Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Fable 5.1 scored 81.2% 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 1.6× cheaper per question as billed ($0.01 vs $0.016), reasoning tokens included.
- GPT-5.3-Codex generates text 1.4× faster: 95 tokens per second against 70.
- GPT-5.3-Codex starts answering sooner: 1.4 s to the first token, against 2.1 s for Claude Fable 5.1.
- Claude Fable 5.1 accepts more context: 1M tokens against 400k.
Which should you choose?
Choose Claude Fable 5.1 for shorter outputs (about 238 tokens per question against 699), a 1M-token context window and more hosts to choose from on OpenRouter (4 against 2). Choose GPT-5.3-Codex for the lower cost per correct answer ($0.013 against $0.02), faster streaming and a quicker first token.
Accuracy gap -1.2 points (z = -0.21). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.