Map at the edges
Shape data on the way in and on the way out — never in the middle. The receive pipeline turns the source format into canonical; the send pipeline turns canonical into the destination format. Between them, the bus carries one stable shape.
Every message has exactly two edges where its shape is allowed to change: the point where it enters the system and the point where it leaves. The inbound map runs in the receive pipeline and produces the canonical message. The outbound map runs in the send pipeline and produces whatever the destination wants. Everything in between is canonical and untouched.
Keeping transformation at the edges is what makes the canonical guarantee real. If a component partway through quietly reshapes the message, every downstream subscriber inherits a surprise and the bus no longer carries a single known shape. It also keeps each map small and one-directional — far easier to read, test and hand to the AI engine than a map that does input parsing and output formatting at once.