API Auth Clerk

Clerk Authentication Patterns

Quick Guide: Clerk provides managed authentication with pre-built UI components, server-side helpers, and organization-based multi-tenancy. Use clerkMiddleware() for route protection, <Show> for conditional rendering, hooks for client state, and auth()/currentUser() for server-side auth. Clerk Core 3 (2026) replaces <SignedIn>/<SignedOut> with <Show>, renames the middleware file to proxy.ts (Next.js 16+), and consolidates packages.

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use @clerk/nextjs/server for ALL server-side imports -- NEVER import server helpers from @clerk/nextjs)

(You MUST verify webhooks using Clerk's verifyWebhook helper -- NEVER trust unverified webhook payloads)

(You MUST use <Show> component instead of deprecated <SignedIn>/<SignedOut>/<Protect> -- these are removed in Core 3)

(You MUST NOT pass the full currentUser() object to the client -- it contains privateMetadata that must stay server-side)

(You MUST protect routes in BOTH middleware AND data access layer -- middleware alone is insufficient)

Auto-detection: Clerk, ClerkProvider, clerkMiddleware, @clerk/nextjs, useUser, useAuth, useClerk, useSession, useOrganization, SignIn, SignUp, UserButton, UserProfile, OrganizationSwitcher, auth(), currentUser(), CLERK_SECRET_KEY, NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY, Show when="signed-in"

When to use:

• Adding authentication and user management to an application
• Building multi-tenant B2B apps with organization-based access control
• Using pre-built sign-in/sign-up UI components with customizable theming
• Protecting routes with middleware and server-side authorization checks
• Syncing Clerk user data to your database via webhooks

Key patterns covered:

• ClerkProvider setup, environment variables, middleware configuration
• Pre-built UI components (<SignIn>, <SignUp>, <UserButton>, <Show>)
• Client-side hooks (useUser, useAuth, useSession, useOrganization)
• Server-side auth (auth(), currentUser()) in Server Components, Route Handlers, Server Actions
• Middleware route protection with clerkMiddleware() and createRouteMatcher()
• Organization-based multi-tenancy with roles and permissions
• Webhook handling with Svix signature verification

When NOT to use:

• Self-hosted auth requirement (need full control over auth data storage)
• Cannot use a third-party auth service (compliance/regulatory constraints)
• Simple API key authentication (custom middleware is sufficient)
• Budget constraints prevent using a managed service

Detailed Resources:

• reference.md - Decision frameworks, hooks quick reference, Core 3 migration cheat sheet
• examples/core.md - ClerkProvider, environment variables, middleware configuration
• examples/components.md - Pre-built components, customization, appearance prop
• examples/hooks.md - useUser, useAuth, useSession, loading states, conditional rendering
• examples/server.md - Server Components, API routes, Server Actions, webhook handling
• examples/organizations.md - Organization management, roles, permissions, RBAC

Philosophy

Clerk is a managed authentication platform that handles the entire auth lifecycle: sign-up, sign-in, session management, user profiles, organizations, and MFA. Instead of building auth from scratch, you integrate Clerk's SDK and pre-built components.

Core principles:

1. Defense in depth -- Protect routes at the middleware layer AND verify auth at every data access point. Middleware alone is insufficient (CVE-2025-29927 demonstrated middleware bypass vulnerabilities).
2. Server-first auth -- Use auth() and currentUser() in Server Components and Route Handlers. Only use client hooks (useUser, useAuth) when you need reactive client-side state.
3. Pre-built over custom -- Use Clerk's <SignIn>, <SignUp>, <UserButton> components. Only build custom flows when the pre-built components genuinely cannot meet requirements.
4. Organizations for multi-tenancy -- Use Clerk Organizations with roles and permissions for B2B apps. Do not build custom tenant systems on top of Clerk's user model.
5. Webhook-driven sync -- Sync Clerk data to your database via webhooks, not by polling. Always verify webhook signatures with verifyWebhook.

When to use Clerk:

• You need auth quickly with minimal custom code
• You want pre-built UI components for sign-in/sign-up/user management
• You need organization-based multi-tenancy with RBAC
• You want managed MFA, SSO (SAML/OIDC), and social login

When NOT to use Clerk:

• You need full control over auth data storage (self-hosted requirement)
• You cannot use a third-party auth service (compliance/regulatory)
• Your app only needs simple API key authentication
• Budget constraints prevent using a managed service

Core Patterns

Pattern 1: ClerkProvider and Middleware Setup

Every Clerk app needs <ClerkProvider> wrapping the app and clerkMiddleware() protecting routes. See examples/core.md for full setup examples.

