Auth Expert

🔑 Skill: auth-expert (v2.0.0)

🌟 Overview & Vision 2026

This skill provides a deep-dive into Auth.js v5, the successor to NextAuth.js. It is specifically optimized for the Next.js 16.1.1 App Router, React 19.2 Server Actions, and Edge Runtime constraints.

In 2026, authentication is no longer just about a login form; it's about a unified session architecture that works across Server Components, Client Components, Middleware, and Server Actions without performance trade-offs.

📚 Table of Contents

1. Core Architecture: The Dual-Config Pattern
2. Quick Start: The 5-Minute Setup
3. Universal Auth: The auth() Protocol
4. Route Protection & Middleware
5. React 19 Server Actions & Authentication
6. The "Do Not" List (Common Pitfalls)
7. Security Hardening Checklist
8. Advanced Patterns: Types & Extensibility
9. Deep Dives (References)

🏗️ Core Architecture: The Dual-Config Pattern

To avoid "Module not found" errors in the Edge Runtime (Middleware), Auth.js v5 requires a split configuration.

1. Edge-Compatible Logic (auth.config.ts)

This file contains only logic that can run on the Edge. Never import database adapters or heavy Node.js libraries here.

import type { NextAuthConfig } from "next-auth";
import GitHub from "next-auth/providers/github";

// This is the base config for Middleware and Edge functions
export const authConfig = {
  providers: [
    GitHub({
      clientId: process.env.AUTH_GITHUB_ID,
      clientSecret: process.env.AUTH_GITHUB_SECRET,
    }),
  ],
  callbacks: {
    authorized({ auth, request: { nextUrl } }) {
      const isLoggedIn = !!auth?.user;
      const isOnDashboard = nextUrl.pathname.startsWith("/dashboard");
      
      if (isOnDashboard) {
        if (isLoggedIn) return true;
        return false; // Redirect unauthenticated users to login
      }
      return true;
    },
  },
} satisfies NextAuthConfig;

2. Full Logic (auth.ts)

This file imports authConfig and adds the database adapter (Node.js runtime).

import NextAuth from "next-auth";
import { PrismaAdapter } from "@auth/prisma-adapter";
import { prisma } from "@/lib/db"; // Your Prisma client
import { authConfig } from "./auth.config";

export const { 
  handlers, 
  auth, 
  signIn, 
  signOut,
  update // Used to update the session manually
} = NextAuth({
  adapter: PrismaAdapter(prisma),
  session: { strategy: "jwt" }, // JWT is mandatory for Edge compatibility
  ...authConfig,
});

⚡ Quick Start: The 5-Minute Setup

Step 1: Install Dependencies

bun add next-auth@beta @auth/prisma-adapter

Step 2: Configure Environment Variables

# Generate with: npx auth secret
AUTH_SECRET=your_ultra_secure_secret_here

# Provider Credentials
AUTH_GITHUB_ID=...
AUTH_GITHUB_SECRET=...

Step 3: Create the Route Handler

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

import { handlers } from "@/auth";
export const { GET, POST } = handlers;

🌍 Universal Auth: The auth() Protocol

One of the most powerful features of v5 is the universal auth() function. It works everywhere.

1. In Server Components (RSC)

import { auth } from "@/auth";

export default async function ProfilePage() {
  const session = await auth();
  
  if (!session) {
    return <div>Not logged in.</div>;
  }

  return (
    <div>
      <h1>Welcome, {session.user?.name}</h1>
      <p>Role: {session.user?.role}</p>
    </div>
  );
}

2. In Server Actions

"use server"
import { auth } from "@/auth";

export async function createPost(content: string) {
  const session = await auth();
  if (!session) throw new Error("Unauthorized");

  // Securely access user data
  const authorId = session.user.id;
  // ... database logic
}

3. In Client Components

Use the useSession hook from next-auth/react. Note: You must wrap your app in <SessionProvider>.

