Mobile application activity detector

The subject matter of this specification generally relates to detecting events that occur within mobile applications without modifying code of the mobile application. In some implementations, a front-end server receives, from a mobile application management application installed on a client device, interaction data specifying (i) a user interaction identifier for a user interaction with an interactive item that presents, at a first mobile application, content for a second mobile application and (ii) an interactive item identifier. The front-end server receives event data specifying an occurrence of a specified user action within the second mobile application and the interactive identifier for the interactive item. A back-end server determines that the specified user action resulted from the user interaction based on a match between the interactive item identifier specified by the event data and the interactive item identifier that links to the user interaction identifier for the user interaction.