Claude Mythos was one of the most hyped models in recent memory. And it is out now. Sort of. Yesterday Anthropic launched Claude Fable 5, a generally available model built on the same weights as Mythos 5 that switches over to the older Opus 4.8 for most tasks that even mention cybersecurity or biology. Early… The post How Claude Fable 5 stacks up against Opus 4.8 and GPT 5.5 appeared first on Research & Development World .