Anthropic Releases Claude Fable 5.1 and Claude Mythos 5.1, Achieving Cost Reductions and Lower False Positives

While these two models possess equivalent performance, they differ in nature. Claude Fable 5.1 is the general release version with security restrictions, whereas Claude Mythos 5.1 is a limited release version with security restrictions removed.

Claude Fable 5.1 features higher performance compared to the previous generation, Claude Fable 5. Regarding costs, Anthropic stated that reductions of 25% for general workloads and up to 45% for complex agent tasks are possible. Lower pricing for cached inputs also contributes to cost suppression.

In terms of performance metrics, the model recorded the highest scores in tests by the third-party organization Artificial Analysis. It scored "66" on the Artificial Analysis Intelligence Index. On the Artificial Analysis Agentic Index, which indicates agent performance, it earned a "61," surpassing Claude Opus 5.

The safety systems have also been updated. According to the company, this has resulted in a 60% decrease in false positives regarding cybersecurity and an 85% decrease in false positives regarding biology and medicine.

Claude Fable 5.1 is available to subscribers of the Pro, Max, Team, and Enterprise plans. It is also available via API.