If you've sat in on an agentic AI architecture discussion recently, you've probably heard MCP, A2A, AG-UI and A2UI thrown around like everyone already knows what they mean. They're four separate protocols solving four separate problems, and understanding the difference is genuinely useful for judging whether a vendor's architecture will still make sense in eighteen months.
MCP — how an agent reaches your systems
Model Context Protocol is how an agent calls a tool or a system — your CRM, your document store, a market-data feed — without a custom integration for every model and every tool combination. Wrap a system once as an MCP server, and any MCP-compatible agent can use it. This is the layer that stops you rebuilding integrations every time you change models or add a use case.
A2A — how agents work with each other
Agent-to-Agent is how one specialist agent hands work to another — a research agent passing findings to a drafting agent, or a planning agent checking with a compliance agent before it proceeds. Each agent publishes a machine-readable 'Agent Card' describing what it can do, so agents can discover and delegate to each other instead of one monolithic agent trying to do everything itself.
AG-UI — how an agent talks to a person
Agent-User Interaction is the streaming layer between an agent and whatever interface a person is looking at — a chat window, a dashboard, a voice channel. It's what lets an agent's reasoning and state show up live in the UI, rather than a person staring at a spinner until a final answer appears.
A2UI — how an agent builds the interface itself
Agent-to-UI goes a step further: instead of a fixed screen with a chat box bolted on, the agent can describe the interface it needs for a given moment — a form, a chart, an approval card — and have it rendered natively on whatever device the person is using. It's the difference between an agent that talks and an agent that can actually hand you the right control at the right moment.
Why this matters when you're evaluating a partner
The practical test: ask any vendor pitching an agentic platform how their system handles a new use case. If the honest answer is 'we build a new integration,' you're looking at a proprietary stack. If the answer is 'we wrap it as an MCP server and it's available to every agent,' you're looking at something that compounds. That's the whole argument for protocol-native architecture over a single clever chatbot.
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