How Starting/Resuming Playback Actually Works in Spotify
PUT /v1/me/player/play starts or resumes playback on a specified device, optionally with a context_uri (like an album or playlist) or a uris array of specific tracks to play.
Real API Details
This endpoint requires Spotify Premium -- it returns an error for free-tier accounts -- and also requires an active device already available (one that's had the Spotify app open recently), since playback can't be started on a device with no active Spotify session.
Practical Considerations
Pass a device_id explicitly if more than one device might be active, since omitting it targets whichever device Spotify currently considers active, which may not be the one actually intended.
Use cases
Auto-start sessions when a user comes online or completes a workflow.