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mahjadan
mahjadan

Posted on • Originally published at devtoolscenter.com

How to Quickly Inspect & Decode JWTs in Postman (The Right Way)

#webdev #jwt #security #api

Introduction

If you’re a developer working with modern APIs, you live and breathe JWTs (JSON Web Tokens). You know the drill: an API call fails, and you’re left scratching your head, wondering if it's the token itself, the expiration, or the claims.

The typical workflow involves using Postman (or a similar tool like Insomnia) to test the API, but when it comes to decoding the JWT you just received, Postman’s native functionality can leave you running to a separate browser tab to debug.

This guide covers the simple, native way to inspect the token in Postman and then shows you the one tool that eliminates the friction of switching between tools for deep, safe decoding and validation.

Method 1: The Quick-and-Dirty Postman Inspect (The Limitations)

Postman is great for sending and receiving tokens, but it's not a true JWT decoder. Here are a couple of ways to quickly inspect the token, and why they often fall short for deep debugging:

  1. Viewing the Token in the Console If you receive the JWT in the response body or headers, the Postman Console is your best bet for a quick peek:

Open the Postman Console (View -> Show Postman Console or Ctrl/Cmd + Alt + C).

Send your request.

In the Console, click on the request/response. You will see the raw headers and body.

Copy the token (the long string of three dot-separated Base64 chunks).

The Catch: You still have the Base64-encoded string. You can manually Base64-decode the middle part (the payload), but you cannot easily verify the token's signature, check the expiration in a human-readable format, or instantly validate the token's structure.

  1. Using Pre-Request/Test Scripts For more advanced users, you can write a Postman script to manually decode the payload section, but this involves writing code and adding libraries. 
// Example of a Postman Test Script to inspect a received JWT
pm.test("Check JWT Expiration", function () {
    const token = pm.response.json().access_token; // Adjust path as needed

    // Split the token into its parts (Header, Payload, Signature)
    const [header, payload, signature] = token.split('.');

    // Decode the payload part (Base64URL safe decoding)
    const decodedPayload = Buffer.from(payload, 'base64').toString('utf8');
    const claims = JSON.parse(decodedPayload);

    // Log the claims to the Console for inspection
    console.log("JWT Payload:", claims);

    // Example: Check if the token is already expired
    const isExpired = claims.exp < Date.now() / 1000;
    pm.expect(isExpired).to.be.false; 
});
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This works, but it's boilerplate code you have to write for every request, and it still doesn't handle the signature verification.

Method 2: The Right Way, Use a Dedicated JWT Decoder Tool

When you are actively debugging, you need speed, safety, and validation. The most efficient workflow is to copy the token from Postman and paste it into a dedicated, developer-focused tool.

A high-quality tool should do three things instantly:

  1. Instant Decode: Decode the header and payload into human-readable JSON.
  2. Signature Verification: Attempt to verify the signature using a provided secret (if applicable).
  3. Error Flagging: Highlight common issues like expired tokens or invalid signatures.

This is where a tool like DevCenterTools JWT Decoder comes in.

Why a Dedicated Tool is Superior to Postman for Debugging:

100% Client-Side Decoding: The best tools perform the decoding entirely in your browser, ensuring your secret keys and tokens never leave your machine. ( This is how the DevCenterTools JWT Decoder is built.)

Built-in Validation: It automatically checks the exp (expiration), iat (issued at), and other registered claims, often highlighting them in red if they are invalid or expired.

Algorithm Support: It correctly handles different algorithms like HS256 and RS256 and guides you on the required input (secret or public key).

Speed: You copy a token, paste it, and the data is instantly available, saving you the time of writing (and maintaining) custom Postman scripts.

Your Debugging Workflow Upgrade

  1. In Postman, send your request and get the JWT.
  2. Copy the full JWT string.
  3. Paste it into the DevCenterTools JWT Decoder.
  4. Instantly see the decoded header and payload, and provide your secret to verify the signature.

This simple step immediately isolates the problem: Is the issue with your token's payload/claims, or is it an issue with the API's token handling?

Conclusion

While Postman is a fundamental tool for API testing, offloading complex tasks like JWT decoding and validation to a specialized utility will significantly speed up your debugging cycles.

If you are constantly switching between Postman and a browser tab for decoding, make sure you are using a tool that is fast, secure (client-side!), and validates the token for you.

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