Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Opus 5.5 scored 85.0% and GPT-5.3-Codex 82.5% on the same 80 questions, a gap within what chance alone could produce.
- GPT-5.3-Codex is 1.4× cheaper per question as billed ($0.01 vs $0.015), reasoning tokens included.
- Claude Opus 5.5 generates text 1.4× faster: 129 tokens per second against 95.
- GPT-5.3-Codex starts answering sooner: 1.4 s to the first token, against 2.3 s for Claude Opus 5.5.
- Claude Opus 5.5 accepts more context: 1M tokens against 400k.
Which should you choose?
Choose Claude Opus 5.5 for faster streaming, a 1M-token context window and more hosts to choose from on OpenRouter (5 against 2). Choose GPT-5.3-Codex for the lower cost per correct answer ($0.013 against $0.017) and a quicker first token.
Accuracy gap +2.5 points (z = 0.43). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.