For decades, API design rested on a reassuring assumption: given valid input and a stable dependency, software should return a predictable result. Large language models break that assumption without breaking the API. A request can receive HTTP 200, perfectly valid JSON, and a confidently wrong answer.That distinction matters. The network contract may still be deterministic, but the semantic contract is now probabilistic. A model endpoint does not promise one correct output; it samples a likely output from a distribution shaped by the prompt, context, model version, retrieval results, and decoding process.