Pull the playback write loop out as relayRTMPSession, against gortmplib's Conn interface, and check what it writes for a stream's start: the replayed SPS/PPS access unit goes out as a sequence header, and the first keyframe is the only frame at its timestamp. A session without an audio track no longer hands the writer a nil track. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>