Anthropic introduces Claude Fable 5.1 and Mythos 5.1

Anthropic introduced two models, Claude Fable 5.1 and Claude Mythos 5.1. Both rest on the same base model and differ in their safety layers. Fable 5.1 is generally available, while Mythos 5.1 goes only to verified researchers and institutions working on cybersecurity and biology. On a scientific research benchmark cited by the company, the success rate rose from 24.7 percent to 52.6 percent. The cost of reusing previously read context fell 75 percent, cutting overall costs about 25 percent in ordinary use and up to 45 percent in heavy agentic use. Unnecessary refusals on harmless topics dropped 85 percent, and unnecessary cybersecurity interruptions in Claude Code 60 percent. Mythos 5.1 can design protein binders and speeds some open-source biology models by up to 2.5 times; Fable 5.1 produced a higher-resolution map of Venus from old NASA data.