A service for providing messaging extension apps can be an online store that can be browsed and searched for the apps. The store uses extension app identifiers which are related to app identifiers that are sent between devices in a conversation of messages so that a receiving device can, when it does not have the extension app installed to interact with received content, use the extension app identifier to download and install the required extension app. In one embodiment, the download and install can occur while the messaging app remains the foreground app, and the messaging app adds an icon of the newly installed extension app into a browsable tray in the UI of the messaging app.