What is JWT and Why Debug It?

JWT (JSON Web Token) is an open standard (RFC 7519) for securely transmitting information between parties as a JSON object. JWTs are commonly used for authentication and authorization in modern web applications and APIs. When you log into a website, the server often gives you a JWT. Your browser stores it and sends it with every API request to prove you're authenticated.

JWT Structure — Three Parts

A JWT looks like: xxxxx.yyyyy.zzzzz — three Base64Url-encoded strings separated by dots. Header contains metadata like the signing algorithm (HS256, RS256) and token type. Payload contains claims — statements about the user (user ID, name, email) and metadata (issued at, expiration time). Signature verifies the token hasn't been tampered with. Our decoder separates all three parts and formats them as readable JSON.

When Do You Need a JWT Debugger?

API Development: When building REST APIs with JWT authentication, you need to inspect tokens to verify user data. Debugging Auth Issues: "Why is my token rejected?" Check if it's expired, has wrong claims, or corrupt format. Learning JWT: Understanding the structure helps you implement authentication correctly. Security Testing: Verify your tokens don't contain sensitive information in the payload (JWTs are encoded, not encrypted!).

Important Security Notes

JWTs are base64url encoded — NOT encrypted. Anyone can decode a JWT and read the header and payload. Never store passwords or secrets in a JWT payload. Use HTTPS to prevent interception. The signature ensures the token hasn't been modified, but it doesn't hide the data. This tool decodes JWTs in your browser — your tokens never leave your computer. You can validate signatures only if you have the secret key (which this tool doesn't require — it's for decoding only).

Why Our JWT Debugger is Different

Most JWT debugging websites send your token to their servers — a serious security risk if you're debugging production tokens containing real user IDs or session data. Our JWT decoder runs entirely in your browser. The token stays on your computer. No server logs, no tracking, no data collection. Perfect for debugging tokens with actual user data.

Our tool automatically checks expiration claims (exp) and shows if the token is expired. It formats the header and payload as pretty-printed JSON for easy reading. The recent history saves your last 5 tokens locally — useful when testing multiple API responses. Everything stays private and works offline. Whether you're a backend developer implementing JWT auth, a security researcher, or just learning how tokens work, our debugger gives you instant insights. Try it now — paste any JWT and see what's inside.