Google

You get a local server that mimics authorization flows, token exchange, PKCE, and the major Google services. Seed it with test users, OAuth clients, calendar events, Gmail messages, and Drive files so your tests are repeatable. It works with google-auth-library, Auth.js, and Passport by pointing them at localhost instead of accounts.google.com. Honestly a lifesaver if you've ever burned through API quotas during development or needed to test edge cases like workspace domains without creating actual Google accounts. Supports strict client validation when you configure OAuth clients, otherwise accepts any client_id for quick prototyping.