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Stop Using Online JWT Decoders That Store Your Tokens

Inspect, debug, and sanity-check your JSON Web Tokens with Ship Advent’s Day 3 JWT Decoder. Client-side decoding, expiry checks, and security warnings built for privacy-conscious developers.

4 min read
JWT decoder tool showing header, payload, and security warnings

JWTs are everywhere in modern auth flows, but pasting live tokens into random websites is a great way to create a security incident. Day 3 of Ship Advent fixes that with a privacy-first JWT Decoder built for developers who care about security.

Meet the Day 3 JWT Decoder

Ship Advent is a free developer advent calendar where a new micro-tool unlocks every day from December 1st to 25th at shipmas-advent.com. On day 3, you get a focused JWT decoder that lets you inspect and sanity-check tokens without ever leaving your browser.

The tool lives at shipmas-advent.com/day/3 and is completely free to use with no account required. All decoding and analysis runs 100% client-side, so your tokens never touch a server.

Key features at a glance

  • Instant decoding – Paste a JWT and immediately see the header, payload, and signature as nicely formatted JSON.
  • Security analysis – Get human-friendly warnings about weak algorithms (like none or HS256) and recommendations for stronger options such as RS256 or ES256.
  • Timing validation – Quickly see whether exp, iat, and nbf claims make sense, so you can spot expired or not-yet-valid tokens.
  • PII detection – Automatic alerts if the payload looks like it contains emails, phone numbers, or other sensitive identifiers that should probably not be inside a token.
  • Per-part copying – Copy header, payload, or signature individually when you need to share minimal context in tickets or documentation.

Why a privacy-first JWT tool matters

Many JWT “debugger” sites log requests, run server-side decoding, or keep analytics that could accidentally capture sensitive tokens. That is the last place your production access token should live.

The Ship Advent JWT Decoder avoids this by doing all work in the browser and never sending your token to a backend. For security-conscious teams, that means you can debug real issues without having to scrub or fake tokens first.

Understand your standard claims quickly

If you are still memorising claim names, the tool helps by making the standard fields obvious:

  • iss – Who issued the token (identity provider or auth server).
  • sub – Who the token is actually about (typically a user ID).
  • aud – Which service or API this token is intended for.
  • exp – When the token expires, shown as a clear date and time instead of a raw timestamp.
  • iat – When the token was issued, useful for debugging clock-skew issues.
  • nbf – “Not before”, meaning the token is invalid until this time.
  • jti – A unique token identifier, handy for revocation lists or audit trails.

Practical workflows this tool unlocks

Here are a few real-world ways to use the Day 3 JWT Decoder:

  • Debug auth failures – When an API suddenly starts returning 401 or 403, drop the token into the decoder and confirm expiry, audience, and issuer at a glance.
  • Review security posture – Check which signing algorithm your provider is using and decide whether it is time to upgrade to stronger options like RS256.
  • Spot bad payload design – If you see emails, phone numbers, or addresses in the payload, that is a signal to move that data out of the token and into your database instead.
  • Onboard new team members – Use the tool as a teaching aid to walk juniors through how JWTs are structured and what each part means.

How it fits into the Ship Advent toolbox

The JWT Decoder is just one door on the Ship Advent calendar. Day 1 gives you a live Regex Tester, day 2 brings a JSON Formatter and validator, and upcoming days cover timestamps, API testing, performance checks, and more. All tools are free, browser-based, and designed to help you ship faster with less friction.

Once a day unlocks, the tool stays available forever, so you can bookmark the JWT Decoder and use it year-round. Think of December as the big reveal and the rest of the year as unlimited reuse.

Try the Day 3 JWT Decoder now

Ready to inspect a token safely? Head to shipmas-advent.com/day/3, paste in your JWT, and get instant insights into its structure, timing, and security posture. No login, no tracking, just a fast, focused tool built for developers who want to keep secrets secret.

Happy shipping, and see you behind the next door of Ship Advent!