Claude Opus 5.5 Writes With 17% Shorter Sentences: How It Reads More Human
Arena.ai measured how Claude Opus 5.5 writes against Opus 5, the version before it, and a large part of the change sits in two punctuation marks it calls familiar tells of AI writing. Opus 5.5 has nearly stopped using one of them. 📉
How Arena measured it.
The comparison uses high-reasoning answers from Text Arena. BleepingComputer, which reported on the analysis, dates those answers to August and September 2026. Arena tracked 12 writing measures. Ten moved in what it calls a better direction.
Neither the sample size nor the full method has been published, so the results describe Arena's set of answers and make no claim about all Opus 5.5 output.
Em dashes and semicolons.
In the Opus 5 answers, the em dash appeared 15.2 times per 1,000 words. Opus 5.5 brought that down to 0.8, about 95% fewer. Semicolons followed a smaller slide, from 6.10 to 1.64 per 1,000...
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