Partial fills on thin books: when the order is only half done

[Shadow Trader]

An order for five lots can come back as two lots at one price and three at another, or just two, with the remainder cancelled. On deep pairs at quiet times it rarely happens. On thinner pairs, or at off hours, it's common.

The platform may show a single position or several separate ones, depending on how it handles aggregation. Trade history might look strange: three tickets at three prices, all opened within the same second.

For a scalper, the average entry now sits slightly worse than the quote at click time. The exit has the same problem in reverse. The cost of size shows up even when the lot count seems small, particularly on exotic crosses.

Brokers differ on fill policies. Some use fill-or-kill, so the whole order executes or nothing does. Others allow partial fills. A few default to "fill or cancel remainder." The setting can sometimes be found in the order dialog or account configuration, and sometimes it can only be changed by asking support.

Testing with a modest size and then stepping up reveals the point where fills start to fragment.

At what size does a partial fill start to appear on your pair of choice?