What Arena measured
Arena compared answers from Claude Opus 5.5 and Claude Opus 5 on Text Arena, at high reasoning level, from August to September 2026. It tracked 12 writing measures. 10 of them moved in a better direction: the new model uses far fewer of the tells that make text read as AI.
| Measure | Opus 5 | Opus 5.5 | Change |
|---|---|---|---|
| Em dashes (per 1,000 words) | 15.2 | 0.8 | 95% fewer |
| Semicolons (per 1,000 words) | 6.10 | 1.64 | 73% fewer |
| Words per sentence | 12.14 | 10.03 | 17% shorter |
| Long content words (share) | 41.7% | 38.6% | lowest of any Claude |
| Praise openers like "great question" (answers) | 3.76% | 2.60% | fewer |
| Agreement openers like "yes, exactly" (answers) | 3.76% | 2.16% | fewer |
| Honesty wording like "frankly" (per 1,000 words) | 0.48 | 0.31 | fewer |
| Stock phrases like "load-bearing" (per 1,000 words) | 0.19 | 0.11 | fewer |
| Abstract nouns (per 100 words) | 4.39 | 3.68 | fewer |
| Hedges like "perhaps", "arguably" (per 1,000 words) | 0.39 | 0.77 | 97% more |
| Answer length (words) | 453 | 481 | 6% longer |
Source:Arena, "How Claude Models Write", Text Arena outputs at high reasoning level, August to September 2026.
The habit that got worse
Hedges and caveats nearly doubled. Words like "perhaps" and "arguably" went from 0.39 to 0.77 per 1,000 words, the highest rate of the seven Claude models on Arena's chart. Opus 5.5 also writes the longest answers in the Opus family: 481 words on average.
So the new model reads cleaner, but it hedges more and says more. A skill fixes both, because it gives Claude the same rules every time you write.
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