JWT Decoder

Introduction

JSON Web Tokens (JWTs) are the standard way to pass identity and authorization claims between services. When a token fails to work, the first debugging step is almost always the same: decode the JWT and look at what is actually inside the header and payload. Doing this by hand means base64url-decoding segments, formatting JSON, and interpreting the exp, iat, and iss claims manually.

The JWT Decoder at AI Developer Toolbox does all of that in one step. Paste a token and get the decoded header and payload as readable JSON, a breakdown of the exp and iat timestamps, and a security check that flags common problems such as missing expiration or the dangerous alg=none value. Everything runs locally in your browser, so tokens never leave your device.

An important note: decoding a JWT is not the same as verifying it. This tool only inspects the token's contents; it does not verify the signature, which requires the secret key held by the issuing service.

Features

• Format detection and parsing：Automatically recognizes a standard three-part JWT, decodes the header and payload into readable JSON, and displays the signature segment.
• exp and iat time analysis：Reads the expiration and issued-at claims, converts them into human-readable local time, and shows expiry status and remaining validity.
• Security checkup：Runs a quick check for common issues including missing exp, missing iat or iss, and the high-risk alg=none value, so you can spot problems fast.
• Private local parsing：All decoding happens locally in your browser. The token is never sent anywhere, making it safe to inspect real tokens from your environment.

Examples

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkFsaWNlIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Header: {"alg":"HS256","typ":"JWT"}
Payload: {"sub":"1234567890","name":"Alice","iat":1516239022}

(token with alg=none header)

⚠ High risk: alg=none detected. The token is unsigned and can be forged.
Note: this tool decodes but does not verify the signature.

How to Use

1. 1Paste your token：Type or paste the JWT into the input area, or load a demo token to see how the tool works with a local sample.
2. 2Decode and inspect：Press Decode Token to see the header, payload, signature, time analysis, and security check results.
3. 3Interpret the findings：Focus on the red and yellow warnings, such as alg=none or an expired token, and remember that signature verification needs the service's secret.

Use Cases

• Inspecting the payload of a JWT returned by an authentication service
• Checking whether a token has expired by reading the exp claim
• Debugging why an API rejects a token during development
• Reviewing the claims inside a token to understand what it authorizes

FAQ