If you have been following this FastAPI series, we have built CRUD operations, connected to a database, and secured our endpoints with JWTs. But what happens when things go wrong?

When a user searches for an item that doesn't exist, or tries to access a route without a token, your API needs to communicate the exact problem. Returning a generic 500 Internal Server Error or a standard 200 OK with an error message in the body is a bad practice.

Today, we are going to look at the most common HTTP status codes and how to raise them cleanly using FastAPI's HTTPException .

The Core Status Codes You Need to Know

HTTP status codes are grouped by their starting number.

2xx (Success): Everything worked perfectly.

4xx (Client Error): The user did something wrong (bad data, missing token, wrong URL).

5xx (Server Error): Your code broke.

Here are the four most critical 4xx errors you will use in FastAPI:

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