Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, calling them its most advanced models yet for coding and knowledge work, while also pointing to early signs of how AI could contribute to scientific research. The two models share identical underlying architecture, differing only in their safety safeguards. Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programs built for cybersecurity and life sciences work.
Alongside the capability jump, Anthropic is addressing recurring customer complaints around cost and privacy. Fable 5.1 will cost roughly 25 percent less than Fable 5 for typical workloads, and up to 45 percent less for heavily agentic tasks, largely due to a 75 percent price cut on cache reads, now priced at $0.25 per million tokens. Base pricing remains unchanged at $10 per million input tokens and $50 per million output tokens.
On data retention, Anthropic is rolling out Enterprise Frontier Safeguards (EFS), a system that stores customer data entirely on infrastructure the customer controls rather than Anthropic's own servers, while still guarding against misuse. EFS begins rolling out this fall across Claude Code, Claude Enterprise, and major cloud platforms including AWS, Google Cloud, and Microsoft Azure.
The company also touted scientific applications for the models. Claude Mythos 5.1 reportedly designed high-affinity protein binders with a hit rate near 50 percent across 12 targets, well above the 10-15 percent typical in the field, with results validated by external labs. Separately, Claude Fable 5.1 was used to build a new high-resolution elevation map covering a third of Venus's surface, based on three-decade-old NASA Magellan radar data, which Anthropic is releasing publicly ahead of upcoming NASA and ESA missions.
On safety, Anthropic said Mythos 5.1 showed stronger cybersecurity capabilities than any previous model but still falls within its lowest risk tier, and that biology-related safeguards now trigger 85 percent fewer false positives on benign medical queries compared to Fable 5's original safeguards. The company noted it has not found evidence of a critical-severity jailbreak for either model to date.
Claude Fable 5.1, Mythos 5.1 availability
Claude Fable 5.1 is available immediately on all platforms, including via the Claude API under the model name claude-fable-5-1. Mythos 5.1 access remains limited to vetted organizations through Anthropic's Cyber Verification Program and Life Sciences Verification Program, the latter developed in coordination with the US government.
