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Participants may request to cancel any open order using the OrderCancelRequest [F] message. As with order amend requests, the order to be canceled is identified using the last-entered ClOrdID (11) value relating to the order into the OrigClOrdID (41) field. If the order has been canceled, the Polymarket US will respond with an ExecutionReport [8] message which echoes many of the key (updated) order attributes, and with ExecType (150) = 4 (canceled).

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Table 18: OrderCancelRequest (F) message

TagNameReqTypeDescription
< Standard Header >Y35 = F
11ClOrdIDYStringFresh, participant-generated, unique reference for this OrderCancelRequest. Must be different from the original ClOrdID (11) for the order.
41OrigClOrdIDYStringThe last (participant-generated) ClOrdID (11) reference for the order. This might be from the original NewOrderSingle or prior amends.
55SymbolYStringInstrument symbol as indicated on the order.
< Standard Trailer >Y

If the request to cancel the order passed validation, then the Polymarket US will acknowledge the order cancellation with an ExecutionReport [8] message indicating OrdStatus (39) = 4 (canceled), and LeavesQty (151) = 0.

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Figure 12: Successful cancelation of existing order

Example 16: Example of order cancel message 
8=FIXT.1.1 | 9=107 | 35=F | 49=SENDER |  56=TARGET | 34=96 | 52=20240517-19:29:50 | 50=SENDERSUB | 41=1182560834 | 11=1182560840 | 55=GOOG | 54=1 | 10=244 |
Rejected attempts to cancel orders for any reason (for example order is no longer alive) are rejected using the OrderCancelReject [9] message, which identifies the reason for the rejection.

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Table 19: OrderCancelReject (9) message

TagNameReqTypeDescription
< Standard Header >Y35 = 9
11ClOrdIDYStringEchoed from the OrderCancelRequest
37OrderIDYStringWill be ‘NONE’ for unknown orders
39OrdStatusYcharStatus of the cancellation request. 8=Rejected
41OrigClOrdIDYStringEchoed from the OrderCancelRequest
58TextYStringFree text string providing additional rejection reason
102CxlRejReasonYintReason for the rejection. 0=Too late to cancel, 1=Unknown order, 6=Duplicate ClOrdID, 18=Invalid price increment (tick size), 99=Other
434CxlRejResponseToYchar1=Order cancel request, 2=Order amend request
< Standard Trailer >Y

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Figure 13: Unsuccessful attempt to cancel existing order


Example 17: Rejection of an unsuccessful cancel attempt 
8=FIXT.1.1 | 9=145 | 35=9 | 34=91 | 49=TARGET | 52=20240521-09:26:30.378549737 | 56=SENDER | 57=SENDERSUB | 11=1886428723 | 37=NONE | 39=8 | 41=1886428676 | 58=Unknown order | 102=1 | 434=1 | 10=198 |