Key rules:

• ClerkProvider goes inside <body>, not wrapping <html> -- Core 3 requires this
• Use NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY env var, never hardcode keys
• Middleware file is proxy.ts on Next.js 16+ or middleware.ts on Next.js <=15
• Webhook endpoint must be in the public routes list (verified separately by verifyWebhook)
• CLERK_WEBHOOK_SIGNING_SECRET is the env var for webhook signing (not CLERK_WEBHOOK_SECRET)

// proxy.ts (Next.js 16+) or middleware.ts (Next.js <=15)
const isPublicRoute = createRouteMatcher([
  "/",
  "/sign-in(.*)",
  "/sign-up(.*)",
  "/api/webhooks(.*)",
]);

export default clerkMiddleware(async (auth, req) => {
  if (!isPublicRoute(req)) {
    await auth.protect();
  }
});

Why good: Public routes explicitly whitelisted, everything else requires auth, webhook endpoint is public (verified separately)

// BAD: clerkMiddleware() with no callback
export default clerkMiddleware(); // All routes are PUBLIC by default!

Why bad: clerkMiddleware() without callback attaches auth data but does not protect any route

Pattern 2: Pre-Built UI Components and <Show>

Use <Show> for conditional rendering (Core 3 replacement for <SignedIn>/<SignedOut>/<Protect>). See examples/components.md for full examples.

<Show when="signed-out"><SignInButton /></Show>
<Show when="signed-in"><UserButton /></Show>
<Show when={{ role: "org:admin" }}><AdminPanel /></Show>
<Show when={(has) => has({ permission: "org:invoices:manage" })}><InvoiceManager /></Show>

Why good: <Show> is the Core 3 API, supports string/object/callback conditions, fallback prop for unauthorized

// BAD: Deprecated -- removed in Core 3
<SignedIn>...</SignedIn>
<SignedOut>...</SignedOut>
<Protect role="admin">...</Protect>

Why bad: <SignedIn>, <SignedOut>, <Protect> removed in Core 3, use <Show> with when prop

Sign-in/sign-up pages require catch-all route segments [[...sign-in]] for multi-step flows (MFA, OAuth callbacks).

Pattern 3: Client-Side Hooks

Use hooks in Client Components for reactive auth state. Always check isLoaded before accessing data. See examples/hooks.md for complete examples.

"use client";
const { isLoaded, isSignedIn, user } = useUser();
if (!isLoaded) return <div>Loading...</div>;
if (!isSignedIn) return <div>Please sign in</div>;
// Now safe to access user.firstName, user.emailAddresses, etc.

Why good: Prevents hydration errors from accessing undefined during init, prevents TypeError from accessing null when signed out

// BAD: Accessing user without guards
const { user } = useUser();
return <p>{user.firstName}</p>; // Runtime error when isLoaded=false or isSignedIn=false

Why bad: user is undefined until loaded and null when signed out

Key hooks:

• useUser() -- user profile data (name, email, avatar)
• useAuth() -- session tokens (getToken()), userId, orgId, has() for authorization
• useOrganization() -- active org data, membership, paginated members list
• useReverification() -- re-authenticate before sensitive actions (Core 3)

In Core 3, getToken() throws ClerkOfflineError when offline instead of returning null -- always wrap in try/catch.

Pattern 4: Server-Side Authentication

Use auth() and currentUser() from @clerk/nextjs/server. See examples/server.md for complete examples.

auth() -- lightweight, no API call, reads session claims:

import { auth } from "@clerk/nextjs/server";
const { userId, orgId, isAuthenticated, redirectToSignIn } = await auth();
if (!isAuthenticated) return redirectToSignIn();

currentUser() -- full user object, makes Backend API call:

import { currentUser } from "@clerk/nextjs/server";
const user = await currentUser();
// CRITICAL: Pick safe fields before passing to client components
const safeData = { firstName: user.firstName, imageUrl: user.imageUrl };

Why critical: currentUser() returns privateMetadata that must never reach the client. Explicitly pick fields.

Defense in depth: Auth MUST be checked in every Server Action and Route Handler, not just middleware:

"use server";
const { userId } = await auth();
if (!userId) throw new Error("Unauthorized");
// userId scopes all queries/mutations to current user

Pattern 5: Authorization with Roles and Permissions

Use auth.protect() or has() for granular access control. See examples/organizations.md for RBAC patterns.

// Middleware: role-based route protection
await auth.protect((has) => has({ role: "org:admin" }));

// Server Component: permission check
const { has } = await auth();
if (!has({ permission: "org:invoices:delete" })) redirect("/dashboard");

// Client: conditional rendering by role
<Show when={{ role: "org:admin" }}><AdminPanel /></Show>

Permission format: org:<feature>:<action> (e.g., org:invoices:create, org:reports:read)

Default roles: org:admin (all permissions), org:member (read-only). Custom roles configured in Clerk Dashboard.

