How to control media playback

Enumerate permitted media endpoints and inspect declared playback, volume and content-launcher capabilities in the capability directory. Select endpoint or native media group only when its declarations support the requested operation. Use REST dispatch with the exact message/payload/version.

Absolute volume and relative volume changes have different retry behavior. Validate units/ranges. After dispatch, query fresh playback/volume observations. Accepted playback is not proof of audible output. On lost relative-command responses, recover the result/read back or report unknown; do not apply the increment twice.