Auth.js (NextAuth v5) Patterns

Quick Guide: Configure Auth.js in a root auth.ts file exporting { auth, handlers, signIn, signOut } from NextAuth(). Use the unified auth() function everywhere (Server Components, Route Handlers, middleware). Default session strategy is JWT (cookie-based); add a database adapter for persistent sessions. Protect routes via middleware or per-page auth() checks.

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<critical_requirements>

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 configure Auth.js in a root auth.ts file and export { auth, handlers, signIn, signOut } from NextAuth())

(You MUST use the unified auth() function for server-side session access - NOT the deprecated getServerSession(), getSession(), or getToken())

(You MUST use AUTH_SECRET environment variable - NEXTAUTH_SECRET is deprecated in v5)

(You MUST use AUTH_ prefixed environment variables for provider credentials (e.g., AUTH_GITHUB_ID, AUTH_GITHUB_SECRET) - they are auto-detected)

(You MUST split auth config into auth.config.ts (Edge-compatible) and auth.ts (with adapter) when using database sessions with middleware)

(You MUST check session inside Server Actions and API routes - middleware alone is NOT sufficient for authorization)

</critical_requirements>

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Auto-detection: Auth.js, NextAuth, next-auth, authjs, auth.ts, auth.config.ts, NextAuth(), signIn, signOut, auth(), handlers, SessionProvider, useSession, AUTH_SECRET, OAuth provider, credentials provider, database adapter, @auth/prisma-adapter, @auth/drizzle-adapter, authorized callback, jwt callback, session callback, proxy auth, middleware auth

When to use:

• Adding authentication to Next.js, SvelteKit, Express, or Qwik apps
• Implementing OAuth login (GitHub, Google, Discord, etc.) with 80+ built-in providers
• Building email/magic link authentication flows
• Need JWT or database-backed session management
• Projects requiring Edge-compatible middleware authentication

When NOT to use:

• Building a custom auth system from scratch (Auth.js is opinionated)
• Need fine-grained organization/team management out of the box
• Mobile-only apps without web frontend
• Need self-hosted auth with plugin architecture

Key patterns covered:

• Auth configuration (auth.ts, auth.config.ts)
• OAuth providers (GitHub, Google, Credentials, Email)
• Session strategies (JWT vs database)
• Session access (Server Components, Route Handlers, Client Components)
• Middleware/proxy route protection
• Database adapters (Prisma, Drizzle)
• Callbacks (jwt, session, signIn, redirect)
• Role-based access control
• Edge compatibility split configuration

Detailed Resources:

• For decision frameworks and anti-patterns, see reference.md

Core patterns:

• examples/core.md - Auth configuration, providers, callbacks
• examples/session.md - Session strategies, session access patterns
• examples/middleware.md - Route protection, middleware, Edge compatibility
• examples/database.md - Database adapters, Prisma, Drizzle
• examples/patterns.md - Role-based access, magic links, account linking

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<philosophy>

Philosophy

Auth.js (v5) consolidates authentication into a single, unified API. The auth() function replaces getServerSession, getSession, withAuth, and getToken from v4 for server-side use. useSession() remains the correct client-side API. Configuration lives in a root file, not in API routes.

Core principles:

1. Framework-agnostic - Works with Next.js, SvelteKit, Express, Qwik
2. Unified API - Single auth() function for all server-side contexts
3. Provider ecosystem - 80+ built-in OAuth providers with auto-detection of AUTH_* env vars
4. JWT by default - Stateless sessions in encrypted cookies, no database required
5. Edge-compatible - Middleware runs on Edge runtime with split configuration (Next.js 16 proxy runs on Node.js)
6. Progressive complexity - Start with OAuth, add database adapter, then customize callbacks

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Auth Configuration

The central configuration file exports everything you need from NextAuth().

Basic OAuth Setup

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

export const { auth, handlers, signIn, signOut } = NextAuth({
  providers: [
    GitHub, // Auto-detects AUTH_GITHUB_ID and AUTH_GITHUB_SECRET
    Google, // Auto-detects AUTH_GOOGLE_ID, AUTH_GOOGLE_SECRET
  ],
});

// app/api/auth/[...nextauth]/route.ts
import { handlers } from "@/auth";

export const { GET, POST } = handlers;

Why good: Single config file exports all auth utilities, providers auto-detect AUTH_* env vars, API route is minimal

Environment Variables

# .env.local
AUTH_SECRET="generate-with-npx-auth-secret"  # Required
AUTH_GITHUB_ID="your-github-client-id"       # Auto-detected by GitHub provider
AUTH_GITHUB_SECRET="your-github-secret"      # Auto-detected by GitHub provider
AUTH_GOOGLE_ID="your-google-id"       # Auto-detected by Google provider
AUTH_GOOGLE_SECRET="your-google-secret"

Why good: AUTH_ prefix is standardized in v5, AUTH_SECRET replaces deprecated NEXTAUTH_SECRET, providers auto-detect credentials

---

Pattern 2: Providers

Auth.js supports OAuth, email/magic link, and credentials authentication. 80+ built-in OAuth providers auto-detect AUTH_* env vars.

