A2A vs. AG-UI vs. MCP: Three Layers of the Same Stack

With all three protocols covered individually, the practical question is how to actually apply this: which protocol handles which part of a real system? This course's own framing is direct — these aren't competing standards, they're layers of the same stack, and the right mental model is matching each protocol to the specific CONNECTION it standardizes, not picking just one.

MCP (Model Context Protocol) is the standard for how agents connect to tools, data, and workflows — started by Anthropic, now adopted broadly. Reach for MCP wherever an agent needs to call external tools, read resources, or use prompt templates (exactly the 6 primitives covered earlier in this topic).

A2A (Agent-to-Agent) is the protocol for multi-agent coordination — how agents delegate tasks and share intent across systems, discoverable via published Agent Cards. Reach for A2A wherever your system involves MULTIPLE distinct agents (potentially built on different frameworks, by different teams) that need to collaborate without sharing internal implementation details.

AG-UI (Agent-User Interaction) is the bidirectional connection between agentic backends and frontends — this is how agents become genuinely interactive inside applications, not just chatbots but collaborative co-workers. Reach for AG-UI wherever a human needs to watch, interrupt, or collaborate with an agent in real time through an actual UI.

Crucially, these compose rather than being mutually exclusive choices: AG-UI can handshake with both MCP and A2A, meaning tool outputs (from MCP) and multi-agent collaboration (via A2A) can flow seamlessly through to the user interface (via AG-UI) — your frontend stays connected to the entire agent ecosystem through one unified protocol layer. A realistic production system uses all three simultaneously: agents call tools via MCP, coordinate with each other via A2A, and stream their combined progress to users via AG-UI — three protocols, three distinct connection points, working together rather than being alternatives to choose between.

def which_protocol(connection_type: str) -> str:
    """A direct lookup matching each connection type in an agent
    system to the protocol that standardizes it — these compose,
    they aren't mutually exclusive choices."""
    mapping = {
        "agent_to_tool": "MCP — connect to tools, data, workflows",
        "agent_to_agent": "A2A — multi-agent coordination, task delegation across systems",
        "agent_to_user": "AG-UI — stream agent execution, tool progress, and state to the frontend",
    }
    return mapping.get(connection_type, "Not one of the 3 standardized connection types")

# A realistic system uses all three at once, for their respective layers:
for connection in ["agent_to_tool", "agent_to_agent", "agent_to_user"]:
    print(f"{connection}: {which_protocol(connection)}")

# AG-UI can handshake with both MCP and A2A — meaning tool outputs and
# multi-agent collaboration results both flow through to the same
# unified AG-UI event stream the frontend already knows how to consume