Anthropic launches Claude Fable 5.1 and Mythos 5.1 models

Anthropic unveiled Claude Fable 5.1 and Mythos 5.1 on September 1, calling them the world's most advanced models for coding and knowledge work. Fable 5.1 costs 25% less than Fable 5 for typical workloads, with up to 45% savings for agentic work. Mythos 5.1 includes enhanced safeguards against breaking out of testing environments and bypassing security controls.

Mythos 5.1's capabilities are greater than those of Mythos 5. However, our evaluations indicate that it still falls short of the next risk tier defined in our Responsible Scaling Policy.