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

OpenAI’s GPT-5.5 matches the performance of Anthropic’s Claude Mythos in identifying security vulnerabilities, according to an evaluation by the UK’s AI Security Institute. The OpenAI model is generally available to the public, while the Institute also found that a smaller, cheaper model can achieve comparable results with additional scaffolding from the user.

OpenAI’s GPT-5.5 is as Good as Mythos at Finding Security Vulnerabilities The UK’s AI Security Institute evaluated GPT-5.5’s ability to find security vulnerabilities, and found that it is comparable to Claude Mythos. Note that the OpenAI model is generally available. Here is the Institute’s evaluation of Mythos. And here is an analysis of a smaller, cheaper model. It requires more scaffolding from the prompter, but it is also just as good.