Next.js 16 Auth Kit Reveals 5 Integration Bugs After Passing Unit Tests

A developer building a complete auth kit for Next.js 16 found five bugs that only appeared when wiring together email/password login, OAuth, and role-based routes. The bugs included a cookie that vanished after being written, silent proxy matcher failures, and a skipped mutation-layer auth check. The fixes involved moving cookie writes to the server side and tightening middleware logic.