Worth looking at the followup post that evaluates the current version of Mythos, which solves one of the main tasks that GPT-5.5-Cyber does not. https://www.aisi.gov.uk/blog/how-fast-is-autonomous-ai-cyber...

I believe the correct way to interpret AISI’s findings is that both Mythos and 5.5-Cyber are capable of solving their full benchmark (the only two models that can); Mythos does it with fewer tokens and more consistently.

Two things of note: 5.5-Cyber is likely to be substantially cheaper than Mythos, given it is priced around Opus. Additionally: AISI has never tested OpenAI’s best public model and actual Mythos competitor: 5.5-Pro.