mythos 5.1 has the strongest cybersecurity capabilities of any model it has released when tested without safeguards, while still remaining in the lower risk category under its framework.

Claude Security is also now powered by Mythos 5.1.

Claude Fable 5.1 is available now through Anthropic, Amazon Web Services, Google Cloud and Microsoft Azure.

Developers can access it through the API using claude-fable-5-1 .

Claude Mythos 5.1 remains limited to vetted cybersecurity and life-sciences users through Anthropic’s trusted-access programs.

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