Debugging 422 Unprocessable Entity Error in FastAPI

It sounds super frustrating to deal with a 422 error, especially when you’re sure your payload is correct. Here are some things you can check that might help you out:

1. Check Pydantic Model:

   Make sure your Pydantic model is defined correctly. For example, if your model looks like this:

   from pydantic import BaseModel
   
   class User(BaseModel):
       name: str
       age: int
       email: str

   Any mismatches in types can cause that error.

2. Inspect Request Payload:

   Double-check the actual data you’re sending. It should be a JSON object like this:

   {
       "name": "John Doe",
       "age": 30,
       "email": "john.doe@example.com"
   }

   Sometimes, small formatting issues (like extra spaces or wrong quotes) can sneak in and cause problems.

3. Review Content-Type Header:

   You mentioned setting it to `application/json`, which is good. Just ensure that it’s set properly for the POST request.

4. Look at FastAPI Logs:

   If you’re logging with FastAPI, make sure to log the errors. FastAPI should give you detailed validation errors in the console. You can extract those messages and check what exactly is failing.

5. Client-Side Tooling:

   If you’re using a tool like Postman or curl, check to see how you’re structuring the request. Sometimes there are subtle things that can go wrong, like incorrect encoding.

6. Match Data Types:

   If `age` is expected to be an integer, ensure you’re not sending a string (e.g., “30”). Keep an eye on these types to avoid mismatches.

Hopefully, one of these suggestions will point you in the right direction. Don’t give up! Debugging can be tough, but these little checks often help uncover the issue.