GPT-5.5: Mythos-Like Hacking, Open to All
xbow.com
April 23, 2026
6 min read
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47/100
Summary
OpenAI's GPT-5.5 is being released freely, offering offensive security capabilities comparable to Anthropic's Mythos, which has limited access. Testing has been conducted across various benchmarks and workflows to evaluate its performance.
Key Takeaways
- GPT-5.5 reduces the miss rate for known vulnerabilities to 10%, significantly improving upon GPT-5's 40% miss rate and Opus 4.6's 18% miss rate.
- In black box testing, GPT-5.5 outperforms GPT-5 even when the latter has access to source code, reversing the expected performance hierarchy.
- In white box testing, GPT-5.5 demonstrates such a substantial performance increase that it effectively nullifies the existing benchmark.
- The model's progression shows that GPT-5.4 focused on speed, while GPT-5.5 emphasizes depth in vulnerability detection.
Coverage & discussions
Hacker News · xbow.com
Community discussions
Community Sentiment
Positives
- Mythos claims to validate vulnerabilities by building and running exploits, making it a functional tool for hacking, unlike smaller models that struggle with false positives.
Concerns
- The presentation of data in the article is misleading, using inappropriate visualizations that confuse rather than clarify important information.
- There are concerns that GPT-5.5 is being marketed as revolutionary without substantial evidence to back its claims compared to existing models.
