The objective of RL changed circa 2024.

The 3.5-4o era models were trained by RLHF primarily to write in a way that's pleasing to humans.

Starting with o1 the focus switched to reasoning, coding and benchmarks. If you remember when GPT-5 came out it introduced a new "colder" writing style (which was partly intentional) which made a certain subsection of Reddit Very Upset that Sam Altman had killed their AI boyfriends and they forced him to reinstate the warmer 4o.

The artificial default voice of modern models is optimized for reasoning efficiency rather than understandability. Unfortunately the artificialness has a tendency to leak out into the output tokens because reasoning tokens and output tokens are all still just tokens. You can prompt recent models to write in a more understandable style and they do a fairly good job of it, but it's not the default anymore, which means you will have to keep re-prompting it.

I don't think it's an unfixable problem, Anthropic in particular are just not very motivated to fix it because Claude is much less popular for chat than ChatGPT is.

You can certainly nudge it directionally, but it has its own style. And verbal tics etc.

So it simply cannot keep documentation or a codebase in the same style, and it cannot write <information> in <style>.

With 3.5 Turbo I used to give it bulleted information and paragraphs of what I wanted it to sound like and it'd replicate that style, then I'd edit the factual details it got wrong.

Now the situation has completely reversed! It can extract the data itself, but I have to write the stuff by hand. Humorously, it will do bulletpoints for me, and I write the thing.