1. A method comprising: detecting a notification condition, the notification condition comprising an event that occurred on a client device with which a server is configured to communicate; identifying a first application that is stored at the client device, wherein the first application is configured to generate data for rendering a first notification message related to the notification condition; generating the first notification message for the notification condition, the first notification message comprising one or more attributes, the one or more attributes at least specifying how the first notification message is to be presented by the first application at the client device; detecting a conflict between transmittal of the first notification message at a time that is concurrent with transmittal of a second notification message, wherein a second application is stored at the client device and is configured to generate data for rendering the second notification message and the first application differs from the second application; applying one or more conflict rules to the first notification message and the second notification message; selecting, based on application of the one or more conflict rules, the first notification message for transmittal to the client device; transmitting the first notification message to the client device; receiving, from the client device, a response to the first notification message, the response comprising data corresponding to audio input to the client device; and performing an action specific to the first application based on the response.