Dynamic content item creation

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for providing a customized content item. A method includes: receiving a content request; identifying one or more user device attributes and/or a context of the request; identifying a content item having distribution parameters matching the context and/or user device attributes and including one or more variables that are resolved based on the context or the one or more user device attributes; selecting, as modifying data, a subset of feed data to which the one or more variables for the content item will be resolved, including: identifying the feed data, evaluating the feed data to locate the subset; and selecting the modifying data; and modifying the content item, at least in part, by including, in place of the one or more variables, the selected modifying data; and providing the modified content item in response to the request.