Firebase Auth errorFirebase Auth: “Unable to process request due to missing initial state”

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Why the redirect sign-in fails, and the fix

1. 01

   authDomain still points at firebaseapp.com

   “This ensures that your app and the auth iframe use the same domain, which prevents the sign-in issue.” Firebase, Redirect best practices (Option 1)

   With the app on its own domain and authDomain on firebaseapp.com, the browser keeps them apart. On Firebase Hosting, set authDomain to your custom domain and add https://your-domain/__/auth/handler to the provider’s redirect URIs.

   const firebaseConfig = {
     // ...
     authDomain: 'app.example.com',
   };

2. 02

   The app isn’t on Firebase Hosting

   “The signInWithRedirect flow starts by redirecting from your app domain to the domain specified in the authDomain parameter in firebase config” Firebase, Redirect best practices (Option 3)

   On Vercel, Netlify or your own server, proxy /__/auth/ to your firebaseapp.com domain as a transparent rewrite, not a 302 redirect. Then set authDomain to your domain and add the redirect URI and authorized domain as above. On Netlify the rule is /__/auth/* https://PROJECT_ID.firebaseapp.com/__/auth/:splat 200.

   // vercel.json
   {
     "rewrites": [
       { "source": "/__/auth/:path*", "destination": "https://PROJECT_ID.firebaseapp.com/__/auth/:path*" }
     ]
   }

3. 03

   A popup would do

   “Use signInWithPopup() instead of signInWithRedirect().” Firebase, Redirect best practices (Option 2)

   Popups avoid the redirect storage problem. Call signInWithPopup from the click handler and get the user from its result. Popups are sometimes blocked, and the flow is less smooth on phones.

4. 04

   Self-hosting the helper files

   Copying the sign-in helper files to your domain also works, but Firebase says it doesn’t support Apple sign-in or SAML. Use it only when a proxy isn’t possible.

Checked against the vendor’s docs on November 9, 2026: Firebase docs: Best practices for using signInWithRedirect.

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Related: Firebase Google sign-in popup blocked · Firebase auth/unauthorized-domain · Google OAuth redirect_uri_mismatch · Firebase auth/billing-not-enabled