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