JWT Tokens Explained: How to Decode and Debug Them

JWTs (JSON Web Tokens) are everywhere in modern web development. They're the standard format for authentication tokens in REST APIs, OAuth flows, session management, and microservice communication. But they look like impenetrable strings of random characters — until you know how to read them. This guide explains the JWT format from scratch: what each part means, how to decode one, how to verify it, how to debug common authentication errors, and what security mistakes to avoid. What Is a JWT? A JWT is a compact, self-contained token that encodes a set of claims as a JSON object. It's digitally signed, so the recipient can verify that the token came from a trusted source and hasn't been tampered with. A JWT looks like this: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkFsaWNlIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c It's three Base64URL-encoded sections separated by dots: [header].[payload].[signature] JWT Structure: The Three Par