Value not valid integer FastAPI

Why This Happens

In FastAPI, a path or query parameter couldn't be converted to the expected type. 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.get('/items/{item_id}')
async def get(item_id: int):
    pass
# GET /items/abc

The Fix

# Ensure URL has integer: /items/123
# Or accept str: item_id: str

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: value is not a valid integer. 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.