A2A vs MCP: Which to Use

Quick answer: Use A2A when one agent delegates work to another agent, because it describes agent messages, tasks, status updates, and artifacts. Use MCP to expose tools and resources that a client can call. They model different relationships rather than compete, and many systems use both, with MCP offering the tools and A2A carrying the agent-to-agent work.

The relationship each models

The cleanest way to choose is to ask who is talking to whom.

MCP is built around a client calling tools and reading resources. It exposes capabilities that something invokes, and the shape is a call against a named tool or a fetch of a resource. A2A is built around an agent handing work to another agent, where the work can take time, move through status, and produce an artifact.

One is a tool exposure model, the other is a task delegation model.

Picture a coding assistant that can search a codebase and run builds. The search and build capabilities are exposed as tools over MCP, and a client invokes them directly. When the same assistant hands a long refactor to a separate planning agent, that is a different relationship: it sends a task, watches the status, and reads the artifact when the work completes. The refactor cannot be reduced to one tool call, because it runs across many steps and needs progress and cancellation. The MCP tools still exist underneath, and the planning agent may call them while handling the A2A task. The two protocols describe two layers of the same system rather than competing options.

Side by side

Why they are not substitutes

A task that belongs in A2A does not collapse into a tool call without losing something. A delegated task can run for a while, report progress, be cancelled, and return an artifact, and a single tool invocation does not express that lifecycle. In the other direction, a simple tool or resource does not need the full task machinery, and wrapping it in A2A adds weight without value.

The honest move is to match the protocol to the shape of the work.

A common mistake is choosing by which protocol is newer rather than by the shape of the call. A short, immediate capability call stays simpler in MCP, and a long, delegated task belongs in A2A, and reversing that either bloats the simple case or starves the complex one.

How they fit together

The two often sit in the same system rather than one replacing the other.

1. Expose tools and resources over MCP for clients to call.
2. Use A2A when an agent delegates a task to another agent.
3. Let an agent use the MCP tools while handling A2A tasks.
4. Keep the long-running lifecycle in A2A and the capability calls in MCP.
5. Verify both surfaces with the same evidence discipline.

This keeps each protocol doing what it is designed for rather than forcing one to cover the other's case.

Powerduck treats both as surfaces to verify, with the A2A work kept as a supporting capability behind the core review and testing workflows rather than a reason to become a general client.

Frequently asked questions

Can I use A2A instead of MCP for tools? It is a poor fit. MCP is designed to expose tools and resources, while A2A models agent tasks and their lifecycle.

When is A2A clearly the right choice? When one agent hands work to another and needs status, cancellation, and artifacts rather than a single immediate call.

Do the two work together? Yes. A system can expose tools over MCP and carry agent-to-agent tasks over A2A at the same time.

The bottom line

Pick the protocol by the relationship. A2A handles agent-to-agent delegation with tasks, status, and artifacts, while MCP exposes tools and resources for clients. They do not substitute well, and forcing one into the other's role either loses the task lifecycle or adds needless weight. In many systems they sit side by side, and both deserve the same verification rather than trust by default. Read more in the A2A guide.