Anthropic officially announced the launch of its new generation AI models Claude Fable5.1 and the restricted variant Mythos5.1, significantly enhancing agent coding and scientific reasoning capabilities while substantially optimizing inference costs.

Both models are based on the same foundation. Fable5.1 fully opens its API to public cloud platforms, while Mythos5.1 is restricted to specific projects in cybersecurity and life sciences. At the same time, both new models are the first Claude products to have built-in AI text watermarks.

Claude

In terms of performance, Fable5.1 demonstrates strong agentic task processing capabilities, achieving a score of 55.8% in the Terminal-Bench4.0 agent coding test (Mythos5.1 scored 60.9%), and 52.6% in the Terminal-Bench-Science0.1 scientific research test. This represents a doubling compared to the previous version Fable5 and surpasses competitors like GPT-5.6Sol.

Additionally, the new models have optimized the safety filtering mechanism, reducing the false positive rate in cybersecurity by 60%, and the false positive rate for biologically harmless queries by 85%, enabling them to autonomously identify software vulnerabilities.

Economic efficiency was the core focus of this upgrade. By significantly lowering the cost of cache reads to $0.25 per million tokens, Fable5.1 can save approximately 25% in costs for typical workloads, and up to 45% for complex agent workflows running over extended periods.