hen you deploy a Next.js app to Vercel, some deployments (especially previews or protected routes) are not public. They’re behind Vercel Authentication → you need to log in with your Vercel account to view them. If you (as a human) open the preview URL → you log in → now you can see it. But if an AI coding agent (say you’re testing an agent that needs to fetch or debug your app) tries to access that preview → it gets blocked because it’s not logged in. Without this, you’d either: Make your deployment public (security risk). Or manually log in and copy data (slow + annoying). With the MCP server, agents can securely act like a logged-in developer, without ever exposing your credentials.