x.com/zephyr_z9/status/2080179599095603511

Anthropic’s own launch materials directly contradict this. In the June 9, 2026 announcement for Claude Fable 5 and Claude Mythos 5, Anthropic says: “It’s the same underlying model as Fable 5, but with the safeguards lifted in some areas.” Anthropic also says that “Given that they are the same underlying model, Fable 5’s level of alignment will be similar.”

Anthropic’s Claude Mythos product page says the reverse the same way: “Claude Fable 5 is the same underlying model as Mythos 5 with robust safeguards for cybersecurity and biology.”

So the company’s official documentation treats Mythos 5 and Fable 5 as two configurations of the same model, not as separate models where Mythos is “not Fable 5/Mythos 5.”

This correction is limited to that separateness claim. Anthropic has not, in the cited materials here, publicly confirmed the specific “800B active params” figure in this post.