Two protocols, two layers
MCP — the Model Context Protocol — standardizes how a single model or agent reaches out to tools and data. It is the layer between one agent and the capabilities it uses: a database, an API, a filesystem.
A2A — agent-to-agent — standardizes how separate, independently-built agents communicate and delegate to one another. It is the layer between agents, letting a scheduling agent hand a task to a research agent that neither team built together.
So they are not alternatives. MCP answers "how does my agent use a tool"; A2A answers "how does my agent talk to your agent". A sophisticated system uses both: each agent uses MCP to reach its own tools, and A2A to coordinate with peers.
Where MCP fits
If you are building one agent and want to give it access to your systems, MCP is what you need. The agent stays a single entity; MCP is the wiring to everything it can touch. The vast majority of "connect my AI to X" problems are MCP problems, not A2A problems.
MCP assumes a client-server shape: your agent is the client, and each tool is a server. It does not attempt to coordinate multiple autonomous agents — that is deliberately out of scope, which is part of why the protocol stayed simple enough to be widely adopted.
Where A2A fits
A2A becomes relevant once you have multiple agents that were not designed as one system and need to interoperate — often across organizational boundaries. Its job is discovery, capability negotiation, and message passing between peers that treat each other as black boxes.
This is a genuinely harder problem than tool access, involving identity, trust, and long-running tasks between parties that do not share a codebase. It is also much earlier in adoption than MCP, so if you are unsure which you need, you almost certainly need MCP first.
Which one you actually need
Building a single agent that uses your tools and data: MCP. This covers most projects, and it is where to start.
Coordinating multiple independent agents, especially ones you don't control: that is the A2A layer, and you would typically add it on top of agents that already use MCP internally.
If the question is which to learn first, the answer is MCP — it is more mature, more widely supported, and solves the problem most teams actually have today. A2A matters as multi-agent systems spanning organizations become common, which is still emerging.