Field required Pydantic FastAPI

Why This Happens

In FastAPI, a required field is missing from the request body. 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

class Item(BaseModel):
    name: str
    price: float
# POST: {'name': 'Widget'}  # missing price

The Fix

class Item(BaseModel):
    name: str
    price: float = 0.0  # Add default

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: field required. 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.