"use client"
import { useSession } from "next-auth/react";

export function UserButton() {
  const { data: session, status } = useSession();

  if (status === "loading") return <div>...</div>;
  if (!session) return <button>Login</button>;

  return <img src={session.user.image} alt="Avatar" />;
}

🛡️ Route Protection & Middleware

The middleware.ts file is the first line of defense.

import NextAuth from "next-auth";
import { authConfig } from "./auth.config";

export default NextAuth(authConfig).auth;

export const config = {
  // Matcher allows excluding static files and api routes if needed
  matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
};

🧪 React 19 Server Actions & Authentication

React 19 introduces improved handling for forms and transitions. Use them for your login flows.

Login Action Example

"use server"
import { signIn } from "@/auth";
import { AuthError } from "next-auth";

export async function authenticate(
  prevState: string | undefined,
  formData: FormData,
) {
  try {
    await signIn("credentials", formData);
  } catch (error) {
    if (error instanceof AuthError) {
      switch (error.type) {
        case "CredentialsSignin":
          return "Invalid credentials.";
        default:
          return "Something went wrong.";
      }
    }
    throw error;
  }
}

🚫 The "Do Not" List (Common Pitfalls)

🏁 Security Hardening Checklist

• Rotate Secrets: Ensure AUTH_SECRET is rotated periodically.
• Secure Cookies: In production, cookies are automatically secure, but verify trustHost.
• Role Validation: Don't just check !!session, check session.user.role === 'admin'.
• Rate Limiting: Apply rate limiting to app/api/auth endpoints.
• HTTPS Only: Ensure your site is served over HTTPS to protect session cookies.

🔧 Advanced Patterns: Types & Extensibility

Extending the session object is a common requirement.

next-auth.d.ts

import { type DefaultSession } from "next-auth";

export type ExtendedUser = DefaultSession["user"] & {
  role: "ADMIN" | "USER";
  id: string;
};

declare module "next-auth" {
  interface Session {
    user: ExtendedUser;
  }
}

Callback Logic for Roles

callbacks: {
  async session({ token, session }) {
    if (token.sub && session.user) {
      session.user.id = token.sub;
    }
    if (token.role && session.user) {
      session.user.role = token.role as "ADMIN" | "USER";
    }
    return session;
  },
  async jwt({ token }) {
    if (!token.sub) return token;
    
    const existingUser = await getUserById(token.sub);
    if (!existingUser) return token;

    token.role = existingUser.role;
    return token;
  }
}

📖 Deep Dives (References)

For more complex scenarios, consult the following specialized guides:

• Edge Compatibility & Runtimes
• Advanced Providers & Custom Logic
• Security Best Practices

🛠️ Troubleshooting

"JWT expired" or "Invalid Signature"

Check if your AUTH_SECRET matches across all environments. If you changed it, all current sessions will be invalidated.

"Redirect recursion"

This usually happens when your middleware.ts logic redirects to /login but doesn't exclude /login from the middleware matcher or the authorized check.

🗄️ Advanced Database Integration: Custom Schemas

When using Prisma or Drizzle, you might need to customize the table names or add extra fields.

Prisma Schema Example

// schema.prisma
model User {
  id            String    @id @default(cuid())
  name          String?
  email         String?   @unique
  emailVerified DateTime?
  image         String?
  role          UserRole  @default(USER)
  accounts      Account[]
  sessions      Session[]
  createdAt     DateTime  @default(now())
  updatedAt     DateTime  @updatedAt
}

enum UserRole {
  ADMIN
  USER
}

model Account {
  id                 String  @id @default(cuid())
  userId             String
  type               String
  provider           String
  providerAccountId  String
  refresh_token      String? @db.Text
  access_token       String? @db.Text
  expires_at         Int?
  token_type         String?
  scope              String?
  id_token           String? @db.Text
  session_state      String?

  user User @relation(fields: [userId], references: [id], onDelete: Cascade)

  @@unique([provider, providerAccountId])
}

Drizzle Schema Example

import {
  timestamp,
  pgTable,
  text,
  primaryKey,
  integer,
} from "drizzle-orm/pg-core"
import type { AdapterAccount } from '@auth/core/adapters'

export const users = pgTable("user", {
  id: text("id").notNull().primaryKey(),
  name: text("name"),
  email: text("email").notNull(),
  emailVerified: timestamp("emailVerified", { mode: "date" }),
  image: text("image"),
})

export const accounts = pgTable(
  "account",
  {
    userId: text("userId")
      .notNull()
      .references(() => users.id, { onDelete: "cascade" }),
    type: text("type").$type<AdapterAccount["type"]>().notNull(),
    provider: text("provider").notNull(),
    providerAccountId: text("providerAccountId").notNull(),
    refresh_token: text("refresh_token"),
    access_token: text("access_token"),
    expires_at: integer("expires_at"),
    token_type: text("token_type"),
    scope: text("scope"),
    id_token: text("id_token"),
    session_state: text("session_state"),
  },
  (account) => ({
    compoundKey: primaryKey({ columns: [account.provider, account.providerAccountId] }),
  })
)

