Request validation error FastAPI Pydantic

Why This Happens

In FastAPI, pydantic couldn't validate the incoming request data. 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

@app.post('/users')
async def create(user: UserCreate):
    pass
# POST: {'name': 123}  # name should be str

The Fix

class UserCreate(BaseModel):
    name: str
    age: int = 0  # Default value

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: RequestValidationError. 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.