Error 400: redirect_uri_mismatch after changing authDomain

You changed authDomain to your domain, and now Google blocks sign-in with Error 400: redirect_uri_mismatch. Google shows this until it knows the new handler address. We reproduced it on 24 Oct 2026 by swapping hosts on an unregistered lab domain.

Fix (a console step you do yourself): Google Cloud console → APIs & Services → Credentials → the auto-created Web client (the one Firebase made) → Authorised redirect URIs → add exactly https://<your domain>/__/auth/handler → Save. Google may take minutes to hours to apply it. Use the host from your address bar, and add one line for each domain you sign in from.

The free check reads only your public page, never your Google Cloud settings, so this error is the signal to look for.

FAQ

What if it doesn’t work for me?

On Cloudflare Workers, it works or you get your money back within 14 days. Our refund policy says: “Works or your money back. If within 14 days you've followed the pack and Google's sign-in still doesn't show your domain, email support with your re-check link. We'll help you fix it, and if we can't, we refund you in full.” It covers apps served by Cloudflare Workers, the hosting setup we’ve tested, and sits alongside your rights under UK consumer law. Full terms: https://pewterkit.com/refunds

Do I need to be a developer?

Your fix pack has a paste-ready prompt for your AI builder, the exact snippet we tested, and a re-check that runs the free check on your app again, so you can see the name Google’s sign-in shows after your change.

Is the check safe?

It’s read-only. It reads your public page and its same-origin scripts, the same page any visitor can open, and sends no cookies. It never asks for a sign-in, a password or access to your accounts.

Which hosts does it cover?

Cloudflare Workers on your own domain. In our lab on Cloudflare Workers, Google’s sign-in showed our own domain (re-checked 24 Oct 2026). A workers.dev address can’t show your own domain at sign-in. You need a domain you own. On Cloudflare Pages, Firebase Hosting, Vercel or Netlify, run the free check, pick your host and leave your email: we’ll email you once when yours is ready.

What’s in the fix pack?

It’s built from the check we ran on the domain you entered: the exact snippet we tested for your host, the two console settings to change with the click path to each, a paste-ready prompt for your AI builder, and a re-check. One-time payment, no subscription.