Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on 2026-09-01, positioning them as its most advanced models for coding and knowledge work. The two are the same underlying model with different levels of safeguards: Fable 5.1 is generally available, while Mythos 5.1 is distributed only through trusted access programs, with safeguards specifically designed to support work in cybersecurity and the life sciences. That split repeats the structure Anthropic used for the Fable 5 and Mythos 5 generation.
The reported benchmark movement is largest in terminal-based scientific and engineering work. Fable 5.1 scores 52.6 percent on Terminal-Bench-Science 0.1 against 24.7 percent for Fable 5 - roughly a doubling. Terminal-Bench 4.0 rises from 42.0 to 55.8 percent, Humanity’s Last Exam without tools from 57.8 to 60.9 percent, CursorBench 3.2.0 from 70.5 to 73.4 percent, and OSWorld 2.0 strict from 36.1 to 41.7 percent. Anthropic frames the evaluation suite as covering software engineering, mathematical and scientific reasoning, long context, agentic search and multi-agent orchestration, multimodal and computer-use tasks, and professional domains.
The commercially significant change is pricing, and it is not on the headline rate. Fable 5.1 remains 10 dollars per million input tokens and 50 dollars per million output tokens, unchanged from Fable 5. What moved is cache reads, cut 75 percent to 0.25 dollars per million tokens. Anthropic measures the effect as roughly 25 percent lower cost on typical workloads and up to about 45 percent on highly agentic work. That asymmetry matters for buyers: the saving accrues to workloads that repeatedly re-read a large stable context, which is precisely the shape of agentic coding and long-running research tasks.
Anthropic describes the release as a direct response to customer feedback on pricing, data retention and safeguards - an unusual framing for a frontier model launch, and a signal that the Fable 5 generation drew commercial friction as well as capability praise. Fable 5.1 is available on all platforms at launch, including Amazon Web Services, Google Cloud and Microsoft Azure.