JWT Decoder: Understand and Debug JSON Web Tokens Easily
Learn what JSON Web Tokens are, how JWT structure works (header, payload, signature), and how to decode JWTs for debugging authentication and API development.
February 20, 2026
What Are JSON Web Tokens?
JSON Web Tokens (JWTs) are compact, URL-safe tokens used to securely transmit information between parties as a JSON object. They are widely used in modern web applications for authentication, authorization, and information exchange. When you log into a web application, the server often issues a JWT that your browser sends with each subsequent request to prove your identity.
JWT Structure Explained
A JWT consists of three parts separated by dots: header.payload.signature. Each part is Base64URL-encoded, making the token safe for use in URLs and HTTP headers.
Header
The header typically contains two fields: the token type (typ), which is usually JWT, and the signing algorithm (alg), such as HS256 (HMAC SHA-256) or RS256 (RSA SHA-256). The header tells the recipient how to verify the token's signature.
Payload
The payload contains the claims, which are statements about the user and additional metadata. Standard claims include sub (subject/user ID), iss (issuer), exp (expiration time), iat (issued at), and aud (audience). Applications can also include custom claims like user roles, permissions, or email addresses.
Signature
The signature is created by combining the encoded header and payload with a secret key using the algorithm specified in the header. This ensures the token has not been tampered with. For HS256, the server uses a shared secret. For RS256, the server signs with a private key and verification uses the public key.
How to Decode JWTs
Decoding a JWT means extracting and reading the header and payload. Since these parts are only Base64URL-encoded (not encrypted), anyone can decode them. Decoding does not mean verification. Verifying a JWT requires the signing key to confirm the signature is valid.
Use Cases for JWT Decoding
- Authentication debugging: When login flows fail, decoding the JWT helps identify issues like expired tokens, wrong audience claims, or missing permissions.
- API development: Developers building API integrations need to inspect tokens to ensure they contain the expected claims and are properly formatted.
- Token inspection: Before processing a JWT, applications may need to read claims like expiration time to decide whether to refresh the token.
- Security auditing: Security teams decode JWTs to verify that sensitive information is not accidentally included in token payloads.
Security Considerations
JWTs are encoded, not encrypted. Never store sensitive data like passwords or credit card numbers in JWT payloads. Always verify JWT signatures on the server side before trusting the claims. Use short expiration times and implement token refresh mechanisms. Store JWTs securely in the browser, preferably in HTTP-only cookies rather than localStorage.
Using Our JWT Decoder Tool
Our free online JWT decoder instantly parses any JWT token. Paste your token into the input field and the tool displays the decoded header, payload, and signature information in a readable format. You can quickly check claim values, verify expiration dates, and inspect the signing algorithm. The tool works entirely in your browser, so your tokens never leave your device.