Definition
A partial fill happens when only a portion of your order executes at the requested price because the order book doesn't have enough volume at that level. The unfilled remainder either stays in the book as a resting order or is cancelled, depending on your order type.
Example
You attempt to buy 100 YES contracts on Polymarket at $0.47. The order book only has 60 contracts available at that price. Your order fills for 60 contracts — you're partially filled. If your corresponding NO order on Kalshi fills for the full 100 contracts, you now hold 60 YES + 100 NO — a net directional short of 40 contracts.
Why partial fills create leg risk
Arbitrage requires both legs to be equal in size. If Leg 1 partially fills and Leg 2 fully fills, the excess on the full-fill side becomes a directional position. If the event resolves against that excess position, you lose money — converting an arbitrage trade into an accidental directional bet.
How Arbitrage Agent handles partial fills
Arbitrage Agent sizes each order against the available book depth before placing either leg. Orders are capped at the volume available at the quoted price, preventing partial fills in normal conditions. For the rare case where the book changes between scan and execution, the circuit breaker automatically equalises both legs.
Avoiding partial fills
- Size orders to available book depth (not your desired position size)
- Use limit orders at or inside the quoted price
- Monitor both sides simultaneously — if one leg partially fills, adjust the other
- Implement a circuit breaker to auto-equalise mismatched legs