Anthropic's Claude Fable 5.1 More Than Doubles Its Score on an Agentic Science Benchmark and Cuts Cache Reads 75%

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, three months after Fable 5. The figure Anthropic emphasizes most is Terminal-Bench-Science 0.1, an agentic benchmark for scientific research tasks run in a terminal, where Fable 5.1 reaches 52.6% against 24.7% for its predecessor. On Terminal-Bench 4.0, Fable 5.1 scores 55.8% and Mythos 5.1 scores 60.9%, according to benchmark summaries of Anthropic's published figures.

The pricing change is what affects deployment costs immediately. List rates hold at $10 per million input tokens and $50 per million output tokens, but cache-read pricing falls 75%, from $1.00 to $0.25 per million tokens. VentureBeat reports that this translates to roughly 25% savings on typical workloads and up to 45% on agentic ones, the workload type where a long system prompt and accumulated tool output are re-read on every turn.

Mythos 5.1 is not a larger model or a different architecture. Anthropic describes it as the same model as Fable 5.1 with more permissive safeguards, available only through its trusted-access programs, and has not published separate pricing for it. The higher Terminal-Bench 4.0 score for Mythos is therefore a measure of what refusal behavior costs on agentic evaluations, not evidence of extra capability. Compared with OpenAI's GPT-6 Astra, which reports 57.9% on the same Terminal-Bench 4.0, the standard-safeguard Claude scores lower and the trusted-access variant scores higher, all on vendor-published numbers with no independent rerun yet.