Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving, from a client device, an ad request for an ad space being for presentation in a user interface of an application on the client device, sending, to bidder systems, a bid request on the ad space, receiving bids from the bidder systems, selecting, from the received bids, a cost-per-view bid corresponding to a first creative and a cost-per-impression bid corresponding to a second creative, and sending a message to the client device wherein the message comprises respective identifiers of the first and second creatives, and wherein the client device is configured to obtain the first creative and present the first creative in the ad space if the ad space is in view one the client device within a specified time period.