Anthropic released new AI models with strong coding and science gains.
Fable 5.1 is broadly available; Mythos 5.1 is limited.
The company also disclosed safety concerns about monitoring difficulty.
What happened
Anthropic announced Claude Fable 5.1 and Claude Mythos 5.1 on September 1. Fable 5.1 is generally available, while Mythos 5.1 is limited to organizations that pass cybersecurity and life sciences reviews.
Why it matters
Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1, far above Fable 5's 24.7%, Claude Opus 5's 29.0%, and GPT-5.6 Sol's 22.4%. It also created a high-resolution Venus elevation map from NASA's Magellan radar images, improving resolution from 10–20 km to 2–3 km.
What to watch
API pricing stays at $10 per million input tokens and $50 per million output tokens, but cache reads dropped 75% to $0.25 per million tokens. Mythos 5.1 can exploit Firefox 147 vulnerabilities in 98.0% of trials, and Anthropic warns it may be harder to monitor.
Ask the AI about this article →
Anthropic's latest models show significant progress in coding and scientific research, as evidenced by scores on Terminal-Bench-Science 0.1 and the creation of a high-resolution Venus map. However, the company also released a System Card detailing risks, including high success in exploiting known vulnerabilities and occasional bypasses of safety checks. The dual release—one for general use and one for vetted organizations—reflects an effort to balance capability with safety. The lower cache pricing and unchanged API rates may make the models more cost-effective for developers. The company also plans to offer Enterprise Frontier Safeguards for customers who want their data kept on their own cloud, with a watermarking requirement for models released after August 2, 2026, per EU AI Act.
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