Auth0's Next.js SDK v4 is designed for Next.js 16's server-first architecture, replacing legacy Pages Router patterns with a new proxy.ts interception model. Key changes include auto-mounted auth routes (/auth/login, /auth/callback, etc.), simplified session access via await auth0.getSession() without request/response arguments, and clear guidance on session validation across Server Components, Server Actions, and API routes. The guide covers three route protection strategies (page-level, layout-level, and proxy-level), explains why every Server Action must independently re-validate authentication, and highlights security considerations like AUTH0_SECRET rotation, allowed callback URL configuration, and never trusting client-side session state for authorization decisions.
