verification: what "done" means

why this is the number one lesson

Anthropic's own team says it plainly: of all their workflow tips, verification is the single most impactful one — if you adopt only one practice, make it this. The reason is mechanical: the agent stops when the work looks done. Without a check it can run, "looks done" is the only signal — and you become the verification loop, noticing every mistake by hand.

Give the agent something that produces a pass or fail, and the loop closes on its own: it does the work, runs the check, reads the result, and iterates until the check passes.