Fixing QNetworkReply::ProtocolFailure in Qt WebAssembly by Handling CORS Headers

Learn how to resolve QNetworkReply::ProtocolFailure errors in Qt WebAssembly by properly setting CORS headers on your Qt HTTP server. --- This video is based on the question https://stackoverflow.com/q/79405700/ asked by the user 'leon' ( https://stackoverflow.com/u/7575782/ ) and on the answer https://stackoverflow.com/a/79416636/ provided by the user 'leon' ( https://stackoverflow.com/u/7575782/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Receiving QNetworkReply::ProtocolFailure to GET request Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to drop me a comment under this video. --- Understanding the Problem: QNetworkReply::ProtocolFailure in Qt WebAssembly When developing Qt applications that run as desktop clients, HTTP communication using QNetworkAccessManager usually works seamlessly. However, when the application is compiled to run in the browser using WebAssembly, network requests often fail with QNetworkReply::ProtocolFailure even though the server properly sends responses. This issue typically occurs because browsers enforce Cross-Origin Resource Sharing (CORS) policies to restrict how web pages interact with resources from different origins. Why Does ProtocolFailure Occur? Desktop applications have direct network access without browser security restrictions. WebAssembly Qt apps run inside browsers and must obey CORS rules. If the HTTP server does not send the correct CORS headers, the browser blocks the response for cross-origin requests. Qt's QNetworkAccessManager reports this as ProtocolFailure. The Correct Way to Fix It: Add CORS Headers on the Server The key is to allow cross-origin requests by adding appropriate HTTP headers in your Qt HTTP server responses. Specifically, you want to add at least the: [[See Video to Reveal this Text or Code Snippet]] header to your GET response. This tells the browser to allow any origin to access the resource. Example Fix Modify your server code to include this header in your GET route: [[See Video to Reveal this Text or Code Snippet]] You should also handle OPTIONS requests for the preflight checks browsers do before sending actual GET/POST requests: [[See Video to Reveal this Text or Code Snippet]] Summary QNetworkReply::ProtocolFailure in WebAssembly Qt apps usually means the browser blocked the response due to CORS. Add Access-Control-Allow-Origin header to your HTTP server responses to fix this. Also handle OPTIONS preflight requests properly to enable full cross-origin support. After these changes, the Qt WebAssembly network requests should succeed as expected. This simple header addition aligns your backend with browser security, enabling smooth network communications in Qt apps running in web browsers.