Verdict
Built from the measurements below
- Accuracy is too close to call: Claude Fable 5.1 scored 81.2% and Claude Sonnet 5.5 78.8% on the same 80 questions, a gap within what chance alone could produce.
- Claude Sonnet 5.5 is 3.0× cheaper per question as billed ($0.0054 vs $0.016), reasoning tokens included.
- Claude Sonnet 5.5 generates text 1.8× faster: 126 tokens per second against 70.
- Claude Sonnet 5.5 starts answering sooner: 0.7 s to the first token, against 2.1 s for Claude Fable 5.1.
Which should you choose?
Choose Claude Fable 5.1 for shorter outputs (about 238 tokens per question against 456). Choose Claude Sonnet 5.5 for the lower cost per correct answer ($0.0069 against $0.02), faster streaming and a quicker first token. The widest subject gap is in law: Claude Fable 5.1 answered 5 of 6 correctly, Claude Sonnet 5.5 3 of 6. With so few questions per subject, treat this as a lead to test on your own prompts, not a result.
Accuracy gap +2.5 points (z = 0.40). We call a winner only when z is at least 1.96, the usual 95% threshold. Speed, latency and price need a 10% margin.