OpenAI’s GPT-5.5 is as Good as Mythos at Finding Security Vulnerabilities

The UK's AI Security Institute tested whether GPT-5.5 (OpenAI's model) could find security vulnerabilities as well as Claude Mythos, and found they perform comparably. A smaller, cheaper model also performed equally well, though it needs more scaffolding (additional structure and guidance provided by the person writing prompts to the AI).