next.js/test/integration/api-support/pages/api/no-parsing.js

export const config = {
  api: {
    bodyParser: false,
  },
}

export default (req, res) => {
  return new Promise((resolve) => {
    if (!req.body) {
      let buffer = ''
      req.on('data', (chunk) => {
        buffer += chunk
      })

      req.on('end', () => {
        res.status(200).json(JSON.parse(Buffer.from(buffer).toString()))
        resolve()
      })
    }
  })
}