I have implemented FIX for a living for a few years. It’s fragmented beyond belief. You can have different standards for different parts of the same brokerage. You have to do separate conformance tests for different asset classes at the same (big, well known, reputable) broker. The proprietary APIs like Bloomberg’s provide additional value rather than being alternatives to FIX. Make no mistake: you cannot interchange a FIX endpoint for another without making changes. The core problem is that you can impose arbitrary semantics on top of FIX messages. For example, a broker might require you to send timestamps in UTC for equities, and local time for FX, on the same FIX connection. Another might have a limit on the number of characters you can put in a ClOrdId - so GUIDs as ClOrdIds go out the window. Another big screw up is extensive use of non-ISO currency codes and the ambiguity of major and minor denomination. Is “GBp” pence? Sometimes yes! Sometimes not. Then - the most colossal screw up of them all - the reverse proxy people. They are companies that will convince management that they have a “network” and “access to liquidity”. It’s actually just a reverse proxy server that _manipulates messages in flight_! It sounds ideal, as if they will standardize your message flow so you don’t have to worry about broker specifics. In practice, it never works, and they opaquely mess with things you don’t want them to. Imagine sending an order denominated in cents, and the reverse proxy forwards it having changed the denomination to dollars. This really happened to me.

BTW, I love working on this stuff and swapping notes with other people. Drop me a line (email in profile) if you want to talk anything related to FIX!