The Agent Protocol Stack: MCP vs. A2A vs. AG-UI (opens in new tab)

If you're building AI agents in 2026, you've probably bumped into at least one of these acronyms: MCP, A2A, AG-UI. Maybe all three. And if you're anything like me, your first reaction was: "Are these competing standards? Do I need all of them? Which one do I actually use?" Here's the short answer: They're not competing — they're complementary. Each one solves a different problem at a different layer of the agent architecture. Think of them like TCP, HTTP, and HTML — different protocols at dif...