Risk Checks Between a Strategy and the Exchange

Pre-trade risk checks cap order size, position, price and message rate before an order reaches the exchange, and the 2012 Knight Capital loss shows what skipping them costs.

Pre-trade risk checks are the fixed set of limits a firm applies to every order between the moment a strategy decides to send it and the moment it reaches the exchange, covering at minimum a maximum order size, a maximum resulting position, a price collar against the last trade, and a cap on message rate. Where those checks sit in the software, as an independent layer the strategy cannot bypass or as a setting inside the strategy's own code, determines whether a bug turns into a loss measured in minutes or one measured in milliseconds.

The order size limit rejects any single order above a fixed quantity, catching an obvious input error such as a size typed with an extra digit. The position limit is checked against the account's aggregate exposure, not against any single order, and it has to account for working orders that have not filled yet rather than confirmed positions alone, since assuming resting orders will never fill is exactly the assumption that lets a strategy accumulate an unintended position.

Both limits are set per strategy and often per instrument, because a limit sized for a liquid futures contract would be meaningless, or dangerously loose, applied to a thinly traded single-name option.

A price collar rejects any order priced too far from the last traded price or the current best bid and offer, protecting against a fat-finger price entry and against a strategy computing a quote from a stale or corrupted print. The collar has to be wide enough that it does not reject legitimate quotes during a fast market, where prices can move a meaningful distance in seconds, and narrow enough that it still catches a genuine error before it reaches the tape.

Collar width is one of the few risk parameters that desks revisit continuously rather than setting once, because a static collar becomes either too loose in a calm market or too tight during an announcement, and both failure directions carry a cost, one in missed quotes and the other in an unchecked bad order.