We get asked this on roughly every other discovery call: "Should we standardize on A2A or MCP?"
The premise of the question is wrong. They aren't competitors. They solve different problems at different layers, and a healthy agentic system uses both.
What MCP is for
MCP (Model Context Protocol) is a standard for tool surfaces. It answers: how does an LLM-based agent discover and call tools that aren't part of its training? Document retrieval, database queries, calendar APIs, internal services — each tool publishes an MCP server, the agent connects, lists capabilities, invokes.
The shape is: one agent ↔ many tools. The tool is passive; the agent decides when to call it.
MCP gets you: schema discovery, typed parameters, structured returns, authentication boundaries, sandboxing. It is, essentially, an RPC standard tuned for agentic clients.
What A2A is for
A2A (Agent-to-Agent protocols, of which there are several — Google's A2A, AGNTCY, others) is a standard for agent collaboration. It answers: how do multiple semi-autonomous agents coordinate, hand off subtasks, and aggregate results?
The shape is: agent ↔ agent, with state. Both sides have their own reasoning loops. Handoffs include context, intermediate results, sometimes ownership of a subtask.
A2A gets you: agent cards (capability advertisements), task lifecycles, message passing, optional streaming and human-in-the-loop. It's not RPC — it's closer to a typed actor system.
Why the confusion
Both involve agents talking to something over a protocol with structured payloads. Both produce JSON. Both have client and server roles. At a 30,000-ft view they look like the same thing.
But the level of autonomy on each side is the real distinction:
- MCP server: passive. Responds to calls. Has no reasoning loop of its own.
- A2A agent: active. Can decline tasks, propose alternatives, hand off to other agents, request clarification.
If the other side is "fetch this row from a database," that's MCP. If the other side is "investigate this anomaly and tell me what you find," that's A2A.
The architecture that actually works
In every multi-agent system we've shipped, the breakdown looks like this:
- Top of the stack: an orchestrator (often itself an agent) routes user requests to specialist agents.
- Specialist agents communicate over A2A. Handoffs preserve context; supervisors track task state.
- Specialists use tools over MCP. The retrieval agent has an MCP connection to a vector DB and an MCP connection to a fulltext search service. The "ops agent" has an MCP connection to a ticket system.
A2A is how the agents coordinate. MCP is how each agent acts.
The pragmatic take
Adopt MCP first — it solves the most immediate problem (talking to tools), it has the broader ecosystem today (most LLM providers ship native MCP support), and the upgrade path from "ad-hoc tool calls" to MCP is straightforward.
Adopt A2A when you have more than one agent that needs to hand work off. If your system has a single agent + lots of tools, A2A is overkill — you're not coordinating anyone. The moment you add a second specialist agent that needs to talk to the first, A2A starts paying for itself.
Don't pick. Stack them.