Why does my Flutter web app get a 500 error from a Firebase Cloud Function in Safari?

Below is a set of recommendations and things to check based on similar setups with Firebase Cloud Functions and Stripe:

Potential Causes

• CORS or Request Headers:
  Safari can sometimes enforce stricter cross‐origin policies. Verify that your Cloud Function endpoint has proper CORS settings so that requests from Safari aren’t being blocked or modified.

• Browser-Specific Behavior:
  Differences in how Safari handles cookies, cache, or redirects may affect your request. Make sure that your Firebase and Stripe integration is not relying on browser-specific cookies that Safari might be blocking.

• Auth and Session Handling:
  When initiating the call via the Firebase SDK in Flutter web, check that the authentication tokens or headers aren’t being stripped or altered in Safari. Sometimes the way browsers handle HTTP credentials can differ.

What to Share When Debugging

• Error Logs (Without Sensitive Data):
  Instead of sharing all the logs, extract and share error messages from the Cloud Function logs (especially the 500 error details). Include stack traces or error messages that don’t reveal API keys or other sensitive information.

• Request Details:
  Share the request method, URL, and headers (if possible), but make sure to redact any sensitive tokens or keys. Note if there are any differences in the payload between Safari and Chrome.

• CORS/Network Console Output:
  Safari’s Web Inspector can show CORS errors or other network-level errors—these may help pinpoint whether the issue is on the client or server side.

Next Steps

1. Review Cloud Function Logs:
   In the Firebase console, look at the detailed error logs for the “POST500 383 B 2.7 s Safari 18 …” request. Look for stack traces or error messages that can help determine whether the problem is a misconfiguration on the server, invalid input, or possibly a CORS issue.

2. Test with Minimal Setup:
   Create a minimal HTML/JS page (outside of Flutter) to call the Cloud Function endpoint. This can help isolate whether the issue lies specifically in Safari’s handling of your Flutter web app or in the Cloud Function integration.

3. Check CORS Configuration:
   Ensure your Cloud Function is sending the correct Access-Control-Allow-Origin header. If you haven’t set up a custom CORS configuration, you might try using a CORS middleware library to help handle these headers.

4. Validate API Calls:
   Confirm your API calls are using HTTPS and that the authentication (via the Firebase SDK) is working as expected in Safari. Double-check if any differences in how Safari or Flutter web handles these calls could affect the request.

By sharing error details (with sensitive information redacted) and focusing on network and Cloud Function logs from Safari, you’ll likely be able to narrow down whether the issue is due to browser-specific behavior (like stricter CORS or cookie policies) or an error in your Cloud Function code.

Let me know if you need further details on any of these steps.