JWT Decoder and Encoder is a free browser-based tool for inspecting and creating JSON Web Tokens (JWTs). JWTs are a standard format for transmitting authentication and authorization information between services, typically used in REST API authentication flows where a server issues a signed token after login and the client sends it with each subsequent request. This tool accepts any JWT and decodes its header, payload, and signature components, displaying them as formatted JSON. Users can also construct new tokens by entering a payload and a secret key. The tool runs entirely in the browser, which is important for security: pasting a production JWT into a third-party server-based tool would expose sensitive credentials. No account or installation is required. JWT Decoder & Encoder is commonly used as a online JWT validator, making it a practical choice for everyday tasks directly in the browser. For complete results, XML Prettify & Formatter can format and validate XML documents, JSON to TypeScript can generate TypeScript interfaces from JSON, and Base64 to Image can decode Base64 strings back into image files.
JWT Decoder and Encoder is used primarily for development and debugging tasks in API-driven applications. When implementing JWT authentication in a backend service, inspecting the decoded payload confirms that the token contains the expected claims: user ID, roles, issued-at time, expiration time, and any custom fields. This is faster than adding debug log statements to the application code and then running the authentication flow. When a JWT is rejected by an API, decoding it reveals whether the issue is an expired token (the exp claim is in the past), a missing required claim, or an unexpected issuer value. The header component shows the algorithm used for signing, which is important for matching the verification logic in the backend. Common algorithms are HS256 (HMAC with SHA-256, using a shared secret) and RS256 (RSA with SHA-256, using a public/private key pair). The tool supports construction of new tokens with a custom payload and a shared secret, which is useful for generating test tokens during development without running the full authentication flow. Because JWTs can contain sensitive information such as user identifiers, roles, and session data, using a browser-based tool that processes everything locally is significantly safer than pasting tokens into a server-based decoder. The tool does not persist any entered values and clears state on page reload.