Two ways to hit one device. For test devices: tag the device in the app with sendTag (for example tag is_test set to true), create a dashboard segment filtered on that user tag, and target the segment instead of All. For a real user's device: grab the player_id from the SDK's ids callback on the device, send that id to your backend with the user record, then call the notifications API with include_player_ids set to that id. Targeting by player_id is the reliable path for user-to-user pushes; tags plus segments are for test cohorts.

Context: Stack Overflow #37738719 (top answer, 18 votes): An Ionic developer wanted to push to one specific device instead of blasting all users, and did not know how to capture the device identifier or which API targets a single device. The answer walks through OneSignal's own guide for individual-device delivery.