I Spent 2 Days Debugging My Vercel Deployment - Here’s What I Missed

I recently deployed a Next.js project to Vercel. The app had both email/password and Google authentication using Supabase. Everything ran smoothly in development. But when I pushed it to production, something weird happened.

Users who signed in successfully were redirected to http://localhost:3000. Not ideal—especially for a live app. At first, I assumed I had messed up my environment variables. So I checked:

NEXTAUTH_URL – looked good

Vercel environment settings – looked fine

My auth config – nothing unusual

I went through the docs, read GitHub issues, browsed StackOverflow, and even started questioning whether the bug was in NextAuth itself.

Still, no fix.

Two days later, after trying nearly everything, I finally found the culprit: Supabase still had the redirect URL set to localhost:3000. Here’s what I missed: Supabase uses the Site URL in the Authentication settings to generate OAuth callback URLs. Mine was still set to the local development default.

To fix it, I just needed to:

1. Go to Supabase

2. Navigate to Authentication → URL Configuration

3. Update the Site URL to https://my-production-url.com

Once I hit save, everything started working as expected.

Takeaway

If your production app keeps redirecting to localhost after login, double-check your Supabase Auth redirect settings - not just your code.