I’m using NextAuth.js with a custom base path /api/v1/auth.
Everything works perfectly in local development, but in production (Vercel) I get a Google OAuth error: redirect_uri_mismatch even though the redirect URL in the Google Cloud Console matches exactly what NextAuth generates locally.

Environment

• Framework: Next.js 15
• Auth library: next-auth@4.24.11
• Deployment: Vercel
• Provider: Google OAuth

.env

NEXTAUTH_URL="https://mydomain.vercel.app/api/v1/auth"
NEXTAUTH_SECRET="super-secret"

app/provider.ts

"use client";
import { SessionProvider } from "next-auth/react";

export default function AppProvider({
	children,
}: {
	children: React.ReactNode;
}) {
	return (
		<SessionProvider basePath="/api/v1/auth">
			<ThemeProvider
				attribute="class"
				defaultTheme="light"
				enableSystem
				disableTransitionOnChange
			>
				{children}
			</ThemeProvider>
		</SessionProvider>
	);
}

File structure
/app/api/v1/auth/[...nextauth]/route.ts

Expected Behavior
Google OAuth should redirect correctly to:
https://mydomain..vercel.app/api/v1/auth/callback/google

Actual Behavior
After sign-in, the redirect URL generated by NextAuth is:
https://mydomain..vercel.app/api/auth/callback/google

Images