$ npx envinfo --system --binaries --browsers --npmPackages "{next,react,next-auth,@auth/*}"
  System:
    OS: macOS 14.0
    CPU: (8) arm64 Apple M2
    Memory: 590.48 MB / 16.00 GB
    Shell: 3.6.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 18.18.0 - ~/.local/share/nvm/v18.18.0/bin/node
    Yarn: 1.22.19 - ~/.local/share/nvm/v18.18.0/bin/yarn
    npm: 9.8.1 - ~/.local/share/nvm/v18.18.0/bin/npm
    pnpm: 8.11.0 - ~/.local/share/nvm/v18.18.0/bin/pnpm
  Browsers:
    Chrome: 119.0.6045.199
    Safari: 17.0
  npmPackages:
    next: latest => 14.0.3 
    next-auth: 5.0.0-beta.2 => 5.0.0-beta.2 
    react: ^18.2.0 => 18.2.0 

import type { NextApiRequest, NextApiResponse } from "next";
import { auth } from "auth";

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse,
) {
  const session = await auth(req, res);
  return res.status(200).json({session});
}

 ⨯ Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/howard/git/next-auth-v5-example/node_modules/next/headers' imported from /Users/howard/git/next-auth-v5-example/node_modules/next-auth/lib/index.js
Did you mean to import next-auth-v5-example/node_modules/next/headers.js?
    at new NodeError (node:internal/errors:405:5)
    at finalizeResolution (node:internal/modules/esm/resolve:327:11)
    at moduleResolve (node:internal/modules/esm/resolve:946:10)
    at defaultResolve (node:internal/modules/esm/resolve:1132:11)
    at nextResolve (node:internal/modules/esm/loader:163:28)
    at ESMLoader.resolve (node:internal/modules/esm/loader:835:30)
    at ESMLoader.getModuleJob (node:internal/modules/esm/loader:424:18)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:77:40)
    at link (node:internal/modules/esm/module_job:76:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}

 ⨯ node_modules/next-auth/lib/index.js (117:0) @ eval
 ⨯ TypeError: Cannot read properties of undefined (reading 'append')
    at eval (webpack-internal:///(api)/./node_modules/next-auth/lib/index.js:128:52)
    at Array.forEach (<anonymous>)
    at cloneSetCookie (webpack-internal:///(api)/./node_modules/next-auth/lib/index.js:128:21)
    at eval (webpack-internal:///(api)/./node_modules/next-auth/lib/index.js:73:13)
    at async handler (webpack-internal:///(api)/./pages/api/endpoint.ts:11:21) {
  page: '/api/endpoint'
}
null

git clone -b api-route-bug-repro https://github.com/howard36/next-auth-v5-example.git
cd next-auth-v5-example
npm install
cp .env.local.example .env.local
npm run dev