Why does my URLSession request return not valid JSON when curl works fine?

Understand why your URLSession request might be returning "not valid JSON" even when curl works fine, and how to troubleshoot the issue in Swift and SwiftUI.
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Why does my URLSession request return "not valid JSON" when curl works fine?

Many developers encounter a perplexing situation where their URLSession request in Swift or SwiftUI returns the error message "not valid JSON," despite the fact that the same request works seamlessly when using curl from the command line. Understanding why this discrepancy occurs can be key to troubleshooting and resolving the issue.

Common Causes

HTTP Headers Mismatch
One common scenario is that the HTTP headers in your URLSession request might not match those used in the curl command. Headers like Content-Type, Accept, or custom authentication might be set differently, resulting in the server responding with invalid JSON.

Data Encoding
Curl, by default, might handle data encoding differently compared to URLSession. Ensure that the data you're sending and receiving is properly encoded and matches the expected format specified by the server.

Request Method Discrepancy
Sometimes, the method (GET, POST, PUT, etc.) specified in your URLSession request might differ from the one used in curl. Double-check to ensure that both methods align.

Troubleshooting Steps

Compare Headers:

Use a tool like Postman to replicate your curl request and examine the headers.

Ensure your URLSession request headers match those seen in the successful curl request.

Verify Request Body:

Examine the body content of both requests. Make sure any data, especially JSON payload, is correctly formatted.

Check Response Handling:

Ensure that your URLSession's response handling code can parse and display raw response data. Sometimes, the issue lies in the response parsing logic rather than the initial request.

Example

Here is a simple way to set up a URLSession request in Swift with headers:

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

In conclusion, ensuring consistency between your curl command and URLSession request is crucial for troubleshooting why a "not valid JSON" error might occur. By comparing HTTP headers, verifying request bodies, and reviewing response handling, you can often pinpoint and resolve the cause of the discrepancy.