Programming Tutorials

  Few things break a backend developer’s flow like a persistent   422 Unprocessable Entity   error. You define your Pydantic models, set up your FastAPI routes, and send a request via Postman or   curl . Instead of a successful   200 OK , you get a   422 . The error implies the server understands the content type (usually  application/json ), but the syntax of the data is incorrect. In the context of FastAPI, this almost exclusively means  Pydantic validation failed . This guide moves beyond generic advice. We will dissect the root causes of validation errors, implement global debugging handlers to expose hidden schema mismatches, and solve the common "Body vs. Query" parameter confusion. The Root Cause: How FastAPI validates Data FastAPI relies heavily on Pydantic (v2 in modern implementations) to parse and validate incoming data. When you define a route, FastAPI reads the type hints and creates a validation schema. If the incoming payload does n...