Develop and Document REST API Endpoints

By the end of this lesson, you'll be able to

  • Explain REST's core building blocks (resources, URIs, the five HTTP methods, idempotency) and its guiding principles, and articulate why FastAPI's ASGI/Pydantic foundation fits an agent backend specifically
  • Define routes with path/query/header/cookie parameters and real validation constraints, and avoid the route-ordering gotcha
  • Validate request bodies (including file uploads) with Pydantic, and shape, status-code, and error-handle responses — per-route with HTTPException, or globally with @app.exception_handler
  • Read the automatically generated /docs and /redoc pages, and explain what they're actually built from
  • Write both async def and plain def routes correctly, know which fits which situation, and avoid blocking the entire server
  • Use Depends() for shared logic, implement API key auth, and describe how JWT-based auth fits REST's statelessness
  • Organize a multi-file app with APIRouter, and add CORS, rate limiting, startup/shutdown logic, and background tasks

Why it matters

This lesson is where nearly everything else in Module 0 converges into one deployable thing: Pydantic validates what a client sends, decorators wire up what happens when a request arrives, async/await lets one route wait on an LLM call without blocking every other request, and custom exceptions from the I/O lesson become real HTTP error responses. An agent that serves tool endpoints, receives webhooks, or exposes results to a frontend needs exactly this — and needs it done with real attention to who’s allowed to call it and how often, not just that it technically works.