Practice. MCP for Tools, A2A for Agents¶

Many teams mix up two different problems too early:

how to connect tools and external systems;
how to organize collaboration between multiple agents.

Because of that, MCP and A2A start to look interchangeable. In reality, they belong to different architectural levels.

1. The Short Rule¶

If you want the shortest version:

MCP is for working with tools, resources, and adapters;
A2A is for exchanging tasks, context, and results between agents.

In other words, one protocol is mainly about agent-to-tool, while the other is about agent-to-agent.¹²

2. When You Almost Certainly Need MCP¶

MCP is useful when you have one or more external capabilities that need to be connected systematically:

document search;
CRM;
helpdesk;
internal APIs;
file resources;
knowledge bases;
execution sandboxes.

In that situation, the main goal is not to "build a society of agents," but to:

standardize the contract;
separate adapters from the core runtime;
simplify policy checks;
centralize logging, auth, and isolation.

That is where MCP fits naturally.

3. When You Actually Need A2A¶

A2A makes sense when you no longer just have tools, but truly distinct agents with separate responsibility:

a coordinator;
a researcher;
an analyst;
an executor;
a domain specialist.

And they need to:

hand tasks to each other;
delegate work;
exchange status;
return results not as tool payloads, but as the work product of another agent.

That means A2A appears not where you need "one more adapter," but where real agent boundaries already exist in the system.

4. The Typical Mistake: Building Multi-Agent Too Early¶

In practice, the confusion usually looks like this:

The team has one runtime.
It needs to connect three systems.
Instead of capability contracts, the team starts designing multi-agent orchestration.

That is almost always unnecessary complexity.

If the system still has no real reason to split responsibility between agents, then:

tools are better connected through MCP;
orchestration is better kept inside one runtime;
multi-agent coordination is better postponed.

A good practical rule is: if the entity does not make its own decisions and does not have its own operational role, it is probably not an agent yet. It is a capability.

4.1. Multi-agent reliability research currently reinforces skepticism¶

This point is worth one more practical note. Recent papers on multi-agent reliability do not yet provide a strong reason to make runtimes more complex by default. If anything, they show how quickly coordination failures, verification gaps, and ambiguity grow when a system is split without a clear need.

The practical reading is:

the research does not say “build more agents”;
the research says “if you already build multi-agent systems, use explicit contracts, verification loops, and diagnosable handoffs”;
the current best practice is still single-agent first.

That is why A2A should be introduced not because it looks architecturally elegant, but because the system already contains separate operational roles that can no longer be honestly described as tools.

4.2. Agreement between several agents is not independent verification¶

Even if several agents reach a similar conclusion, that still does not prove the system has produced an independent verification signal.

The common problems are familiar:

agents see the same contaminated context;
the reasoning paths are too similar;
the manager agent nudges downstream agents toward the expected answer;
the consensus looks convincing while resting on the same bad assumption.

That is why multi-agent agreement is better treated as a signal, not as proof of correctness.

5. Decision Table¶

Question	More likely `MCP`	More likely `A2A`
Do you need to connect an external API or resource?	Yes	No
Do you need a typed contract for tools?	Yes	No
Is there a separate agent with its own role and lifecycle?	No	Yes
Do you need delegation between agents?	No	Yes
Do you need to isolate adapter behavior and policy path?	Yes	Sometimes
Do you need to pass a task to another agent runtime?	No	Yes

6. What This Looks Like in a Mature Architecture¶

In a mature platform, these ideas do not compete. They sit on different layers.

MCP and A2A complement each other instead of replacing each other

flowchart LR
    A["Coordinator agent"] --> B["A2A handoff"]
    B --> C["Specialist agent"]
    A --> D["MCP client"]
    C --> E["MCP client"]
    D --> F["Tool / resource server"]
    E --> G["Tool / resource server"]

The healthy pattern is:

an agent works with tools through MCP;
an agent works with another agent through A2A;
policy and audit should cover both directions.

7. When Not to Use A2A¶

There are a few red flags:

you want to use a second agent only as an API wrapper;
the second agent has no separate policy surface;
it has no separate operational identity;
it can be described more simply as a capability contract;
parallelism and specialization do not produce clear value.

In all those cases, it is better to stay with MCP or even an ordinary gateway/adapter layer.

8. Minimal Code Sketch¶

Below is a very small sketch that shows the difference in thinking:

def call_tool_via_mcp(tool_name: str, payload: dict) -> dict:
    return {"kind": "tool_result", "tool": tool_name, "payload": payload}


def delegate_via_a2a(agent_name: str, task: dict) -> dict:
    return {"kind": "agent_result", "agent": agent_name, "task": task}

The point is not the code itself, but the interaction type:

a tool call returns a capability result;
an A2A handoff returns the work result of another agent.

Those are different operational semantics, and it is useful not to blur them together.

9. What to Do Right Away¶

Start with this short list and mark every "no" explicitly:

Do I need a new agent or just a new capability?
Does this entity have its own role, policy surface, and lifecycle?
Is this a delegation problem or an integration problem?
Can I explain why MCP is not enough here?
Am I building a multi-agent topology earlier than I actually need?

If those questions are hard to answer, it is usually safer to choose MCP first, not A2A.