Google sign-in failures are almost always configuration mismatches between your app and Google Cloud Console. The most common causes are redirect URIs that don't exactly match, including protocol, domain, port, and trailing slash; an app behind a proxy that generates an http callback instead of https; an OAuth consent screen still in testing mode; different client IDs or secrets between environments; and session or cookie settings that lose the login state during the redirect.

Signs Your Google Sign-In Has This Problem

• Error 400: redirect_uri_mismatch after clicking Sign in with Google

• Access blocked: This app's request is invalid

• Google login works on localhost but fails in production

• invalid_client or unauthorized client errors

• Users return from Google and land back on the login page

• state mismatch or CSRF errors after the redirect

• Only certain test accounts can sign in

• Sign-in fails inside in-app browsers or embedded webviews

Why Google OAuth Login Fails

1. Redirect URI doesn't match exactly

Google compares the callback URL character by character with the list in your OAuth client. A different protocol, www versus non-www, a port number, a trailing slash, or a missing preview domain triggers redirect_uri_mismatch.

2. App behind a proxy sends an http callback

On platforms with load balancers or reverse proxies, apps that don't trust forwarded headers think they're running on http and build the callback URL with http instead of https, which Google rejects.

3. Consent screen still in testing mode

Apps in testing mode only allow listed test users, so everyone else is blocked. Apps requesting sensitive scopes may also need Google verification before public use.

4. Wrong client or secret for the environment

Using the development OAuth client in production, or a client ID from one project with a secret from another, causes invalid client errors.

5. Login state lost during the redirect

OAuth relies on a state value and session cookie surviving the round trip to Google. Cookie domain, SameSite, or secure settings, or multiple app instances without shared sessions, cause state mismatch errors and login loops. Google also blocks sign-in inside many embedded webviews.

How We Diagnose the Sign-In Failure

1. Capture the exact callback URL. Read the redirect URI your app actually sends in the authorization request.

2. Compare Google Cloud settings. Check authorized redirect URIs and JavaScript origins character by character.

3. Check environment credentials. Confirm client ID and secret match per environment.

4. Review consent screen status. Check publishing status, test users, scopes, and verification needs.

5. Inspect proxy headers. Confirm the app detects https correctly behind load balancers.

6. Trace cookies and state. Follow session cookies and the state parameter through the full redirect.

How We Fix It

We test sign-in for new and returning users on every production and preview domain you use.

Example Fix

Situation: A SaaS app on a managed cloud platform lost most new sign-ups after launch because Sign in with Google failed for everyone except the founders.

Cause: The app sat behind the platform's load balancer and generated an http callback URL, causing redirect_uri_mismatch. The OAuth consent screen was also still in testing mode, allowing only listed test users.

Fix: Configured the app to trust proxy headers so callbacks used https, added the production and www redirect URIs, published the consent screen, and fixed the session cookie settings.

Result: New users could sign in with Google on the first try, and sign-up conversion recovered.

How to Keep It From Happening Again

• Keep a list of every domain your app runs on and add each to Google settings before launch.

• Use separate OAuth clients for development and production.

• Test sign-in with a brand-new Google account before every release.

What You Get

• Root cause confirmed and explained

• Working Google sign-in on all production domains

• Correct environment and consent screen setup

• Session and cookie configuration documented

Frequently Asked Questions

How do I fix Error 400: redirect_uri_mismatch? 

Make the callback URL your app sends exactly match an authorized redirect URI in Google Cloud Console, including protocol, domain, port, path, and trailing slash.

Why does Google login work locally but not in production? 

Production usually needs its own redirect URIs, credentials, and https handling. Apps behind proxies often send http callbacks unless configured to trust forwarded headers.

Why can only some users sign in with Google? 

The OAuth consent screen is probably in testing mode, which allows only listed test users. Publishing the app removes that restriction.

What causes a state mismatch error? 

The session or cookie holding the state value was lost during the redirect, often due to cookie settings, domain differences, or multiple servers without shared sessions.

Do you fix Google sign-in in mobile apps too? 

Yes, including flows in React Native, Flutter, and native apps, where sign-in must use the system browser rather than an embedded webview.

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