A2A vs MCP: Comparing Google's Agent-to-Agent Protocol and Anthropic's Model Context Protocol

If you are building AI agents, you have probably heard about two emerging protocols: Anthropic's Model Context Protocol (MCP) and Google's Agent-to-Agent (A2A) protocol. Both standardize how AI systems communicate. Both are open source. Both have major industry backing.

But they are not interchangeable. MCP defines how an agent connects to tools and data sources. A2A defines how agents connect to each other. Understanding the boundary between them is critical for anyone designing multi-agent architectures, because choosing the wrong protocol for the wrong layer creates unnecessary complexity and brittle integrations.

This article compares A2A and MCP across every dimension that matters: purpose, creator, transport, discovery, authentication, and primary use case. At the end, you will know exactly where each protocol fits in your agent stack and how to use both together through a single gateway.

The Core Distinction: Agents-to-Tools vs Agents-to-Agents

The simplest way to understand the difference: MCP is vertical and A2A is horizontal.

MCP connects an agent downward to the tools and data it needs to do its job. When an AI coding assistant calls a file search tool, executes a database query, or reads a document from cloud storage, that is an MCP interaction. The agent is the client. The tool server is the provider. The protocol standardizes how the agent discovers available tools, sends structured inputs, and receives results.

A2A connects an agent sideways to other agents that can help with parts of a larger task. When an orchestrator agent delegates a research subtask to a specialist research agent, or when a customer service agent hands off a billing question to a billing agent, that is an A2A interaction. Both participants are agents. The protocol standardizes how they discover each other's capabilities, negotiate task handoffs, exchange messages, and manage long-running collaborative workflows.

Think of it this way: MCP is how an agent uses a screwdriver. A2A is how an agent asks another agent to handle the electrical work while it focuses on the plumbing.

Side-by-Side Comparison

How Discovery Works in Each Protocol

Discovery is where the two protocols diverge most clearly, because they are discovering fundamentally different things.

MCP Discovery: What Can This Server Do?

In MCP, an agent sends a {"method":"tools/list"} request to a server and receives a list of tools. Each tool has a name, a human-readable description, and a JSON Schema defining its inputs. The agent can then call any tool by posting a tools/call message with the tool name and arguments. No pre-configuration. No wrapper code. The agent reads what is available and decides what to use based on the task at hand.

A2A Discovery: What Can This Agent Do?

In A2A, every agent publishes an Agent Card at a well-known URL (typically /.well-known/agent.json). The Agent Card describes the agent's name, description, supported skills, accepted input formats, authentication requirements, and capabilities (like streaming or push notifications). A client agent reads the Agent Card to understand what the remote agent can do, then sends a task with messages describing what it needs done.

The key difference: MCP discovery returns a list of atomic operations (tools) with precise input schemas. A2A discovery returns a high-level description of an agent's skills and what kinds of tasks it can handle. MCP is "here are the specific functions you can call." A2A is "here is what I am good at, send me a task and I will figure out how to do it."

Task Lifecycle: Request-Response vs Long-Running Collaboration

MCP interactions are typically request-response. The agent calls a tool and gets a result. Even with streaming, the pattern is: send a request, receive a response (possibly in chunks). The server does not initiate communication. The agent drives every interaction.

A2A interactions are task-based and potentially long-running. A client agent sends a task to a remote agent, which may require minutes, hours, or days to complete. The remote agent can push status updates, request clarification (via messages), stream partial results (via artifacts), and eventually mark the task as complete or failed. Both sides can send messages. The remote agent is an autonomous participant, not a passive tool.

This distinction matters for architecture decisions. If your agent needs to query a database and get a result in milliseconds, that is a tool call (MCP). If your agent needs to delegate a research report to a specialist that will take 10 minutes of autonomous work, that is a task delegation (A2A).

Where They Overlap and Where They Do Not

There is a gray zone. A2A supports "instant" tasks that look like request-response. MCP supports session-based interactions with progress notifications. In theory, you could model tool calls as A2A tasks or agent delegation as MCP tool invocations.

In practice, trying to use one protocol where the other fits better creates friction:

• Using A2A for tool calls adds unnecessary overhead. Agent Cards, task lifecycle management, and message-based communication are heavy machinery for what should be a simple function call with typed inputs and outputs.
• Using MCP for agent delegation forces a tool-shaped interface onto what is fundamentally a conversation. You lose the ability for the remote agent to ask clarifying questions, push intermediate results, or manage long-running state.

The clean architecture: MCP for the tool layer, A2A for the agent coordination layer. Each protocol does its job without contorting to cover the other's domain.

The Two-Protocol Stack in Practice

Here is how A2A and MCP work together in a real multi-agent system:

In this flow, A2A handles the horizontal coordination between agents. MCP handles the vertical connection from each agent down to its tools. Neither protocol is doing the other's job.

When to Use MCP Alone

MCP is sufficient for the vast majority of single-agent applications. If your agent does not need to delegate work to other autonomous agents, you do not need A2A. An MCP gateway gives your agent access to dozens or hundreds of tools through one connection, which covers most production use cases.

When to Add A2A

A2A becomes valuable when you move from single-agent to multi-agent architectures. The protocol shines in enterprise environments where different teams build and deploy agents independently, and those agents need a standard way to find and collaborate with each other.

How ToolRoute Supports Both Protocols

ToolRoute is built as a protocol-agnostic gateway. The same tool catalog, the same authentication, and the same billing work across both MCP and A2A:

One API key works across all endpoints. Credits are deducted the same way regardless of which protocol your agent uses. This means you can start with MCP for single-agent tool calling, then add A2A for multi-agent orchestration later, without changing your authentication or billing setup.

The gateway handles protocol translation under the hood. Whether your agent sends a JSON-RPC tools/call via MCP, an A2A task message, or a REST POST, the request is routed to the same adapter, executed against the same underlying service, and billed at the same rate. Read the full documentation for integration examples in both protocols.

The Future: Convergence or Coexistence?

Some in the AI community have asked whether A2A and MCP will eventually merge into a single protocol. The short answer is probably not, and that is a good thing. The two protocols solve genuinely different problems. Merging them would create a bloated specification that does both jobs poorly.

What is more likely is convergence at the infrastructure layer. Gateways, registries, and orchestration platforms will support both protocols natively, letting developers pick the right protocol for each interaction without managing separate infrastructure. This is already happening. ToolRoute supports both today. Expect more platforms to follow.

The winning architecture for AI agent systems in 2026 and beyond is not MCP or A2A. It is MCP and A2A, each doing what it was designed for, connected through a unified infrastructure layer that handles the protocol boundaries so developers do not have to.

Frequently Asked Questions