We probably need something like the WET principle for skills. If you need to explain the same thing to an agent more than twice, turn it into a skill (or add it to AGENTS.md, or CLAUDE.md, or to you docs folder, or your guides folder, or whatever method you use). If you haven't needed to explain it more than twice, it's probably fine. The context pollution from the skill would likely be worse than not having the skill

Of course exceptions apply. Some basic information that will reliably be discovered is still worth adding to your AGENTS.md to cut down on token use. But after a couple obvious things you quickly get into the realm of premature optimization (unless you actually measure the effects)