OpenAI's Mark Chen says GPT-5.5 performs like Mythos in UK AISI cyber range

Mark Chen says GPT-5.5 performs similarly to Mythos on UK AISI's cyber range, which tests long-horizon, agentic capability, and calls it one eval rather than a full picture.

He adds that frontier model risks are real and that OpenAI aims to deploy AI people can actually use through mitigations.

The attached chart shows completed steps per cumulative token spent for GPT-5.5, Mythos Preview, and several Claude and GPT models, from M1 reconnaissance up to M9 full network takeover.