Formula: JWT = base64url(header) + '.' + base64url(payload) + '.' + signature
JSON Web Tokens (JWT) are a compact way to transmit claims between parties. A JWT has three base64url-encoded parts separated by dots: the header (algorithm and token type), the payload (claims like user ID, expiration, issuer), and the signature (cryptographic verification). This tool decodes the header and payload for inspection — the signature is shown raw since it requires the secret key to verify.