Author here. Five 2026 studies, read side by side. The short version: a hand-written file helps a little (ETH v2: +2.4 points, not significant), a generated one comes out lower, both cost about 20% more. McMillan's 1,650 Claude Code sessions found file size, rule position and layout made no detectable difference; compliance tended to fall as the session went on. Vercel's docs index in AGENTS.md hit 100% where a skill left to the agent hit 53%. Every source is linked. I work on Harbor, which published it.