Architecture decision

MCP vs A2A

Does an agent need a tool, or does it need to collaborate with another agent?

Get the short answerReview evidence

The short answer

Use MCP for capabilities the calling agent should invoke as tools. Use A2A when the remote system owns its own agent loop, task lifecycle, and independently managed trust boundary.

Choose

Tools and MCP

Typed tools connect an agent to actions and data. The Model Context Protocol standardizes how compatible clients discover and invoke those capabilities.

  • The capability has typed inputs, outputs, and understandable side effects.
  • The caller should decide when and how the capability is used.
  • Several compatible clients should discover the same server.
Open the Tools and MCP guide

Choose

Agent-to-agent protocols

Protocols for discovering remote agents, negotiating supported interfaces, exchanging messages, and tracking long-running tasks across organizational boundaries.

  • The remote party decides how to complete a delegated outcome.
  • Tasks can be long-running, asynchronous, or require additional input.
  • The remote agent is independently deployed, secured, and operated.
Open the Agent-to-agent protocols guide
Use both

How the layers combine

  1. 01

    Use A2A for collaboration across agent boundaries.

  2. 02

    Let each participating agent use MCP internally for its own tools and data.

  3. 03

    Keep identity, authorization, and audit evidence explicit at both boundaries.

Decision review

Questions to answer before implementation

  • Is the remote capability deterministic enough to be a tool?
  • Who owns task planning and lifecycle state?
  • Can discovery happen before sensitive data is shared?
  • Are retries, cancellation, and duplicate work handled?
  • Can each system authenticate and authorize the other?

Evidence ledger

Primary documentation for both sides

Reviewed July 26, 2026. Use the provider documentation for current configuration and security behavior.

MCP specification

Model Context Protocol

The official protocol model, lifecycle, capabilities, and security guidance.

Connect Codex to MCP servers

OpenAI

Current Codex MCP configuration and client behavior.

A2A protocol specification

A2A Project

Agent discovery, messages, tasks, artifacts, transports, and security.