My default AGENTS.md/CLAUDE.md/etc. is a few sentences from Strunk and White, to try to make all the models not suck at writing. It helps keep the models brief, but it doesn't actually make models with shitty prose have good prose. The relevant portion of my agents file is: "Omit needless words. Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts." Which might add up roughly the same as "be brief" in the weights, I don't know.

If you have a prompt that makes GPT a decent-to-good writer, I would like to see it.

Gemini produces decent-to-good prose without prompting, which improves if instructed to be concise. The other models, even the frontier models, do not have decent-to-good prose without prompting, and even with prompting, rarely elevate to what I would consider Good Enough. Part of this may be that GPT and Claude models get used a lot more heavily, and so I'm highly tuned into their idiosyncrasies. The heavy use of emojis, the click-bait headline style, etc. that they both use unprompted. All of that is repugnant to me, so anything that doesn't do all that by default, or at least not as aggressively, has a huge leg up.