principle-mechanical-gateslisted

# Mechanical Gates Where a rule must hold, do not ask the model to remember it. Enforce it with a deterministic check that runs whether or not the model cooperates. A rule enforced only by good behavior is not enforced at all. **Why:** Models forget instructions roughly one time in five. A prompt line is a request; a gate is a guarantee. The deterministic layer outranks the model: a mechanical check can fail a step that every agent in the run believes is fine. **Pattern:** - Push every check to the cheapest, most deterministic layer that can catch it. Never an expensive judge for something a regex could decide. - A check at the wrong layer is worse than no check: slow, flaky, or costing money to learn nothing. - Detect errors early, surface them loudly, never mask them silently. - Prefer a check that makes the violation impossible over one that observes it, and a check on the artifact over a check on the intent. - A check that only passes when the model happens to be well-behaved is not a check.