Claude Fable 5.1

Claude Fable 5.1 is the Anthropic flagship, released September 1, 2026 alongside its restricted-access sibling Claude Mythos 5.1. The headline rate is unchanged from Fable 5 at $10 per million input tokens and $50 per million output, but the cache economics moved: cache reads on 5.1 are priced at 0.025x base input rather than the usual 0.1x, which is $0.25 per million instead of $1. For agent workloads that replay a large stable prefix on every turn, that is the line that actually moves the bill, and it is a 75 percent cut on the dominant cost of a long-running session. On capability, 5.1 more than doubles Fable 5 on Terminal-Bench-Science (24.7 to 52.6 percent), lifts Terminal-Bench 4.0 coding from 42.0 to 55.8, and reaches 65.0 percent on Humanity’s Last Exam with tools. Mythos 5.1 is described by Anthropic as the same model with more permissive safeguards, and it carries the same $10 and $50 rate, which is worth stating plainly because a gated tier is often assumed to cost more.