Pattern 6: Webhook Handling

Use Clerk webhooks (via Svix) to sync user data to your database. See examples/server.md for complete handler.

import { verifyWebhook } from "@clerk/nextjs/webhooks";

export async function POST(req: Request) {
  const evt = await verifyWebhook(req); // reads CLERK_WEBHOOK_SIGNING_SECRET env var
  // evt.type: "user.created" | "user.updated" | "user.deleted" | "organization.*" | ...
  // evt.data: UserJSON | OrganizationJSON | etc.
}

Why good: verifyWebhook validates Svix signature, always returns 200 to prevent retries, webhook route is public in middleware (verified by signature not by auth)

// BAD: No signature verification
const body = await req.json(); // Anyone can POST fake data!
await db.insert(users).values(body.data); // Security vulnerability

Why bad: No signature verification means anyone can send fake webhook events to your endpoint

Import types from @clerk/shared/types (Core 3): UserJSON, OrganizationJSON, OrganizationMembershipJSON

Decision Framework

Client vs Server Auth

Where do you need auth data?
|-- Server Component, Route Handler, Server Action
|   |-- Need just userId/sessionId? --> auth()
|   |-- Need full user object? --> currentUser()
|   |-- Need to protect the route? --> auth().protect()
|   +-- Need org context? --> auth() returns orgId, orgRole
|
+-- Client Component (interactive UI)
    |-- Need user profile data? --> useUser()
    |-- Need session/token data? --> useAuth()
    |-- Need org data? --> useOrganization()
    +-- Need low-level Clerk API? --> useClerk()

Route Protection Strategy

What kind of route is it?
|-- Public (landing, sign-in, sign-up, webhooks)
|   +-- Add to isPublicRoute matcher, skip auth.protect()
|
|-- Authenticated (dashboard, profile, settings)
|   +-- auth.protect() in middleware + auth() check in data layer
|
+-- Authorized (admin, org-specific, permission-gated)
    |-- Role-based? --> auth.protect({ role: "org:admin" })
    +-- Permission-based? --> auth.protect((has) => has({ permission: "org:feature:action" }))

Component Choice

What auth UI do you need?
|-- Full sign-in page --> <SignIn /> on catch-all route
|-- Full sign-up page --> <SignUp /> on catch-all route
|-- Sign-in button (modal) --> <SignInButton />
|-- User avatar + menu --> <UserButton />
|-- Full profile editor --> <UserProfile />
|-- Org switcher --> <OrganizationSwitcher />
+-- Conditional content --> <Show when="signed-in"> or <Show when={{ role: "..." }}>

RED FLAGS

High Priority Issues:

• Importing server helpers from @clerk/nextjs instead of @clerk/nextjs/server (breaks in Server Components)
• Using deprecated <SignedIn>/<SignedOut>/<Protect> components (removed in Core 3)
• Passing full currentUser() object to client components (leaks privateMetadata)
• Trusting webhook payloads without verifyWebhook signature verification
• Relying solely on middleware for route protection (middleware can be bypassed)
• Importing types from @clerk/types instead of @clerk/shared/types (Core 3 rename)

Medium Priority Issues:

• Not checking isLoaded before accessing hook data (causes hydration errors)
• Using currentUser() on the client side (it is server-only)
• Hardcoding Clerk keys instead of using environment variables
• Using authMiddleware() (deprecated, replaced by clerkMiddleware())
• Not making webhook endpoint public in middleware matcher
• Using CLERK_WEBHOOK_SECRET instead of CLERK_WEBHOOK_SIGNING_SECRET

Common Mistakes:

• Naming middleware file middleware.ts on Next.js 16+ (should be proxy.ts) or proxy.ts on Next.js <=15 (should be middleware.ts)
• Forgetting catch-all segments [[...sign-in]] on sign-in/sign-up pages (breaks multi-step flows)
• Using getToken() without try/catch in Core 3 (throws ClerkOfflineError when offline instead of returning null)
• Not adding prefetch={false} to <Link> components pointing at protected routes from public pages

Gotchas & Edge Cases:

• currentUser() counts against Backend API rate limits -- prefer useUser() hook on the client when possible
• auth() in Server Components is deduplicated per request (safe to call multiple times)
• <Show when={{ role: "org:admin" }}> requires an active organization in the session
• Organization roles use the org: prefix (e.g., org:admin, org:member, org:billing)
• Clerk Core 3 requires Node.js 20.9.0+, Next.js 15.2.3+
• @clerk/clerk-react renamed to @clerk/react in Core 3 -- update imports after upgrade

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use @clerk/nextjs/server for ALL server-side imports -- NEVER import server helpers from @clerk/nextjs)

(You MUST verify webhooks using Clerk's verifyWebhook helper -- NEVER trust unverified webhook payloads)

(You MUST use <Show> component instead of deprecated <SignedIn>/<SignedOut>/<Protect> -- these are removed in Core 3)

(You MUST NOT pass the full currentUser() object to the client -- it contains privateMetadata that must stay server-side)

(You MUST protect routes in BOTH middleware AND data access layer -- middleware alone is insufficient)

Failure to follow these rules will create authentication vulnerabilities or break on Clerk Core 3.