Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for distributing content items. In one aspect, a method includes identifying a content item that is eligible to be provided in response to a content item request and is associated with a bid. It is determined that the identified content item includes two or more different interactive elements that each cause different actions to be initiated in response to user interaction with the different interactive elements. An auction score is determined for the content item based on a function of the bid, a bid modifier for each different interaction, and a probability that each interaction will be invoked. The content item is selected to be provided based on the auction score. Data that cause presentation of the selected content item at a user device are output.