Next.js Patterns

Version: Check package.json for the SDK version — see clerk skill for the version table. Core 2 differences are noted inline with > **Core 2 ONLY (skip if current SDK):** callouts. For basic setup, see setup/.

Impact Levels

• CRITICAL - Breaking bugs, security holes
• HIGH - Common mistakes
• MEDIUM - Optimization

References

Reference Impact references/server-vs-client.md CRITICAL - await auth() vs hooks references/middleware-strategies.md HIGH - Public-first vs protected-first, proxy.ts (Next.js <=15: middleware.ts) references/server-actions.md HIGH - Protect mutations references/api-routes.md HIGH - 401 vs 403 references/caching-auth.md MEDIUM - User-scoped caching

Mental Model

Server vs Client = different auth APIs:

• Server: await auth() from @clerk/nextjs/server (async!)
• Client: useAuth() hook from @clerk/nextjs (sync) Never mix them. Server Components use server imports, Client Components use hooks. Key properties from auth():
• isAuthenticated — boolean, replaces the !!userId pattern
• sessionStatus — 'active' | 'pending', for detecting incomplete session tasks
• userId, orgId, orgSlug, has(), protect() — unchanged

Core 2 ONLY (skip if current SDK): isAuthenticated and sessionStatus are not available. Check !!userId instead.

Minimal Pattern

// Server Component
import { auth } from '@clerk/nextjs/server'
export default async function Page() {
  const { isAuthenticated, userId } = await auth()  // MUST await!
  if (!isAuthenticated) return <p>Not signed in</p>
  return <p>Hello {userId}</p>
}

Core 2 ONLY (skip if current SDK): isAuthenticated is not available. Use if (!userId) instead.

Conditional Rendering with <Show>

For client-side conditional rendering based on auth state:

import { Show } from '@clerk/nextjs'
<Show when="signed-in" fallback={<p>Please sign in</p>}>
  <Dashboard />
</Show>

Core 2 ONLY (skip if current SDK): Use <SignedIn> and <SignedOut> components instead of <Show>. See custom-ui/core-3/show-component.md for the full migration table.

Common Pitfalls

Symptom Cause Fix undefined userId in Server Component Missing await await auth() not auth() Auth not working on API routes Missing matcher Add `'/(api Cache returns wrong user's data Missing userId in key Include userId in unstable_cache key Mutations bypass auth Unprotected Server Action Check auth() at start of action Wrong HTTP error code Confused 401/403 401 = not signed in, 403 = no permission

See Also

• setup/
• orgs/

Docs

Next.js SDK

GitHub Owner

Owner: clerk

GitHub Links

• Website: https://clerk.com
• Twitter: https://twitter.com/ClerkDev
• Email: hello@clerk.com
• Verified domains: clerk, clerk.com