FIX Protocol

FIX, the Financial Information eXchange protocol, is the de facto messaging standard for electronic trading of securities. Born in 1992 out of a need to standardize how buy-side and sell-side firms talked to each other, it defines a tag-value message vocabulary that carries orders, executions, and market data through the pre-trade, trade, and post-trade lifecycle. For decades it has been the plumbing underneath equities, fixed income, and derivatives markets.

• Tag-value messaging - A compact, session-oriented format where each field is an integer tag paired with a value, tuned for low-latency trading.
• Full trade lifecycle - Covers indications of interest, order entry, execution reports, and allocations across pre-, at-, and post-trade.
• Session and application layers - A persistent session protocol for reliable delivery, with the application layer carrying the business messages.
• A governed industry standard - Maintained by the FIX Trading Community, with widespread adoption across exchanges and broker-dealers.

In practice, FIX is the incumbent that most trading infrastructure still speaks, which is exactly why it matters to the way I think about market data, financial regulation, and modern API consumers. It is fast and battle-tested, but it is also session-bound and specialized — a world apart from the stateless, HTTP-native, JSON-over-REST interfaces that agentic consumers expect. As I cover in the market data work, the agentic turn is increasingly routing around FIX toward web-friendly APIs, even as FIX remains the backbone of the trading floor.