Partial Fill

A partial fill occurs when an order for a given quantity executes in pieces rather than all at once — for example, an order to sell 5 contracts fills 2 immediately and leaves 3 still working because only 2 contracts of resting liquidity were available at that price. The unfilled remainder either stays working (waiting for more liquidity or a better price) or gets cancelled, depending on the order type and what the trader or platform does next.

Partial fills are a normal feature of order-book trading, not a malfunction: they happen because the market only has so much size resting at a given price at a given moment, and a large order can consume all of it and still have quantity left over.

Partial fills matter most on lower-liquidity contracts or during size that's large relative to what's resting on the book — a thin market in RTY or a less-active month can leave a multi-contract order working for longer, or filling across several prices, compared to a highly liquid front-month ES or NQ order.

For anyone running or following a copy strategy, understanding partial fills also matters because it shapes expectations about timing: a leader's order isn't necessarily an instant, complete event — it can take multiple fill reports to fully execute, and what a follower sees depends on how the copier handles that sequence.

MimikTrader copies a market-on-fill leader order once — when the leader's order reaches its terminal full fill — sized to the leader's complete original order quantity (scaled by the follower's multiplier), not to each individual partial. If the leader's order partially fills and is then cancelled before reaching a full fill, no copy is sent for that order, since it never reached the terminal fill that triggers replication. This is a deliberate design choice, not a bug: it avoids sending followers a stream of small, staggered orders that don't match what the leader actually ended up with. Separately, a follower's own order can experience its own partial fill against its own broker's liquidity — that fill outcome depends on that account's own market conditions, not on how the leader filled.