`getServerSideProps()` surfaces a `ServerResponse` object through the `res` property of its `context` arg. This object is not intended to be accessed or changed after `getServerSideProps()` resolves.
This is because the framework tries to optimize when items like headers or status codes are flushed to the browser. If they are changed after `getServerSideProps()` completes, we can't guarantee that the changes will work.
For this reason, accessing the object after this time is disallowed.
#### Possible Ways to Fix It
You can fix this error by moving any access of the `res` object into `getServerSideProps()` itself or any functions that run before `getServerSideProps()` returns.
If you’re using a custom server and running into this problem due to session middleware like `next-session` or `express-session`, try installing the middleware in the server instead of `getServerSideProps()`.
