Anthropic wrote a style guide against its own model's prose, and the examples are Opus 5's tics.

The Fable 5.1 guide's writing-density section defines mannered prose as an anti-pattern with examples reading as a direct answer to months of complaints: "a dial worth turning" instead of "a parameter worth varying," "this point earns its keep" instead of "this point still matters." Arena measured the underlying drift from tens of thousands of real responses on August 12: 510 words per Opus 5 response against 158 for Opus 4.5, sentences 58% longer with 46% more clauses, em dashes 2.3 times as frequent. The vendor's fix is a paragraph you paste into your own prompt.