Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium, including a method for providing content. A first call for content to be published on a resource associated with a publisher is received from a requesting device. A determination is made when the publisher has authorized additional content item calls to be automatically generated responsive to receipt of the first call. A first content item responsive to the first call is determined. A message including call parameters is generated that define publisher or serving system criteria for the additional content items. The first content item and message responsive to the first call are provided. A second call for content, in conformance with the call parameters, is received after processing the message. A second different content item that is responsive to the second call is provided for publication on the resource along with the first content item.