GPT-5.5 Matches Top-Tier Model in Cybersecurity Benchmarks, UK Agency Reveals

GPT-5.5 Matches Top AI in Finding Flaws

OpenAI's latest model, GPT-5.5, has proven as effective as Anthropic's Claude Mythos at identifying security vulnerabilities, according to a new evaluation by the UK's AI Security Institute. The widely available model now matches a previously unmatched specialist tool in this critical domain.

“These results are a significant milestone,” said Dr. Elena Marchetti, lead researcher at the Institute. “A general-purpose model now rivals a dedicated security AI, which could democratize vulnerability discovery.”

Evaluation Details

The Institute tested GPT-5.5 on a range of common and emerging security flaws. The model scored equivalently to Mythos on accuracy and recall, with no major gaps in detection. The same test had previously shown smaller, cheaper models requiring extensive human scaffolding to reach similar performance.

“The fact that GPT-5.5 is generally available means any organization can now leverage top-tier vulnerability scanning,” Marchetti added. “This lowers the barrier for proactive security.”

Background

Anthropic's Claude Mythos has long been the gold standard for automated vulnerability discovery, trained specifically on security datasets. OpenAI's GPT-5.5, by contrast, is a general-purpose large language model used for everything from coding to customer support.

Earlier evaluations by the Institute compared Mythos with smaller models, finding that they required detailed prompts and multiple iterations. GPT-5.5 achieves comparable results with far less guidance.

What This Means for Security

The convergence of general-purpose and specialized AI performance could reshape cybersecurity workflows. Teams no longer need exclusive access to niche models to conduct deep vulnerability assessments.

“We are entering an era where the most advanced security tools are available to all,” said Marchetti. “But this also means attackers will have the same access, so defensive measures must evolve.”

Next Steps

The UK AI Security Institute plans to extend its evaluation to other general-purpose models, including Google's Gemini and Meta's Llama. A public dataset of benchmark results will be released later this month.

Organizations are advised to integrate GPT-5.5 into their security pipelines and to monitor the Institute's background reports for updated comparisons.