FastAPI

FastAPI is a modern, high-performance Python web framework for building APIs. Created by Sebastián Ramírez, FastAPI has become the de facto standard for serving AI models and building agent backend services due to its speed, automatic documentation, and native support for async operations and type validation.

FastAPI's popularity in the AI ecosystem stems from its Python-native design (matching the language most AI/ML code is written in), automatic OpenAPI schema generation (useful for agent tool descriptions), and high performance through async support and Starlette/Uvicorn. Most AI startups use FastAPI as their primary API framework.

In the agent stack, FastAPI serves as the connective tissue between AI models and the applications that consume them — the framework that turns model inference into callable APIs and agent tools.