Improved techniques for testing an application to identify bugs. An API request body, which includes input data, is transmitted to a service to exercise the service. An error type response is received from the service, where the response indicates how the service handled the input data. The response is then used to determine an error type response coverage of the service. The coverage is then expanded by repeatedly performing a number of operations until a threshold metric is satisfied. For instance, in response to learning how previously-used input data impacted the coverage, new input data is generated. This new input data is designed to trigger new types of responses from the service. The new input is sent to the service, and a new error type response is received. These processes are repeated in an effort to expand the error type response coverage of the remote service.