> It might be my AGENTS.md requiring clearer, simpler language

If you gave the exact same markdown file to me and I posted ed the exact same prompts as you, would I get the same results?

I'm not sure if the model (under its temperature/other settings) produces deterministic responses. But I do think models' style and phrasing are fairly changeable via AGENTS.md-style guidelines.

5.4's choice of terms and phrasing is very precise and unambiguous to me, whereas 5.3-Codex often uses jargon and less precise phrases that I have to ask further about or demand fuller explanations for via AGENTS.md.

So sharing markdown files is functionally useless, or no?

you probably can't and asking agents.md to "make it clearer" will likely give you the illusion of clearer language without actual well structured tests. agents.md is to usually change what the llm should focus on doing more that suits you. Not to say stuff like "be better", "make no mistakes"