GPT-5.5 Matches Mythos in Security Vulnerability Detection, UK Institute Confirms

Breaking: GPT-5.5 Achieves Parity with Claude Mythos in Vulnerability Hunting

The UK AI Security Institute has released findings showing that OpenAI's GPT-5.5 is as effective as Anthropic's Claude Mythos at identifying security vulnerabilities. The evaluation, conducted under controlled conditions, found no statistically significant performance gap between the two models.

"GPT-5.5 performs at a level equivalent to Mythos in both breadth and accuracy of vulnerability discovery," said Dr. Helena Marsh, lead researcher at the Institute. "This is a notable milestone given the model's broader public availability."

The assessment involved a standardized set of over 1,500 known software vulnerabilities across multiple programming languages. Each model was tasked with analyzing source code and patch notes to identify potential exploits.

Background

AI-powered vulnerability identification has become a critical tool for cybersecurity teams. Earlier benchmarks, such as the Institute's November 2024 report, placed Mythos as the top performer among commercial models. GPT-5.5 was not included in that evaluation.

The detailed Mythos evaluation published alongside this report shows that the model excelled in detecting memory-safety issues and logic flaws, a strength now mirrored by GPT-5.5.

The Institute also examined a smaller, cost-efficient model that required more human prompting to achieve similar results. That analysis is available here.

What This Means

Security teams can now rely on GPT-5.5, a generally available model, as a viable alternative to specialized tools. The removal of barriers—such as licensing restrictions—could accelerate adoption in smaller organizations.

"This levels the playing field," commented Raj Patel, a cybersecurity analyst not affiliated with the Institute. "If a low-cost, widely accessible model can perform as well as a premium one, the entire threat-detection landscape will shift."

The Institute noted that GPT-5.5 required no additional scaffolding beyond standard query formatting, unlike the smaller model which needed careful prompt engineering.

Key Findings

• Detection accuracy: GPT-5.5 achieved 87% recall and 91% precision, statistically identical to Mythos (88% recall, 90% precision).
• Speed: Both models processed each vulnerability in under 10 seconds on average.
• False positives: Rates remained below 3% for both, well within acceptable operational thresholds.

The report emphasizes that while GPT-5.5 matches Mythos in vulnerability detection, other factors such as ethical constraints and response consistency require further study.