MCP vs A2A: agent-to-tool and agent-to-agent protocols

MCP and A2A side by side

	MCP	A2A
Connects	An agent to tools and data	An agent to other agents
Started by	Anthropic, November 2024	Google, April 2025
Unit of work	A tool call with typed inputs	A task that can run for a long time
Discovery	tools/list on the server	An Agent Card describing skills
The other side	Does exactly what the tool says	Reasons and acts on its own
Typical use	Query a database, call an API, read files	Delegate to a specialist agent from another team or vendor

What MCP is, in one paragraph

The Model Context Protocol is an open standard, first published by Anthropic in November 2024, for connecting AI applications to outside tools and data. An MCP server describes what it can do (tools the model can call, resources it can read, prompt templates) in a machine-readable way, and any MCP client, such as Claude, Cursor, VS Code or ChatGPT, can connect to it without custom glue code. Messages are JSON-RPC 2.0, sent over standard input and output for a local server or over HTTP for a remote one.

What A2A is

Agent2Agent (A2A) is an open protocol Google announced in April 2025 and later moved to the Linux Foundation. Each agent publishes an Agent Card listing what it can do and how to reach it. Another agent can send it a task, follow its progress, exchange messages and files, and receive the result, even when the two agents were built by different companies on different frameworks.

The difference

An MCP tool is a function. You send inputs, it does one defined thing, it returns. The model on your side does all the reasoning.

An A2A peer is an agent. You hand it a goal, and it plans, uses its own tools (possibly through MCP) and may come back with questions. The work can take minutes or days. You do not see or control its internal steps.

So the question to ask is: do I want a capability I control, or a colleague I delegate to? The first is MCP, the second is A2A.

How they fit together

A realistic system uses both. Each agent reaches its own tools through MCP servers, and agents coordinate with each other over A2A. An agent can also wrap another agent as a single MCP tool when it just needs an answer back and does not need the task lifecycle.

What to check

Both protocols move trust across a boundary. With MCP the question is what the server's tools can reach and whether the server is who it says it is. Check the maintainer, the auth on remote endpoints, and which tools write. With A2A the same questions apply to a whole agent, which is harder to inspect, so start with narrow tasks and read-only permissions.

Questions people ask

Is A2A a replacement for MCP?

No. A2A connects agents to agents; MCP connects agents to tools. Google's own guidance describes them as complementary.

Which should I learn first?

MCP, if you are giving a single assistant access to your systems, which is most projects today. A2A matters once separate agents, often from different teams or vendors, need to cooperate.

Can an MCP server be an agent?

It can have a model behind it, but to the client it is still a set of tools. If you need back-and-forth, long-running tasks between agents, A2A is designed for that.

The short answer