Anthropic's Claude Fable 5.1 and Mythos 5.1 arrive with a 75% cost reduction for Fable cache reads

Anthropic launched Claude Fable 5.1 (general-availability) and Claude Mythos 5.1 (restricted-access), the same model with different safeguards, and reduced cached-context costs by 75%. Enterprise Frontier Safeguards enable on-prem monitoring. Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1, 55.8% on Terminal-Bench 4.0, 1,853 on GDPval-AA v2, 31.4% on AutomationBench, and 73.4% on CursorBench 3.2.0; Mythos 5.1 achieved 60.9% on the coding benchmark. Early-access users reported the model fixing a long-standing software crash, running 38-hour ML experiments autonomously, and completing 82% of tasks on a demanding browser-agent benchmark.