JWT Debugger

Analyze JWT tokens for expiry, format, and claims problems.

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Input
Output
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About jwt debugger

JWT Debugger loads once and then runs on its own. Close the tab and there is nothing left behind, on the machine or anywhere else.

No server sees this. That is a consequence of how the tool is built rather than a policy someone wrote down, which is the version worth trusting.

You will find it under Tokens, alongside 9 other tools for the same sort of job.

Detects expired tokens, missing claims, invalid algorithms, malformed headers, and structural issues.

How to use it

01
Start it
Every control is on the tool itself. Keyboard shortcuts, where there are any, are listed beside it.
02
Watch it run
It updates in the page as you go. Nothing is sent anywhere, so it keeps working with the network off.
03
Close when done
Nothing is stored between visits. Reloading gives you a clean start rather than picking up where you left off.

Examples

Analyze an expired JWT token
Debug a JWT to find expiration issues and missing claims
Input
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiZXhwIjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
Output
=== JWT Debug Report ===

Parts count: 3

--- Header ---
{
  "alg": "HS256",
  "typ": "JWT"
}
Algorithm: HS256

--- Payload ---
{
  "sub": "1234567890",
  "name": "John Doe",
  "exp": 1516239022
}
Expires: 2018-01-18T01:30:22.000Z
Subject: 1234567890

--- Signature ---
Signature length: 43 characters
Signature (base64url): SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

--- Summary ---
Issues found: 1

Questions

Does JWT Debugger work offline?+

Yes. It is JavaScript running in this tab, so once the page has loaded it needs nothing further from the network. Put the machine in flight mode and it carries on. Nothing you do here is sent anywhere, because there is nowhere for it to go.

Does it remember anything between visits?+

No. Reload the page and it starts from nothing. There is no account, no saved state and no cookie holding your last session. That is a deliberate trade: it cannot lose your data because it never keeps any.

Is it accurate?+

Timing comes from performance.now(), the high resolution clock the browser provides, rather than from setInterval, which drifts by a few milliseconds every tick and compounds over minutes. What can still shift things is the browser throttling a tab in the background, so leave it visible if the timing matters.

Does it make a sound?+

Where a sound makes sense, it is generated with the Web Audio API rather than loaded as a file, so there is nothing to download and nothing to block. Browsers refuse to start audio until you have interacted with the page, which is why the first press is what switches it on.

Can I use JWT Debugger on a phone?+

Yes. The controls are sized for touch and the layout collapses to one column on a narrow screen. Bear in mind that a phone locking its screen will suspend the page, and timing stops with it.