Systems of the present disclosure provide a versatile, reusable mock server to respond to Application-Programming-Interface (API) requests. The mock server receives an API request and a cookie associated with the API request. The API server identifies response instructions found in the cookie. The response instructions may include a static response value, a name of an API server for the mock server to imitate, or code for the mock server to execute in the process of generating a mock API response. The mock server generates a mock API response based on the response instructions and sends the mock API response in reply to the API request.