Four new protocol acronyms in eighteen months, and most engineers still can't explain how they relate to each other. That's not a knowledge gap — it's a documentation failure.
I've had this exact conversation three times in the past month. Each time, a smart engineer asks: "Wait, if we're using A2A, do we still need MCP?" The answer is yes, always yes — and the fact that the question keeps coming up tells you something about how poorly this space is being explained.
MCP, A2A, A2UI, and AG-UI are not competing standards. They don't solve the same problem. They don't even operate at the same layer of the architecture. What looks like a crowded, confusing standards landscape is actually the beginning of a layered interoperability stack — the TCP/IP moment for agentic systems.
This article draws the map. No hype, no winner-takes-all narrative — just the architecture.
TL;DR — The Four-Line Summary
