nextjs-authentication

SKILL.md

Next.js Authentication

Overview

Provides authentication implementation patterns for Next.js 15+ App Router using Auth.js 5 (NextAuth.js), covering the complete authentication lifecycle from initial setup to production-ready role-based access control implementations.

When to Use

Setting up Auth.js 5 from scratch or adding OAuth providers

Implementing protected routes with Middleware

Handling authentication in Server Components and Server Actions

Implementing role-based access control (RBAC)

Creating credential-based or OAuth sign-in/sign-out flows

Instructions

1. Install Dependencies

Install Auth.js v5 (beta) for Next.js App Router:
npm install next-auth@beta

2. Configure Environment Variables

Create .env.local with required variables:

# Required for Auth.js
AUTH_SECRET="your-secret-key-here"
AUTH_URL="http://localhost:3000"

# OAuth Providers (add as needed)
GITHUB_ID="your-github-client-id"
GITHUB_SECRET="your-github-client-secret"
GOOGLE_CLIENT_ID="your-google-client-id"
GOOGLE_CLIENT_SECRET="your-google-client-secret"

Generate AUTH_SECRET with:

openssl rand -base64 32

3. Create Auth Configuration

Create auth.ts in the project root with providers and callbacks:

import NextAuth from "next-auth";
import GitHub from "next-auth/providers/github";
import Google from "next-auth/providers/google";

export const {
  handlers: { GET, POST },
  auth,
  signIn,
  signOut,
} = NextAuth({
  providers: [
    GitHub({
      clientId: process.env.GITHUB_ID!,
      clientSecret: process.env.GITHUB_SECRET!,
    }),
    Google({
      clientId: process.env.GOOGLE_CLIENT_ID!,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
    }),
  ],
  callbacks: {
    async jwt({ token, user }) {
      if (user) {
        token.id = user.id;
      }
      return token;
    },
    async session({ session, token }) {
      if (token) {
        session.user.id = token.id as string;
      }
      return session;
    },
  },
  pages: {
    signIn: "/login",
    error: "/error",
  },
});

4. Create API Route Handler

Create app/api/auth/[...nextauth]/route.ts:

export { GET, POST } from "@/auth";

5. Add Middleware for Route Protection

Create middleware.ts in the project root:

import { auth } from "@/auth";
import { NextResponse } from "next/server";

export default auth((req) => {
  const { nextUrl } = req;
  const isLoggedIn = !!req.auth;
  const isApiAuthRoute = nextUrl.pathname.startsWith("/api/auth");
  const isPublicRoute = ["/", "/login", "/register"].includes(nextUrl.pathname);
  const isProtectedRoute = nextUrl.pathname.startsWith("/dashboard");

  if (isApiAuthRoute) return NextResponse.next();

  if (!isLoggedIn && isProtectedRoute) {
    return NextResponse.redirect(new URL("/login", nextUrl));
  }

  if (isLoggedIn && nextUrl.pathname === "/login") {
    return NextResponse.redirect(new URL("/dashboard", nextUrl));
  }

  return NextResponse.next();
});

export const config = {
  matcher: ["/((?!_next/static|_next/image|favicon.ico|.*\\.png$).*)"],
};

6. Access Session in Server Components

Use the auth() function to access session in Server Components:

import { auth } from "@/auth";
import { redirect } from "next/navigation";

export default async function DashboardPage() {
  const session = await auth();

  if (!session) {
    redirect("/login");
  }

  return (
    <div>
      <h1>Welcome, {session.user.name}</h1>
    </div>
  );
}

7. Secure Server Actions

Always verify authentication in Server Actions before mutations:

"use server";

import { auth } from "@/auth";

export async function createTodo(formData: FormData) {
  const session = await auth();

  if (!session?.user) {
    throw new Error("Unauthorized");
  }

  // Proceed with protected action
  const title = formData.get("title") as string;
  await db.todo.create({
    data: { title, userId: session.user.id },
  });
}

8. Handle Sign-In/Sign-Out

Create a login page with server action:

// app/login/page.tsx
import { signIn } from "@/auth";
import { redirect } from "next/navigation";

export default function LoginPage() {
  async function handleLogin(formData: FormData) {
    "use server";

    const result = await signIn("credentials", {
      email: formData.get("email"),
      password: formData.get("password"),
      redirect: false,
    });

    if (result?.error) {
      return { error: "Invalid credentials" };
    }

    redirect("/dashboard");
  }

  return (
    <form action={handleLogin}>
      <input name="email" type="email" placeholder="Email" required />
      <input name="password" type="password" placeholder="Password" required />
      <button type="submit">Sign In</button>
    </form>
  );
}

For client-side sign-out:

"use client";

import { signOut } from "next-auth/react";

export function SignOutButton() {
  return <button onClick={() => signOut()}>Sign Out</button>;
}

