A JSON Web Token contains a header, payload, and signature. Reading the first two parts is useful for debugging, but it does not prove that the token was issued by a trusted party.
Decode is not verification
A JWT can be decoded without a key because its header and payload are Base64URL-encoded JSON. Verification requires the correct shared secret or public key and the expected algorithm.
Check time-based claims
Review `exp`, `nbf`, and `iat` when debugging authentication. Compare them with the current time, account for clock skew, and confirm that the application enforces the claims rather than merely displaying them.
Keep keys out of shared tools
Never paste production signing keys or private keys into an online tool unless you have verified its processing model and security controls. A local browser tool can reduce exposure, but it cannot replace a controlled security workflow.