422 validation error FastAPI

Why This Happens

In FastAPI, the request body or parameters failed pydantic validation. This is one of the most common FastAPI errors developers encounter. Understanding the root cause helps you fix it quickly and prevent it from recurring.

The Problem

# POST with wrong types

The Fix

# Check request body matches the Pydantic model
# Use Optional[] for nullable fields

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: 422 Unprocessable Entity. This tells you exactly what went wrong.

2. 2

   Find the cause

   Check the stack trace to find which line of your FastAPI code triggered this error.

3. 3

   Apply the fix

   Use the corrected code pattern shown above. Test to confirm the error is resolved.