9. Implement Role-Based Access

Check roles in Server Components:

import { auth } from "@/auth";
import { unauthorized } from "next/navigation";

export default async function AdminPage() {
  const session = await auth();

  if (session?.user?.role !== "admin") {
    unauthorized();
  }

  return <AdminDashboard />;
}

10. Extend TypeScript Types

Create types/next-auth.d.ts for type-safe sessions:

import { DefaultSession } from "next-auth";

declare module "next-auth" {
  interface Session {
    user: {
      id: string;
      role: "user" | "admin";
    } & DefaultSession["user"];
  }

  interface User {
    role?: "user" | "admin";
  }
}

declare module "next-auth/jwt" {
  interface JWT {
    id?: string;
    role?: "user" | "admin";
  }
}

Examples

Example 1: Complete Protected Dashboard

Input: User needs a dashboard accessible only to authenticated users

Implementation:

// app/dashboard/page.tsx
import { auth } from "@/auth";
import { redirect } from "next/navigation";
import { getUserTodos } from "@/app/lib/data";

export default async function DashboardPage() {
  const session = await auth();

  if (!session?.user?.id) {
    redirect("/login");
  }

  const todos = await getUserTodos(session.user.id);

  return (
    <main>
      <h1>Welcome, {session.user.name}</h1>
      <p>Email: {session.user.email}</p>
      <TodoList todos={todos} />
    </main>
  );
}

Output: Dashboard renders only for authenticated users, with their specific data.

Example 2: Role-Based Admin Panel

Input: Admin panel should be accessible only to users with "admin" role

Implementation:

// app/admin/page.tsx
import { auth } from "@/auth";
import { unauthorized } from "next/navigation";

export default async function AdminPage() {
  const session = await auth();

  if (session?.user?.role !== "admin") {
    unauthorized();
  }

  return (
    <main>
      <h1>Admin Panel</h1>
      <p>Welcome, administrator {session.user.name}</p>
    </main>
  );
}

Output: Only admin users see the panel; others get 401 error.

Example 3: Secure Server Action with Form

Input: Form submission should only work for authenticated users

Implementation:

// app/components/create-todo-form.tsx
"use server";

import { auth } from "@/auth";
import { revalidatePath } from "next/cache";

export async function createTodo(formData: FormData) {
  const session = await auth();

  if (!session?.user?.id) {
    throw new Error("Unauthorized");
  }

  const title = formData.get("title") as string;

  await db.todo.create({
    data: {
      title,
      userId: session.user.id,
    },
  });

  revalidatePath("/dashboard");
}

// Usage in component
export function CreateTodoForm() {
  return (
    <form action={createTodo}>
      <input name="title" placeholder="New todo..." required />
      <button type="submit">Add Todo</button>
    </form>
  );
}

Output: Todo created only for authenticated user; unauthorized requests throw error.

Best Practices

Use Server Components by default - Access session directly without client-side JavaScript

Minimize Client Components - Only use useSession() for reactive session updates

Cache session checks - Use React's cache() for repeated lookups in the same render

Middleware for optimistic checks - Redirect quickly, but always re-verify in Server Actions

Treat Server Actions like API endpoints - Always authenticate before mutations

Never hardcode secrets - Use environment variables for all credentials

Implement proper error handling - Return appropriate HTTP status codes

Use TypeScript type extensions - Extend NextAuth types for custom fields

Separate auth logic - Create a DAL (Data Access Layer) for consistent checks

Test authentication flows - Mock auth() function in unit tests

Constraints and Warnings

Critical Limitations

Middleware runs on Edge runtime - Cannot use Node.js APIs like database drivers

Server Components cannot set cookies - Use Server Actions for cookie operations

Session callback timing - Only called on session creation/access, not every request

Common Mistakes

// ❌ WRONG: Setting cookies in Server Component
export default async function Page() {
  cookies().set("key", "value"); // Won't work
}

// ✅ CORRECT: Use Server Action
async function setCookieAction() {
  "use server";
  cookies().set("key", "value");
}

// ❌ WRONG: Database queries in Middleware
export default auth(async (req) => {
  const user = await db.user.findUnique(); // Won't work in Edge
});

// ✅ CORRECT: Use only Edge-compatible APIs
export default auth(async (req) => {
  const session = req.auth; // This works
});

Security Considerations

Always verify authentication in Server Actions - middleware alone is not enough

Use unauthorized() for unauthenticated access, redirect() for other cases

Store sensitive tokens in httpOnly cookies

Validate all user input before processing

Use HTTPS in production

Set appropriate cookie sameSite attributes

References

references/authjs-setup.md - Complete Auth.js 5 setup guide with Prisma/Drizzle adapters

references/oauth-providers.md - Provider-specific configurations (GitHub, Google, Discord, Auth0, etc.)

references/database-adapter.md - Database session management with Prisma, Drizzle, and custom adapters

references/testing-patterns.md - Testing authentication flows with Vitest and Playwright