Originally published on NextFuture Anthropic's Mythos and OpenAI's GPT-5.5-Cyber both shipped in April–May 2026 as purpose-built cybersecurity models, and both landed behind strict allowlists within days of each other. For AI engineers evaluating them honestly, the core problem is the same: most practitioners can't get direct API access, so any comparison relies on third-party evals, CTF leaderboard data, and structured capability disclosures from partner briefings. This piece pulls those threads together and gives you the clearest signal available as of May 4, 2026. For the full geopolitical backdrop, see our cluster anchor Inside the AI Cyber Arms Race (May 2026) .…