CORS not working after adding custom domain to Netlify site

I can help you troubleshoot this CORS issue! It sounds frustrating, but this is a common scenario when adding custom domains. Here are several things to check:

1. Check the exact format of your CORS origin

The most common issue is that the CORS origin needs to match exactly - including the protocol. Make sure you're adding:

• https://yourdomain.com (not just yourdomain.com)
• Match the exact protocol your site uses (http vs https)

According to the CORS documentation, even small differences like http:// vs https:// will cause requests to be blocked.

2. Enable "Allow credentials"

If you're using authentication tokens with your Sanity client (which is common), you need to check the "Allow credentials" checkbox for your custom domain in the CORS settings. Without this enabled, authenticated requests will fail even if the domain is whitelisted.

You can find this in: Sanity Manage → Your Project → Settings → API → CORS Origins

3. Clear browser cache and try incognito

Sometimes browsers cache CORS responses aggressively. Try:

• Hard refresh (Ctrl+Shift+R or Cmd+Shift+R)
• Test in an incognito/private window
• Clear your browser cache completely

4. Verify DNS propagation

Since you just set up the custom domain, DNS changes might not be fully propagated. You can check if your domain is properly pointing to Netlify using tools like nslookup or online DNS checkers. If DNS isn't fully propagated, requests might be coming from unexpected origins.

5. Check for www vs non-www

Make sure you've added both variations if your site is accessible via both:

• https://yourdomain.com
• https://www.yourdomain.com

6. Wait a few minutes

CORS changes can sometimes take a few minutes to propagate on Sanity's side. If you just added the origin, give it 5-10 minutes and try again.

7. Verify in browser console

Check your browser's developer console for the exact error message. It should tell you which origin is being blocked. Sometimes the origin being sent doesn't match what you think (like if there's a redirect happening).

If none of these work, double-check that your Netlify site is actually serving from the custom domain correctly and that the requests are indeed coming from that domain. You can verify this by looking at the Origin header in your network tab when the request fails.