I don't think most of it has anything to do with intelligence and what I need the AI to do. In our own Claude.md files, we aggressively cut down on things that are irrelevant. Claude code has rules/ feature. Most of the content of Opus 5's system prompt can probably live in a rule or a progressively disclosed file (e.g. when asked about Mythos, read [md file link].).

In my personal experience, the claude.md file heavily influences the LLM's responses. It makes judgement from any and all information you give it, and try to tailor the response to fit the stereotype it gave you. After deleting my claude.md file (ironically, on the advise of Boris), Claude Opus 5's language slop issues drastically went down.