Anthropic releases Claude Fable 5.1 and Mythos 5.1

Background

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on 1 September 2026. ^[2][3]

What happened

Anthropic called Fable 5.1 the world's most advanced model for coding and knowledge work. ^[1]

Prices stayed at $10 per million input tokens and $50 per million output tokens. Cache reads were cut by 75%, and Anthropic estimated savings of up to about 45% on agentic work. ^[1]

Key details

Fable 5.1 scored 55.8% on Terminal-Bench 4.0, against 42.0% for Fable 5. ^[1]

Mythos 5.1 became available through the Cyber Verification Program and a new Life Sciences Verification Program. ^[1]

Anthropic said models released after 2 August 2026 include an invisible text watermark. ^[1]

A cache read is a lower-cost charge for text the model has already processed, which is why a cut to that price matters most for long, repeated inputs. Terminal-Bench is a benchmark that tests whether a model can complete tasks in a command-line terminal. A verification programme gives access to a sensitive capability only after the organisation has been checked.

Impact

Fable 5.1 is generally available, while Mythos 5.1 is offered through verification programmes. ^[1][2][3]