Claude Opus 4.6 vs GPT 5.3 Codex: Who Comes Out on Top?

On benchmarks, Opus 4.6 outperforms GPT-5.2 by about 144 Elo points on real-world work tasks (finance, legal, etc.), and scores highest on Humanity's Last Exam, a tough multidisciplinary reasoning test. GPT-5.3-Codex is OpenAI's answer, and it's leaning hard into coding and computer use.