How do you handle auth in new projects — build or buy?

Curious how other builders approach this. Every new project needs login, OAuth, sessions, password resets, user management… and it's never the fun part.

Do you roll your own each time, reach for a library, or use a hosted service? What's worked and what's bitten you?

(Full disclosure — I'm on the Alternative Auth team, we built a white-label auth service for exactly this. But genuinely interested in how everyone else solves it.)