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Default is JSON. However, a connection can request MessagePack as follows:

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Client → Server

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Server → Client

Valid values: "json", "msgpack". The switch takes effect immediately: the server reads the connection’s current encoding per outbound frame, so every message after it processes your set_format, including the format_set confirmation itself is in the new format. Any frames that were already queued when you switched also go out in the new encoding, so switch while the connection is quiet (right after connect, before subscribing) and flip your client’s decoder as soon as you send the request. Inbound is direction-independent: the server decodes any binary frame you send as MessagePack regardless of the current outbound setting.