Next.js App Router + Supabase: browser client for Client Components, fresh server client per request
# Next.js App Router + Supabase: two clients, used in the right places
The number one way agents break this setup: importing one shared client into both Client and Server Components. Server Components run per request with per-request cookies, so a singleton leaks sessions between users and silently drops auth.
## The rule
- Client Components (`'use client'`): `createBrowserClient` from `@supabase/ssr`. One module-level instance is fine here, it runs in the user's browser.
- Server Components, Server Actions, Route Handlers: `createServerClient` from `@supabase/ssr`, created fresh inside each function call, wired to that request's cookies via `next/headers`.
## Checkable procedure
1. Install the current packages: `@supabase/supabase-js` and `@supabase/ssr`. Do not install `@supabase/auth-helpers-nextjs`, it is deprecated.
2. Create `lib/supabase/client.ts` with a `createClient()` that returns `createBrowserClient(url, publishableKey)`. Import this only from Client Components.
3. Create `lib/supabase/server.ts` with an async `createClient()` that reads `await cookies()` from `next/headers` and returns `createServerClient(url, publishableKey, { cookies: { getAll, setAll } })`. Call it inside every Server Component, Action, and Route Handler, never at module scope.
4. In `setAll`, wrap the cookie writes in try/catch. Server Components cannot write cookies, only the proxy/middleware can, so the write throws there and that is expected.
5. Grep your app for `createClient` from `supabase-js` used directly in `app/` server code. Every hit is a bug: that client has no cookie storage, so every request looks logged out.
## Why the singleton breaks
`createClient` from `@supabase/supabase-js` keeps the session in memory. On the server that memory is shared across requests, so user A's session can serve user B's page. The `@supabase/ssr` server client stores the session in cookies scoped to the request instead.
## Quick test
Sign in as user A in one browser and user B in another. Load a Server Component that shows the user email on both. If either shows the wrong email or a stale session, a shared client is leaking across requests.Maintainer review
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