Anthropic Releases Claude Fable 5.1 at 55.8 Percent on Terminal-Bench 4.0 and Cuts Cache Read Prices by 75 Percent

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1. On Terminal-Bench 4.0, the agentic coding benchmark, Fable 5.1 scores 55.8 percent and Mythos 5.1 reaches 60.9 percent, against 42.0 percent for Fable 5 and 37.3 percent for OpenAI's GPT-5.6 Sol. On Terminal-Bench-Science 0.1, the newer model reports 52.6 percent compared with 24.7 percent for its predecessor, a result Russian-language technical channels picked up within hours.

The pricing change is the part engineers will feel first. Base rates are unchanged at $10 per million input tokens and $50 per million output tokens, but cache reads now cost 75 percent less. Anthropic puts the effect at roughly 25 percent lower cost on typical workloads and up to 45 percent on heavily agentic ones, which favors long-running agents that re-read a large context on every step rather than one-shot chat traffic.

The two model names describe one underlying model. Anthropic says the gap between Fable 5.1 and Mythos 5.1 reflects where earlier and less precise cyber safeguards intervened, and it keeps Mythos behind its Cyber Verification and Life Sciences Verification programs while Fable is generally available to anyone with a Claude account. That is the same access architecture OpenAI applied to Astra, developed separately by each company.

Every number here is vendor-reported. Terminal-Bench is a public benchmark, so third-party runs are possible, but the doubling on the science variant in particular has not yet been reproduced outside Anthropic.