simonwillison.net·Apr 30, 2026
Simon Willison's blog features an evaluation of OpenAI's GPT-5.5 cyber capabilities by the UK's AI Security Institute, which found it comparable to Claude Mythos in identifying security vulnerabilities and is currently available for use.
The most valuable insight for you is the evaluation by the UK's AI Security Institute of OpenAI's GPT-5.5, highlighting its capabilities in finding security vulnerabilities comparable to Claude Mythos. This presents a unique angle to create content around the practical applications and immediate availability of GPT-5.5 in cybersecurity, contrasting it with other AI models like Mythos.