Understanding CORS Preflight Requests Not Showing in Safari Dev Tools

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When developing web applications, one crucial aspect developers often grapple with is Cross-Origin Resource Sharing (CORS). This security feature implemented in browsers restricts web pages from making requests to a different domain than the one that served the web page, unless the server explicitly allows it. One component of CORS is the preflight request, particularly an HTTP OPTIONS request sent by the browser to check if the CORS protocol is understood and allowed by the server before the actual request is made.

However, a common issue developers face is that these preflight OPTIONS requests sometimes do not show up in the browser's Dev Tools, specifically in Safari. This can be frustrating when debugging, as visibility into these requests is essential for ensuring proper CORS configuration.

Why Are Preflight Requests Not Showing in Safari?

Historically, developers could see CORS preflight OPTIONS requests in the network tab of Safari’s Dev Tools. If you remember seeing them previously, you're not mistaken. However, changes in how browsers handle CORS requests, particularly around efficiency and privacy improvements, have led to some preflight requests being hidden from view in recent versions of Safari and other browsers like Chrome.

Safari, like other browsers, might be optimizing the handling of these preflight requests. One reason they might not show up could be due to caching mechanisms where the browser remembers the outcome of a preflight request for a certain period, thus not needing to repeat it or display it. Additionally, certain optimizations or developer tool updates might have altered the visibility of these requests.

Getting Safari to Display Preflight Requests

Unfortunately, Safari does not provide a direct flag or setting, like Chrome's chrome://flags/#out-of-blink-cors, that can be toggled to force the display of preflight requests. However, here are some potential approaches to make these requests visible:

  1. Disable Caching in Safari Dev Tools:

    • Open the Safari Dev Tools (Cmd + Option + I).
    • Go to the Network tab.
    • Ensure that "Disable Caches" is checked. This forces Safari to bypass any cached preflight responses and make the OPTIONS request anew, which should show up in the Network tab.

  2. Use a Proxy Tool:

    • If Safari's Dev Tools are not displaying the requests, consider using a network proxy tool like Charles Proxy or Fiddler. These tools can capture all HTTP traffic, including hidden preflight requests, providing a more detailed look into what's happening under the hood.

  3. Inspect the Server Logs:

    • Since you mentioned that the server logs do show the preflight requests, continue to monitor them there. This can be a reliable alternative for confirming that the requests are being made and understanding their behavior.

  4. Check Safari Experimental Features:

    • Safari regularly updates and includes experimental features that can be toggled via the Develop menu in the menu bar. Make sure all relevant experimental features related to networking and developer tools are enabled, as they might affect how requests are displayed.

  5. Use Another Browser:

    • As a last resort, if Safari continues to obscure these requests, you might temporarily switch to a different browser like Chrome or Firefox for debugging. Both offer more explicit flags and settings to handle CORS behavior.

Conclusion

The disappearing CORS preflight requests in Safari’s Dev Tools can indeed be a challenge, especially when these requests are critical for debugging cross-origin issues. While Safari does not currently offer a straightforward setting to make these requests visible, employing the methods mentioned above can help you work around this limitation. As browser technologies evolve, keeping an eye on updates to developer tools and understanding the underlying mechanisms will be key to effective debugging.

For Chrome users, disabling the out-of-blink-cors flag has been a known solution to a similar problem, though it's worth noting that this flag might not exist or behave similarly in future versions of Chrome. Therefore, as a best practice, always keep your browser and developer tools updated, and leverage external tools when necessary to maintain visibility into all network traffic.

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