Has anyone had success using the Web API to purchase fractional shares by defining the notional value of the purchse (i.e. the cashQty parameter)? I can do it through the UI but when using the API it seems like it always rounds down to whole shares.
Don't use the WebAPI, it is unreliable. Use the IB gateway. You have to get the definition of the contract through the reqContractDetails request. https://ibkrcampus.com/campus/ibkr-api-page/twsapi-doc/#tws-contract-details There you will see two attributes: - MinSize - SizeIncrement That will tell you how to build the quantity you are after.