On September 1, 2026, Anthropic announces Claude Fable 5.1 and Claude Mythos 5.1, presented as the most advanced models for coding and knowledge work. Both are the same underlying model; only the levels of safeguards differ. Fable 5.1 is in general availability; Mythos 5.1 is accessible only through trusted-access programs, with safeguards designed for cybersecurity and the life sciences.
The announcement explicitly responds to three pieces of customer feedback. Pricing: cache reads drop by 75% to $0.25 per million tokens, with input and output unchanged at $10 and $50; total cost falls by around 25% on typical workloads and up to 45% on heavily agentic workloads. Data retention: the new Enterprise Frontier Safeguards store data on the customer's own infrastructure, offering the privacy of a zero-retention agreement while preserving detection of adversarial usage; phased rollout starting in the fall. Safeguards: cyber classifiers produce 60% fewer false positives, and Fable 5.1 is now permitted to identify software vulnerabilities — without developing exploits.
On performance, Fable 5.1 reaches 52.6% on Terminal-Bench-Science 0.1 (versus 24.7% for Fable 5 and 29.0% for Opus 5), 55.8% on Terminal-Bench 4.0 (60.9% for Mythos 5.1), 1853 on GDPval-AA v2, 73.4% on CursorBench 3.2.0, and 31.4% on AutomationBench. Results are presented as cost/accuracy curves across five effort levels; at low or medium effort, the model matches or exceeds Fable 5 at a much lower cost. Twenty-two partners give testimonials, including Millennium, where the model diagnosed a one-in-a-million crash that no one had explained in four to five years.
The science section documents three results. In molecular design, Mythos 5.1 achieves a success rate of nearly 50% across 12 protein targets, with affinities ten times higher than the best submissions from Adaptyv Bio. In modeling, Fable 5.1 produced a carte altimétrique de Vénus covering one third of Venus from Magellan radar data, published under a Creative Commons license. In computational biology, Mythos 5.1 accelerated seven open-source models by up to 2.5× by writing GPU kernels, cutting costs by 30 to 60%.
On safety, Mythos 5.1 stays below the next risk threshold of the Responsible Scaling Policy in biology and in the lower category of the Frontier Compliance Framework in cyber. The alignment audit finds it better aligned than Mythos 5, while acknowledging limited coverage of long-context, multi-agent, and impossible tasks.