We've also wasted 1-2 man days trying to debug this issue. We're 90% sure this is a problem on Apple's side. You can verify by testing with a known "good" client_id. For example - when we navigate to this URL: https://appleid.apple.com/auth/authorize?response_type=code&client_id=OUR_APP_ID&scope=email%20name&response_mode=form_post ... which contains our app ID, we get "invalid_client". When we substitute a third-party app ID, such as Uber's: https://appleid.apple.com/auth/authorize?response_type=code&client_id=com.ubercab.UberClient&scope=email%20name&response_mode=form_post ... we get "redirect_uri is required", which is the expected response. We've reached out via Feedback Assistant and Apple's contact form, but no response yet.

Same here. Didn't change anything, it's working now. No response or update from Apple on Feedback Assistant, email or developer forum. Very frustrating.

Getting the same issue. Can't get even the most basic example working: html !-- example from https://developer.apple.com/documentation/sign_in_with_apple/sign_in_with_apple_js -- script type="text/javascript" src="https://appleid.cdn-apple.com/appleauth/static/jsapi/appleid/1/en_US/appleid.auth.js"/script script type="text/javascript" AppleID.auth.init({ clientId: 'com.myappid', scope: 'name email', redirectURI: 'https://myapp.dev/test/apple/callback' }); const buttonElement = document.getElementById('appleid-signin'); buttonElement.addEventListener('click', function () { AppleID.auth.signIn(); }); /script Created and recreated app IDs and service IDs many times, always results in invalid_client.