🌐 Internationalization (i18n) & Authentication

Handling redirects and error messages in multiple languages.

i18n-Aware Redirects

If you are using next-intl or a similar library, your auth logic should respect the current locale.

// auth.config.ts
callbacks: {
  authorized({ auth, request: { nextUrl } }) {
    const locale = nextUrl.pathname.split('/')[1] || 'en';
    const isLoggedIn = !!auth?.user;
    
    if (nextUrl.pathname.startsWith(`/${locale}/dashboard`) && !isLoggedIn) {
      return Response.redirect(new URL(`/${locale}/login`, nextUrl));
    }
    return true;
  }
}

🛡️ Multi-Factor Authentication (MFA) in 2026

While Auth.js doesn't have a "one-click" MFA, you can implement it using the callbacks and a custom session state.

Implementation Strategy

1. First Factor: Standard login (OAuth/Credentials).
2. MFA Check: After login, check if MFA is enabled for the user.
3. Temporary Session: If MFA is required, mark the JWT as mfa_pending: true.
4. Verification: Redirect to a /verify-mfa page.
5. Finalize: Once verified, update the JWT to mfa_verified: true.

// Example JWT Callback for MFA
async jwt({ token, user }) {
  if (user) {
    token.mfa_required = user.hasMfa;
    token.mfa_verified = false;
  }
  return token;
}

// In Middleware
if (token?.mfa_required && !token?.mfa_verified && pathname !== "/verify-mfa") {
  return Response.redirect(new URL("/verify-mfa", nextUrl));
}

🧪 Testing Authentication

Testing auth flows is critical for preventing regressions.

1. Integration Testing with Vitest

Mock the auth function to test protected components.

import { vi, test, expect } from 'vitest';
import { auth } from '@/auth';

vi.mock('@/auth', () => ({
  auth: vi.fn(),
}));

test('shows secret content to logged-in users', async () => {
  (auth as any).mockResolvedValue({ user: { name: 'Test User' } });
  // Render component and assert
});

2. E2E Testing with Playwright

Use a dedicated test user and the storageState feature to stay logged in across tests.

import { test, expect } from '@playwright/test';

test('can access dashboard after login', async ({ page }) => {
  await page.goto('/login');
  await page.fill('input[name="email"]', 'test@example.com');
  await page.fill('input[name="password"]', 'password');
  await page.click('button[type="submit"]');
  
  await expect(page).toHaveURL('/dashboard');
});

🔗 External Resources

• Official Auth.js v5 Documentation
• Auth.js GitHub Repository
• Prisma Adapter Documentation

📜 Complete Implementation Example (The "Gold Standard")

auth.config.ts

import type { NextAuthConfig } from "next-auth";
import GitHub from "next-auth/providers/github";
import Credentials from "next-auth/providers/credentials";

export const authConfig = {
  providers: [
    GitHub,
    Credentials({
      async authorize(credentials) {
        // Validation logic here
        return null; 
      }
    })
  ],
  callbacks: {
    authorized({ auth, request: { nextUrl } }) {
      // Logic here
      return true;
    }
  }
} satisfies NextAuthConfig;

auth.ts

import NextAuth from "next-auth";
import { DrizzleAdapter } from "@auth/drizzle-adapter";
import { db } from "@/db";
import { authConfig } from "./auth.config";

export const { auth, handlers, signIn, signOut } = NextAuth({
  adapter: DrizzleAdapter(db),
  session: { strategy: "jwt" },
  ...authConfig,
});

middleware.ts

import NextAuth from "next-auth";
import { authConfig } from "./auth.config";

export default NextAuth(authConfig).auth;

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

Updated: January 22, 2026 - 15:18