Anthropic launches Claude Fable 5.1 and Mythos 5.1 with cache price cut, removed data retention limits
On September 1, 2026, Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, positioned for long-running agentic coding, knowledge work, and research, both with a 1M-token context window, 128k max output tokens, and always-on adaptive thinking. List pricing stays at $10/$50 per MTok input/output (same as Fable 5), but prompt cache read price drops 75% to $0.25/MTok. Anthropic also introduced Enterprise Frontier Safeguards (EFS) for agent observability. Per Stratechery, Fable's prior data retention policy was removed rather than merely altered. Anthropic's own benchmark table reports Fable 5.1 scoring 52.6% on the new Terminal-Bench-Science 0.1 benchmark versus 24.7% for Fable 5, 29.0% for Opus 5, and 22.4% for GPT-5.6 Sol. TechCrunch reported the release also reduces false-positive safeguard restrictions. Per Latent Space's AINews recap, Artificial Analysis measured roughly 1.7x more output tokens per task, offsetting the cache savings for a reported ~20% net per-task cost increase.