The invention discloses a method for specifying Web service behavior that includes a step of exposing an internal logic flow of a Web service to clients. The internal logic flow can include at least one activity, which is defined by a set of machine-readable instructions. A special invocation command for the Web service can be received from a client. The special invocation command can specify a modification for the activity. An instance of the Web service can be invoked for the client, where the Web service instance implements the modification instead of the activity.