Definition
FIX is the electronic communications protocol that institutional financial markets use to exchange pre-trade, trade, and post-trade messages between counterparties. Originating in the 1990s for equity markets, FIX has become the lingua franca for buy-side to sell-side connectivity across equities, fixed income, foreign exchange, and increasingly digital assets. FIX 4.4 remains the most widely deployed version; FIX 5.0 SP2 extends the protocol with custom tags and session management improvements. For institutional digital asset trading, FIX connectivity is typically a procurement precondition rather than a nice-to-have.
Example
A sell-side broker's OMS sends a FIX NewOrderSingle (35=D) message to a digital asset venue. The venue acknowledges with ExecutionReport (35=8) messages as fills occur. All communication is structured under FIX semantics, including custom tags documented per counterparty.
How Liquid Mercury Handles This
Mercury Pro exposes FIX 4.4 and 5.0 SP2 as a first-class connectivity surface, with custom tags documented per counterparty and a single canonical order model that FIX, REST, and WebSocket surfaces all bind to.