UK AI Security Institute Evaluates GPT-5.5 Security Capabilities

The UK's AI Security Institute has released its evaluation of OpenAI's GPT-5.5, focusing on the model's ability to identify security vulnerabilities. According to the evaluation, GPT-5.5 performs at a level comparable to Anthropic's Claude Mythos in finding security flaws.

The critical difference: GPT-5.5 is generally available to users now, while Claude Mythos remains in limited release.

Previous Evaluations

This marks the second major security evaluation from the UK's AI Security Institute. The organization previously assessed Claude Mythos for similar capabilities, establishing a baseline for comparing frontier models' performance in cybersecurity tasks.

The evaluations focus on models' abilities to identify and analyze security vulnerabilities, a capability that has implications for both defensive security operations and potential misuse concerns.

Model Availability

While both models demonstrate similar technical capabilities in vulnerability detection, their availability differs significantly. GPT-5.5's general availability means security researchers, developers, and organizations can access these capabilities immediately, while Mythos users must wait for broader release.

Pricing details, specific benchmark scores, and the evaluation methodology were not disclosed in the available information.

What This Means

The comparable performance between GPT-5.5 and Claude Mythos in security vulnerability detection suggests frontier models are converging in this specific capability. The UK AI Security Institute's focus on evaluating these capabilities independently provides valuable third-party assessment beyond vendor claims.

GPT-5.5's general availability creates an immediate practical advantage for security teams needing these capabilities in production environments. However, the lack of detailed benchmark scores and methodology in the public summary limits full assessment of the models' relative strengths and weaknesses in different vulnerability types or code contexts.