Hacker News front pageApr 24, 2026, 02:16 UTC+8

GPT-5.5: Mythos-Like Hacking, Open to All

XBOW says GPT-5.5 cut miss rate to 10% on its real-vulnerability benchmark, versus 40% for GPT-5 and 18% for Opus 4.6. It scored 97.5% on visual acuity and used about half the login iterations of the next-best model. The key point is black-box testing: GPT-5.5 without source beat GPT-5 with source.

Why it matters: HKR-H/K/R all pass: a major OpenAI model claim, concrete security benchmark numbers, and a clear practitioner safety nerve. The source is XBOW rather than an OpenAI launch post, so it stays below 95.

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Take

GPT-5.5 does not read like a minor bump in XBOW’s numbers; it lowers the default difficulty of automated pentesting. Miss rate drops from GPT-5’s 40% to Opus 4.6’s 18%, then to GPT-5.5’s 10%. The sharper datapoint is black-box GPT-5.5 beating GPT-5 with source access, which makes many white-box evals look stale fast.

I don’t fully buy XBOW’s framing, though. XBOW sells security automation, and the benchmark runs inside its own agent workflows on frozen open-source vulnerable apps. The article gives enough shape to trust the direction, not enough to treat it as a public leaderboard. The 97.5% visual-acuity score and roughly half the login iterations versus the next-best model point to production usability, not only exploit reasoning. If GPT-5.5 is broadly available while Anthropic’s Mythos stays gated, governance becomes the bottleneck before capability demos do.

Source: Hacker News front page · xbow.com
Published: Apr 24, 2026, 02:16 UTC+8 · Apr 24, 2026