Methods, systems, and apparatus, including computer programs encoded on computer storage media, for generating a video stream preview. One of the methods includes receiving, from a user device, a request for a video stream preview. A payload of data is transmitted from a session system to the user device over a network. A first measure of network transmission quality is determined based on a result of transmitting the payload to the user device over the network from the session system. A second measure of network transmission quality is determined for data transmitted over the network from a session host to the session system. A video stream preview is generated from an example video stream obtained from the user device by degrading the quality of the example video stream according to the first measure of network transmission quality and the second measure of network transmission quality. The video stream preview is displayed on the user device.