Auth.js Knowledge Patch

Claude Opus 4.6 knows Auth.js (NextAuth.js) through v4 and early v5 betas. This skill covers v5 changes that postdate training.

Index

Topic	Reference	Key features
v5 migration	references/v5-migration.md	Next.js 16 proxy.ts, idToken: false behavioral change
WebAuthn / Passkeys	references/webauthn-passkeys.md	Passkey provider, SimpleWebAuthn setup, Authenticator table, custom signin

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Quick Reference

Next.js 16: proxy.ts replaces middleware.ts

Next.js 16 renames middleware.ts to proxy.ts. Auth.js setup changes accordingly:

// Simple
export { auth as proxy } from "@/auth"

// Advanced: wrap with custom logic
import { auth } from "@/auth"

export const proxy = auth((req) => {
  // req.auth contains the session
})

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

For Next.js < 16, keep using middleware.ts with export { auth as middleware }.

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idToken: false behavioral change

In v5, idToken: false on a provider no longer disables ID token processing entirely. It now signals Auth.js to also visit the userinfo_endpoint for final user data. Previously it opted out of id_token validity checking altogether.

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WebAuthn / Passkeys (experimental)

Requires next-auth@5.0.0-beta.8+, @auth/prisma-adapter@1.3.0+, Node 20+.

Peer dependencies:

npm install @simplewebauthn/server@9.0.3 @simplewebauthn/browser@9.0.1

@simplewebauthn/browser is only needed for custom signin pages.

Auth config:

import Passkey from "next-auth/providers/passkey"
import { PrismaAdapter } from "@auth/prisma-adapter"

export default {
  adapter: PrismaAdapter(prisma),
  providers: [Passkey],
  experimental: { enableWebAuthn: true },
}

Custom signin page — signIn("passkey") to login, signIn("passkey", { action: "register" }) to register:

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

export default function Login() {
  const { status } = useSession()
  return (
    <div>
      {status === "authenticated" ? (
        <button onClick={() => signIn("passkey", { action: "register" })}>
          Register new Passkey
        </button>
      ) : status === "unauthenticated" ? (
        <button onClick={() => signIn("passkey")}>Sign in with Passkey</button>
      ) : null}
    </div>
  )
}

See references/webauthn-passkeys.md for the required Authenticator table schema.

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Reference Files

File	Contents
v5-migration.md	Next.js 16 proxy.ts setup, idToken: false behavioral change
webauthn-passkeys.md	Passkey provider setup, SimpleWebAuthn deps, Authenticator table DDL, custom signin page