Introduction Imagine an AI that approves a second refund on the same order. Its reasoning is clear, and the tool call is correct. Yet, the refund remains wrong, and system knowledge isn’t enough to stop it, since the rule “one order gets one refund” exists only in someone’s mind. That rule belongs in an ontology. By the end of this article, you’ll understand what an ontology is, why developers have always needed one, and why agents and LLMs need a robust one more than ever.