Why am I seeing a redirect URI mismatch error?

This error means the OAuth callback URL your provider is trying to send the user to doesn't match any of the allowed redirect URIs you've configured. Check two things: (1) The exact URL in your browser matches one of your configured URIs in Supabase Authentication → URL Configuration, and (2) For local dev, ensure you've included http://localhost:3000 and /auth/callback. Trailing slashes and https/http must match exactly.