Fractional Shares with Alpaca Brokerage Model

Currently, the QuantConnect documentation and API references for the AlpacaBrokerageModel do not explicitly confirm support for fractional shares. However, the Alpaca platform itself does support fractional trading, subject to specific conditions and securities being eligible. Since Alpaca allows fractional trading, it is very likely that the AlpacaBrokerageModel in QuantConnect would accommodate this, as QuantConnect aims to mirror the brokerage's capabilities. For verification or implementation of fractional trades, you might consider consulting recent updates directly from QuantConnect's LEAN engine or community forums for the latest developments.

Sources: Alpaca - QuantConnect.com, Supported Models - QuantConnect.com, QuantConnect Paper Trading - QuantConnect.com

Has anybody had success with the fractional shares via Alpaca brokerage so far?

Hi Lemuel,

Short answer: no, and it is not a limitation of the Alpaca integration specifically. Fractional equity shares are not supported on QuantConnect for any brokerage, in backtesting or in live trading.

The reason is where the minimum tradable unit is defined. LEAN reads it from the Symbol Properties database, and the entry that covers every US equity sets a lot size of 1 share:

market,symbol,type,description,quote_currency,contract_multiplier,minimum_price_variation,lot_size
usa,[*],equity,,USD,1,0.01,1

Two things follow from that. An order for less than one share is rejected with OrderQuantityLessThanLotSize, and SetHoldings rounds the quantity it computes down to a whole number of shares. That second one is the usual surprise: a small account targeting an expensive stock can end up placing no order at all.

Crypto through Alpaca is different. Those assets are registered with their real minimum trade increment, so fractional quantities do work there.

This is tracked upstream in Lean issue #7282, Support for Fractional Equities. The fix is to let the brokerage model decide whether fractional orders are allowed, rather than the Symbol Properties database, since in practice it depends on the account and not on the asset. It is open, so there is nothing to enable on your side yet.

One correction while I am here: the automated reply earlier in this thread guessed that support was "very likely" present. That was not accurate, and I am sorry it stood unchallenged for so long.