Claude Mythos 5.1

Anthropic has released Claude Fable 5.1 and its restricted-access sibling Mythos 5.1, more than doubling Fable 5's score on Terminal-Bench-Science and cutting cache-read pricing from $1 to $0.25 per million tokens. The models are the first Claude release to ship with built-in watermarking and a private-preview detection API.