Definition
A fill or kill order is an order type that requires the entire quantity to execute immediately, or the entire order to cancel. FOK is used when a trader needs certainty of execution at a specific size and is unwilling to accept partial fills. FOK is stricter than IOC, which accepts partial fills. Both order types are standard in institutional order management and are supported under FIX as well as modern REST and WebSocket APIs.
Example
A desk submits a FOK buy order for 100 BTC at $95,000. If the full 100 BTC is available at $95,000 or lower, the entire order fills at once. If only 80 BTC is available, the entire order cancels rather than partially filling.
How Liquid Mercury Handles This
Mercury Pro supports FOK and the full institutional order-type vocabulary across FIX, REST, and WebSocket surfaces, with identical semantics on every surface so integrations do not need to reimplement order-type handling.