Our evaluation of OpenAI's GPT-5.5 cyber capabilities

2026-04-30 · Source: Simon Willison's Weblog · Field: Technology & Digital — Artificial Intelligence & Machine Learning, Cybersecurity & Data Privacy, Emerging Technologies & Innovation · Depth: Advanced, quick

Summary

The UK's AI Security Institute (AISI) evaluated OpenAI's GPT-5.5 for its cyber capabilities, specifically its ability to identify security vulnerabilities. This assessment follows a previous evaluation of Anthropic's Claude Mythos. The AISI found GPT-5.5's performance in vulnerability detection to be comparable to that of Claude Mythos. A key distinction noted is that GPT-5.5 is currently generally available, whereas Claude Mythos was a preview model at the time of its evaluation. The findings were posted on April 30, 2026, at 11:03 pm, indicating an ongoing effort by the AISI to assess the security implications of advanced AI models.

Key takeaway

For cybersecurity teams evaluating AI tools for vulnerability assessment, your current options include GPT-5.5, which the UK's AISI found comparable to Claude Mythos. Given GPT-5.5's general availability, you should consider integrating it into your security workflows to augment human analysts in identifying potential system weaknesses.

Key insights

GPT-5.5 demonstrates cyber vulnerability detection capabilities comparable to Claude Mythos.

Principles

• AI models can assist in vulnerability identification.
• Availability impacts practical AI security assessments.

Method

The UK's AISI conducts evaluations of large language models to assess their cyber security capabilities, focusing on vulnerability detection.

In practice

• Use GPT-5.5 for security vulnerability scanning.
• Compare AI model performance for cyber tasks.

Topics

• OpenAI GPT-5.5
• Cyber Capabilities
• Security Vulnerability
• AI Security Institute
• Claude Mythos

Best for: CTO, VP of Engineering/Data, Director of AI/ML, AI Security Engineer, AI Scientist, Policy Maker

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• Scaling Trusted Access for Cyber with GPT-5.5 and GPT-5.5-Cyber — OpenAI's new GPT-5.5 and GPT-5.5-Cyber models, with Trusted Access for Cyber, empower verified defenders.
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Editorial summary, takeaway, and curation by AIssential. Original article published by Simon Willison's Weblog.