// OAuth: customize profile mapping
GitHub({
  profile(profile) {
    return {
      id: String(profile.id),
      name: profile.name ?? profile.login,
      role: "user",
    };
  },
});

// Credentials: validate input, return null on failure
Credentials({
  async authorize(credentials) {
    const parsed = LoginSchema.safeParse(credentials);
    if (!parsed.success) return null;
    const user = await getUserByEmail(parsed.data.email);
    if (
      !user ||
      !(await verifyPassword(parsed.data.password, user.hashedPassword))
    )
      return null;
    return { id: user.id, name: user.name, email: user.email };
  },
});

Key rules: Validate input before DB lookup, always hash passwords, return null on failure (never throw - it leaks info). See examples/core.md for complete implementations.

---

Pattern 3: Callbacks

Four callbacks customize auth behavior. Data flows: jwt callback (enrich token) -> session callback (expose to client).

callbacks: {
  jwt({ token, user }) {
    if (user) { token.id = user.id; token.role = user.role ?? "user"; }
    return token;
  },
  session({ session, token }) {
    session.user.id = token.id as string;
    session.user.role = token.role as string;
    return session;
  },
}

Key rules: JWT callback runs on EVERY auth() call (keep lightweight), user param is only present at sign-in, never expose OAuth tokens to client. See examples/core.md for complete callback implementations including signIn and redirect.

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Pattern 4: Session Access

The unified auth() function replaces getServerSession, getSession, and getToken from v4 for server-side use. useSession() remains for Client Components.

Context	How to access session
Server Component	const session = await auth()
Route Handler	export const GET = auth(function GET(req) { req.auth })
Server Action	const session = await auth()
Middleware/Proxy	export { auth as middleware } or authorized callback
Client Component	useSession() (requires SessionProvider in layout)

Key rules: Server-side imports come from @/auth, client-side imports from next-auth/react. Never call auth() in Client Components. See examples/session.md for complete implementations.

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Pattern 5: Sign In / Sign Out Actions

Two approaches: Server Actions (recommended, progressive enhancement) or client-side.

// Server-side (recommended): import from @/auth, use Server Actions in forms
import { signIn, signOut } from "@/auth";
// In form action: await signIn("github", { redirectTo: "/dashboard" })
// In form action: await signOut({ redirectTo: "/" })

// Client-side: import from next-auth/react, use onClick handlers
import { signIn, signOut } from "next-auth/react";
// onClick: signIn("github", { callbackUrl: "/dashboard" })

Key rules: Server-side uses redirectTo, client-side uses callbackUrl. signIn() throws a NEXT_REDIRECT exception internally -- don't wrap in try/catch expecting a return value. See examples/core.md for complete implementations.

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Pattern 6: TypeScript Extensions

Extend session and JWT types via declaration merging in types/next-auth.d.ts. Declare custom fields (e.g., id, role) on Session, User, and JWT interfaces using & DefaultSession["user"] to preserve defaults. See examples/core.md for the complete type declaration example.

</patterns>

---

<integration>

Integration Guide

Auth.js is the authentication layer. It handles identity verification, session management, and route protection. It does NOT handle authorization logic (role checks, permission systems) -- that is application code.

Framework support: Auth.js works with multiple web frameworks via framework-specific packages (next-auth, @auth/sveltekit, @auth/express).

Database adapters: For database sessions, Auth.js provides adapter packages (@auth/prisma-adapter, @auth/drizzle-adapter, etc.) that integrate with your ORM. See examples/database.md.

Session strategy depends on your needs:

• JWT (default) - No database needed, works on Edge, stateless
• Database - Requires adapter, server-side session store, supports immediate revocation

Auth.js does NOT handle: fine-grained authorization/RBAC, rate limiting, or database queries beyond auth -- those are application-level concerns.

</integration>

---

<red_flags>

RED FLAGS

• Using getServerSession(authOptions) -- deprecated in v5; use auth() from your auth.ts
• Using NEXTAUTH_SECRET or NEXTAUTH_URL -- deprecated; use AUTH_SECRET (URL is auto-detected)
• Credentials provider without rate limiting -- vulnerable to brute-force attacks
• Exposing OAuth tokens to client via session callback -- keep accessToken/refreshToken server-side only
• Middleware/proxy as sole authorization -- runs before rendering but does not replace per-route checks in Server Actions/API routes
• Database adapter imported in middleware -- database ORMs can't run on Edge runtime (Next.js 14/15); split config into auth.config.ts + auth.ts
• Wrapping signIn() in try/catch -- it throws a NEXT_REDIRECT exception internally (this is intentional)
• JWT callback querying database on every call -- runs on EVERY auth() invocation; keep it lightweight

See reference.md for the complete anti-pattern list, gotchas, and migration table.

</red_flags>

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<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST configure Auth.js in a root auth.ts file and export { auth, handlers, signIn, signOut } from NextAuth())

(You MUST use the unified auth() function for server-side session access - NOT the deprecated getServerSession(), getSession(), or getToken())

(You MUST use AUTH_SECRET environment variable - NEXTAUTH_SECRET is deprecated in v5)

(You MUST use AUTH_ prefixed environment variables for provider credentials (e.g., AUTH_GITHUB_ID, AUTH_GITHUB_SECRET) - they are auto-detected)

(You MUST split auth config into auth.config.ts (Edge-compatible) and auth.ts (with adapter) when using database sessions with middleware)

(You MUST check session inside Server Actions and API routes - middleware alone is NOT sufficient for authorization)

Failure to follow these rules will cause authentication failures, expose deprecated patterns, or create security vulnerabilities.

</critical_reminders>