You send a request to an API. The endpoint looks correct, the server is running, and the payload appears valid. Yet the response is 400 Bad Request , the authentication fails, or the application returns an error that tells you almost nothing useful. Sound familiar? API debugging is a regular part of backend and full-stack development. Even experienced developers can lose time investigating a problem that eventually turns out to be a missing JSON property, an incorrect content type, an expired token, or a mismatch between a request payload and a C# model. The challenge is not simply finding the error. It is identifying which layer of the request is failing and why . In this guide, we'll walk through seven common API debugging problems, explain how to investigate them systematically, and show practical techniques you can use in .NET and other web applications. We'll also look at a few lightweight developer tools that can simplify everyday tasks such as formatt...