The agent's instinct is to start typing. You ask for a change and it is three files deep before you have said a word about direction. Often that is fine. But on anything real, it has quietly committed to an approach — and you only find out it chose wrong after a hundred lines already lean on it. Now the fix is expensive: unwinding, not redirecting.
The wrong moment to catch a wrong approach is after the code exists. The right moment is before the first line — when changing direction costs a sentence, not a rewrite. Here is what the gate buys you:
✕ Without the rule
✓ With the rule
Drop it into your CLAUDE.md. For a one-line edit it adds a sentence and gets out of the way; for real work it turns a wrong-direction rewrite into a ten-second course-correction. The trade is deliberate: a little latency up front to kill the most expensive kind of rework — where the code was clean, but built toward the wrong goal.