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Discover why your JWT token differs between Curl and Flutter HTTP calls and learn how to address the issue effectively.
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Why is My JWT Token Different in Curl and Flutter? Here's the Fix!

If you're working with authentication and APIs, you may have run into a curious problem: why does your JWT (JSON Web Token) token return different results when making the same HTTP call using Curl and Flutter? It's a common issue, especially when dealing with AWS services such as Cognito for authentication. In this guide, we'll explore the root cause of this discrepancy and provide a straightforward solution for ensuring consistent token responses.

The Problem Explained

Your Scenario

You have a back end set up with C# and AWS Lambda that authenticates users through AWS Cognito. After a successful login, it returns an ID token like this:

[[See Video to Reveal this Text or Code Snippet]]

However, when you make the same HTTP request from a Flutter application and from Curl, you notice that the ID tokens returned are different in length:

Curl Response Length: 1063 characters.

Flutter Response Length: 1023 characters.

This discrepancy leads to authentication failures when you attempt to use the ID token received in Flutter but not in Curl. It seems like Flutter is "skipping" or truncating some characters, but why?

The Root Cause

Upon examination, it turns out that the entire ID token is being received by the Flutter app; the issue arises when you try to print out the token using debugPrint(IDToken). Flutter has a character limit on what it can display in a single statement, which defaults to 1023 characters. Therefore, anything beyond this limit is simply not displayed, leading you to think the token is shorter than it actually is.

The Solution

How to Fix the Print Statement

You can work around this limitation by splitting your debug output into smaller parts, allowing you to see the entire token. Here’s how you can achieve that:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

When debugging long strings in Flutter, remember the 1023 character limit for debugPrint().

Always ensure that your validation checks account for the full length of the data returned by your API, rather than relying solely on printed output.

When in doubt, look at the actual response body, not just what gets displayed in the console.

Conclusion

In summary, while it may seem puzzling at first when your JWT token length differs between Curl and Flutter, the culprit is often straightforward: the way data is being printed and displayed. By adjusting how you handle long strings in Flutter, you can ensure that your ID tokens are fully visible and available for authentication.

Stay tuned for more insights on working with APIs, authentication, and debugging in Flutter!