
GPT-5.5 Instant vs Claude Mythos Preview: Frontier Models Just Crossed a Dangerous Cyber Threshold in 2026
OpenAI released GPT-5.5 Instant in May 2026 as the new default ChatGPT model with significantly better accuracy and ~52.5% fewer hallucinations. At nearly the same time, Anthropicās Claude Mythos Preview demonstrated strong autonomous offensive cybersecurity capabilities, including zero-day discovery and full end-to-end attack chains. Both models have now matched each other on UK AISIās toughest benchmarks by successfully completing complex 32-step enterprise attack simulations ā something no AI could do before 2026. OpenAI and Anthropic are carefully restricting full capabilities to vetted defenders only. This marks the true start of the AI-powered cyber defense race.





