JWT Token Decoder: How to Debug Authentication Issues

Authentication bugs are among the most frustrating to debug. A 401 Unauthorized response gives you nothing to work with. The request looks right. The token looks right. Something is wrong — but what? JWT tokens contain the answer. Every claim, expiration time, issuer, and audience value is encoded inside the token itself. Decoding the token shows exactly what the server is receiving and often makes the bug obvious immediately. This guide shows how to use a JWT decoder effectively, explains what each part of a token means, and walks through the most common authentication issues with step-by-step debugging approaches. JWT Structure: What You're Looking At A JWT looks like this: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ1c2VyXzEyMyIsImVtYWlsIjoidXNlckBleGFtcGxlLmNvbSIsInJvbGUiOiJhZG1pbiIsImlhdCI6MTcxMTA1ODQwMCwiZXhwIjoxNzExMTQ0ODAwfQ.signature_here It's three Base64url-encoded sections separated by dots: Header — algorithm and token type Payload — claims (the actual data) Signature