Say I established a long position of 100 contracts using a bracket order. The parent of the bracket has filled, and the take profit / stop loss are outstanding (in a quantity of 100 each). I now want to trim this position to 80 contracts. Is there a way I can place an order to sell 20 contracts, and as that fills, have it reduce the quantities of the outstanding take profit / stop loss? Obviously, I could just wait for the sell order to fill and modify the take profit / stop loss. But was wondering if there was an automated / more robust way to do this.
That is a good question. Maybe there is a way to do it via programming on your platform? Since the parent order has been filled, adjust your OCO manually for now. This is my recommendation in order to avoid an error. Each software has a different function so it is up to the software functionalities to make such adjustments that many may consider a discretionary function. Ask the software provider or post it here and some traders can comment.
It is indeed possible via the API to change certain parameters of an existing order (e.g. price). However, I am not 100% sure whether you can change the quantity of the order. The rule of thumb is that if you can change it manually in TWS you would also be able to change it via software and the API. Alternatively, if it turns out that you can't modify the quantity for an existing bracket order you could cancel it and submit a new bracket order, using the same prices as before but with the new quantity. This can of course be done via the API.
So, canceling the existing orders and submitting a new bracket order in a quantity of 20 doesn't quite do what I need. (I want the children to close out the net position of 80.) What would work, but I don't think is possible, is canceling existing orders and submitting a funny bracket order with a SELL 20 parent, and SELL 100 stop loss / take profit children, assuming IB made the children "longer" as the SELL parent filled. (I.e., as SELL parent went from 0 to 20, the SELL children would go from 100 to 80.) Pretty sure IB won't like a parent on the same side as the children though, and adjusting parent/child sizes doesn't work that way. Think my easiest option is to wait for the SELL 20 to fill, then cancel the outstanding orders and replace them with a SELL 80 OCA stop loss / take profit pair.