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The V3 perps WebSocket is separate from the event and prediction-market orderbook sockets.
One subscribe request can be partially accepted. Inspect ack.channels and ack.rejected. Each perps_book event is a complete replacement, not an incremental patch. perps.book("BTC-USD") returns the latest managed copy, or None until the first book event arrives. A successful acknowledgement confirms the channel, not book readiness; use the V3 perps orderbook when startup needs immediate state. The client reconnects and resubscribes after non-terminal disconnects, but replay is not guaranteed. Reconnect notices set gap_possible=True; use V3 REST to backfill when completeness matters. Authentication (4401) and connection-limit (4429) closes are terminal until the failing condition changes. See the shared perps guide for channel definitions, lag behavior, and lifecycle details.