This document describes systems, methods, devices, and other techniques for determining media items to insert in a media stream. A first media client can play a primary media stream that is multicast to various media clients including the first media client and other media clients. The first media client identifies a start of a primary programming timeslot in the primary media stream, and in response, obtains and stores a secondary media item. After storing the secondary media item at the first media client, the first media client detects an insertion signal in the primary media stream that indicates a secondary programming timeslot is about to begin in the primary media stream. The first media client can then play the secondary media item in place of the primary media stream during the secondary programming timeslot.