You open ChatGPT. You type a prompt. Within seconds: your request reaches a backend server, the backend communicates with an LLM, retrieves memory, queries vector databases, processes context, and streams responses back to you in real time. Modern AI applications are no longer just “apps.” They are systems made up of multiple services constantly communicating with each other through APIs. And one framework has quietly become the default choice for building these modern AI backends: FastAPI. In this article, we’ll understand: why APIs are essential, why modern AI systems depend heavily on them, what FastAPI actually is, and why it became the preferred backend framework for AI engineers. Modern Applications Are API Systems Most applications today are distributed systems. Your frontend, backend, database, authentication service, payment gateway, and AI models continuously exchange data with one another.…