Splits an order's shipment into two, moving the items that split_by selects to a separate shipment. The body is required and names the kind of split; out_of_stock is the only kind today, and more may be added later.

With split_by: out_of_stock, the out-of-stock items are split off the order, so the items in stock can ship first. This is "Split order" in the merchant portal: the out-of-stock items of the order's shipment that is still waiting_for_picking move to a separate shipment that ships once they are back in stock, and the items in stock stay where they are and are fulfilled as usual. The separate shipment is a new one, or the order's existing out-of-stock shipment from an earlier split. Add-ons from packaging rules and items kitted at packing do not move.

A shipment is split only when it keeps at least one item in stock. When out-of-stock items sit in more than one shipment or warehouse of the order, the request is refused; split such orders in the merchant portal. If the out-of-stock items are back in stock while the original shipment is still waiting for picking, Hive may move them back into it.

A request sent as application/json without a body, with a body that is not valid JSON, without split_by, or with a value that is not listed is rejected with 400 and nothing is split; a request whose Content-Type is missing or is not application/json is rejected with 415. On success the response lists every shipment of the order.