All the functionality from Alpaca Trading API for direct users is supported under Broker API. Please read the documentation for more details. There are additional capabilities for Trading API in the broker setup.

Fractional Shares

Under the correspondent authentication, Trading API is extended for fractional shares trading.

Asset API

Asset entity has an extra field named fractionable with type boolean, it is set to true if the asset is marked for fractional trading. Check out more about our Assets API here.

Order API

The POST method (to submit order) has an extra parameter called notional to denote the dollar amount to buy. This parameter is mutually exclusive to the qty parameter. You can also specify the qty parameter with fractional quantity. Note that the fractional shares trading is currently supported only for market day orders and any other type of orders will result in API error. You can submit such fractional share orders in the night to execute at the next market open.

The order entity includes the notional value if the order was submitted with the notional value. In this case, the order entity omits the qty field.

Account Configuration API

Account configuration API adds another configuration key called fractional_trading and defaults to true. If you want to disable fractional trading for a specific account for any reason, you can set this to false.


You have the option to charge the commission for each order. You will need to contact Alpaca first to set up the commission structure, but once it’s set up, you can submit customer orders with a commission parameter indicating the dollar amount to charge. The respective field is attached in the order entity in the API response.

Order Sub-tagging (Omnibus)

If you are an omnibus setup, we ask you to submit a “sub-tag” value in each order. This is for us to understand the order flow better from the trade surveillance requirements. In case you fail to attach proper sub-tags, we may need to reject all of the order flows coming from you as we may not be able to segregate particular malicious activities.