About the JWT Decoder
JSON Web Tokens carry claims between services as three Base64URL-encoded segments: header, payload, and signature. This decoder splits and decodes the first two locally in your browser — no verification, no network calls — so you can inspect exactly what a token asserts: issuer, audience, expiry, scopes, and custom claims.
It is the fastest way to answer the debugging questions that dominate auth incidents: is the token expired, does the aud match what the API expects, which alg signed it, and did the identity provider actually include the claim your code is looking for.
Frequently asked questions
Is it safe to paste a JWT into a decoder?
This tool decodes entirely client-side with no network transmission. But remember that a live JWT IS a credential — anyone holding it can replay it until expiry. Treat production tokens like passwords: prefer decoding expired tokens or ones from non-production environments when possible.
Why does decoding work without the secret key?
The header and payload are only Base64URL-encoded, not encrypted — by design, so any service can read claims. The secret/private key is needed only to verify the signature (third segment), which proves the token wasn't tampered with. Decoding tells you what a token says; only verification tells you whether to trust it.
What are the claims I should always check when debugging auth failures?
exp (expired tokens are the #1 cause — compare against current Unix time), aud (must match the API's expected audience), iss (must match the trusted issuer URL exactly, including trailing slashes), and alg in the header (an unexpected algorithm like none or HS256-instead-of-RS256 indicates misconfiguration or an attack).
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