Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Fable 5.1 scored 81.2% and Claude Opus 5.5 85.0% on the same 80 questions, a gap within what chance alone could produce.
- They cost about the same per question ($0.016 vs $0.015).
- Claude Opus 5.5 generates text 1.8× faster: 129 tokens per second against 70.
- Claude Fable 5.1 starts answering sooner: 2.1 s to the first token, against 2.3 s for Claude Opus 5.5.
Which should you choose?
Choose Claude Fable 5.1 for a quicker first token and shorter outputs (about 238 tokens per question against 645). Choose Claude Opus 5.5 for the lower cost per correct answer ($0.017 against $0.02) and faster streaming.
Accuracy gap -3.7 points (z = -0.63). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.