I believe the issue is as Michael Logothetis suggested. You are having issues because of the %2F. Flask is automatically decoding the parameter you're passing as a string by default (see https://flask.palletsprojects.com/en/stable/api/#url-route-registrations). You can specify a converter to accommodate for the possibility that your string will include a / by using /getExposeIds/<path:cookie> as your endpoint.