Without `@types/node`, the bundled declarations type every `req.headers` value as a string.
Reading `headers["set-cookie"]` is then refused, and a `{ ...req.headers }` copy holds `set-cookie` as one string joined with `", "`, where Node holds an array.
Read `headersDistinct["set-cookie"]`, or install `@types/node`, whose `headers["set-cookie"]` is `string[]`.