MCP vs A2A: Tools vs Agents

The most common question about the two big agent protocols is which one to use — and the answer is almost always "both," because they solve different problems: MCP connects an agent to its tools, A2A connects an agent to other agents.

By Pratik Dhanave · August 19, 2026 · 5 min read

#AI Architecture #MCP #A2A #AI Agents #AI Engineering #Software Architecture #LLMs

The Model Context Protocol and the Agent2Agent protocol are frequently framed as competitors, and choosing between them is treated as an either/or. It isn’t. They operate at different layers of an agentic system, and the useful decision is not “which one” but “which one for this connection.” This fifth post in the AI Architecture Decisions series settles the distinction. (I’ve written full series on both — MCP from Scratch and the Agent2Agent Protocol — this is the chooser.)

Two layers, not two competitors

The clean mental model:

MCP connects an agent to its tools and context — the vertical link. A single agent reaching down to functions, data, and resources it uses to do its own work. A database, a code interpreter, a document: things an agent uses.
A2A connects an agent to other agents — the horizontal link. An agent reaching across to peer agents it delegates work to. A specialist agent in another team or company: a collaborator an agent works with.

The one-line mnemonic: MCP is how an agent uses tools; A2A is how an agent talks to agents. A tool is a capability you invoke and control; an agent is an autonomous, opaque peer you delegate to and coordinate with. They are complementary layers of the same system, not two options for the same job.

How to tell which a dependency is

When you have something your agent needs to interact with, ask what it is:

Is it a discrete capability you invoke and fully control — a function, a query, a resource fetch, with no autonomy of its own? That’s a tool → expose it via MCP.
Is it an autonomous system that reasons, has its own tools, and produces deliverables over a task lifecycle — possibly owned by another team or vendor, whose internals you neither know nor want to know? That’s an agent → connect via A2A.

Two more tells that resolve edge cases. If you’d need to know and manage its internals to use it, you’re building a tool; if you deliberately do not want to know its internals and just want its results, you’re delegating to an agent. And if the interaction is a quick synchronous call, MCP’s shape fits; if it’s long-running, interruptible work with its own progress and artifacts, A2A’s task model fits.

Why real systems use both

Because the two protocols address different layers, a mature system uses them together. Picture an orchestrating agent handling “plan and book a client offsite.” It uses MCP to call tools — query the company calendar, look up the travel budget in a database, read the attendee list from a file. And it uses A2A to delegate to other agents — hand venue research to a specialist events agent, flights to a travel agent, catering to a vendor’s agent — each of which is itself an autonomous system with its own tools.

The elegant part is that they nest. Each remote A2A agent can, internally, use its own MCP tools and delegate to its own downstream A2A agents — because A2A treats remote agents as opaque, the orchestrator neither knows nor cares how the events agent does its job. The boundary is clean: tools within an agent (MCP), agents between (A2A). That composition is how systems larger than any single agent get built.

When you need only one

You won’t always need both. If you’re building a single self-contained agent that uses tools but doesn’t collaborate with other autonomous agents, you need MCP and not A2A — most single-agent applications are here. Conversely, if you’re exposing an existing agent for other organizations’ agents to delegate to, or orchestrating across autonomous agents you don’t own, you need A2A (and each agent still uses MCP internally for its own tools). The decision is per-connection: MCP for every agent-to-tool link, A2A for every agent-to-agent link, and most systems have more of the former than the latter.

Pick this when

MCP — for connecting an agent to a tool, data source, or resource it invokes and controls (the common case; nearly every agent needs it).
A2A — for connecting an agent to another autonomous agent it delegates to, especially across teams or organizations, for long-running collaborative work.
Both — for any multi-agent system: MCP for each agent’s tools, A2A for the agent-to-agent delegation. This is the norm for anything beyond a single tool-using agent.
Neither as a rivalry — never choose “MCP or A2A” for the same connection; they’re different layers, and the question is always which layer a given dependency lives at.

Key takeaways

MCP and A2A are complementary layers, not competitors: MCP connects an agent to its tools/context (vertical), A2A connects an agent to other agents (horizontal).
Tell which a dependency is by what it is: a discrete capability you invoke and control is a tool (MCP); an autonomous, opaque, task-lifecycle system is an agent (A2A).
Real systems use both, and they nest cleanly — each A2A agent internally uses its own MCP tools and downstream A2A agents, because A2A treats remote agents as opaque.
The rule: tools within an agent (MCP), agents between (A2A); decide per-connection, and most systems have more MCP links than A2A ones.
Single tool-using agents need only MCP; cross-organization or multi-agent delegation needs A2A (with MCP still inside each agent) — never frame them as an either/or for one connection.