Next.js Pages Router with the Auth0 SDK: v3 patterns vs v4

# Next.js Pages Router auth

## Which SDK version

The `@auth0/nextjs-auth0` v4 SDK is App Router first. Pages Router apps historically used v3 (`handleAuth`, `withPageAuthRequired`, `getSession(req, res)`). If the app is on Pages Router, either stay on v3 patterns or migrate the app to App Router first. Do not import v4 App Router helpers into `pages/api` routes.

## v3 setup

`pages/api/auth/[...auth0].js`:

```
import { handleAuth } from "@auth0/nextjs-auth0";
export default handleAuth();
```

This creates `/api/auth/login`, `/api/auth/callback`, `/api/auth/logout`, `/api/auth/me`.

Protect a page:

```
import { withPageAuthRequired } from "@auth0/nextjs-auth0";

export const getServerSideProps = withPageAuthRequired();

export default function Protected({ user }) { ... }
```

Read the session in API routes:

```
import { getSession } from "@auth0/nextjs-auth0";

export default async function handler(req, res) {
  const session = await getSession(req, res);
  if (!session) return res.status(401).end();
  ...
}
```

## Env vars (v3)

```
AUTH0_SECRET [your value]
AUTH0_BASE_URL [your app base URL]
AUTH0_ISSUER_BASE_URL https://YOUR-TENANT-DOMAIN
AUTH0_CLIENT_ID [your value]
AUTH0_CLIENT_SECRET [your value]
```

Note the v3 names differ from v4 (AUTH0_BASE_URL vs APP_BASE_URL). Copying a v4 env file into a v3 app (or the reverse) silently breaks everything.

## Callback registration

Allowed Callback URLs: `AUTH0_BASE_URL` + `/api/auth/callback`. Same exact-match rules as everywhere else.

## Checklist

- Router version and SDK version agree; v4 helpers are App Router only.
- Env var names match the SDK major version in package.json.