Claude Fable 5.1 and Mythos 5.1: The System Card

At the time of its release Claude Fable 5.1 was, by a healthy margin, the most capable publicly available AI model in the world. As per usual, we have a 200+ page model card, and the assessments start there. We have now done a lot of these, including recently for Mythos 5 and Opus 5 . Also highly re