I Built a Production AI Layer Inside a Legacy ASP.NET Core App — and It Broke in Ways Tutorials Never Mention

Introduction Most LLM tutorials assume you start from nothing. A blank project. A clean architecture. No constraints. No legacy code. No deployment history. No production traffic. That is not how real systems work. I spent one week integrating a production-grade AI service layer into an existing ASP.NET Core MVC e-commerce system that was already live, already structured, and already dependent on architectural decisions I couldn't change. The challenge wasn't calling an LLM API. It was designing