The same UK security research body ran the same CTF against GPT5.5. GPT5.5 got the same result as Mythos.

Anthropic promised us that Mythos was such an existential threat that it would compromise "every OS and browser on devices across the planet". They've held conferences and meetings with banks and govts across the world, shouting how critical this issue is.

GPT5.5 has been out for a month. Every device on earth has not been breached yet. It's very fair to criticize Anthropic's maximalist posturing when it's becoming exceedingly clear their models are fairly behind OpenAI's in capability.

In my opinion, the original commenter's statement stands, and the UK govt data point only helps support that due to the equal result between Mythos and GPT.

I'd advise reading into the specifics of what happened with Firefox; the TL;DR is a reduced safety version of its code was scanned by Opus 4.6 (yes Opus) and found a multitude of bugs and 4 high severity vulns that did not escape sandbox. The Mythos system card test describes running Mythos against the same issues Opus found to see if it could reliably replicate and chain together an attack.