The SSR State Management Trap: defineStore vs defineContextStore

You ship a Next.js-style SSR app with a global user store. A month later, a user files a bug: "I logged in and saw someone else's profile." You can't reproduce it locally. Production logs are useless. The session cookie looks fine. What happened: your global store is a singleton on the server. Under concurrent requests, one request's setUser() wrote to the same object another request was reading. The second user hit a warm server, got a hydration payload containing the first user's data, and saw