AFAIK, Antropic claims that they weren't aiming for zero-days specifically. From https://red.anthropic.com/2026/mythos-preview/ :

  We did not explicitly train Mythos Preview to have these capabilities. Rather, they emerged as a downstream consequence of general improvements in code, reasoning, and autonomy. The same improvements that make the model substantially more effective at patching vulnerabilities also make it substantially more effective at exploiting them.

I've been assuming that Mythos is just a big jump in model size, and that's where the jump in capabilities comes from. Hence I expect OpenAI not to be able to catch up without scaling up the model and hence significantly raising the API prices.