MCP vs A2A: Tools and Agent-to-Agent Communication Are Not the Same

Evidence check: 25 October 2026. Product details reflect the referenced documentation observed at this cutoff; exact plans, prices, regional eligibility and unverified release dates are not claimed. This is an editorial guide, not a hands-on test.

Two levels of integration

Model Context Protocol (MCP) defines a way for applications and agents to discover and invoke tools or access supplied context. Agent2Agent (A2A), announced by Google on April 9, 2025, focuses on interoperability between autonomous agents and the tasks they exchange. The concepts can complement each other: one agent may use MCP tools internally while communicating with another agent through an agent-to-agent interface. Neither protocol specifies your business authorization policy by magic.

A tool connection is not permission

Tool descriptions make capabilities discoverable, but a host must decide whether a user can actually invoke them. For a remote MCP server, evaluate OAuth and token scope, server trust, network origin and data shared with the remote operator. MCP documentation illustrates authorization at the protected HTTP resource and also advises handler-level verification. When an agent has an MCP client, check whether each tool is read-only, writes data or can chain into further actions.

Agent collaboration adds responsibility

An A2A-style boundary may exchange task descriptions, status and results between systems run by different teams. A receiving agent should not automatically inherit the sending agent’s account privileges. Authenticate both sides, record who delegated the task, restrict the accepted scope and specify an expiry. Shared narrative context should be stripped of hidden credentials and irrelevant user messages.

Versions and compatibility

MCP has versioned specifications and SDK release lines. The TypeScript v1 documentation describes support up to the 2025-11-25 spec, while separate documentation references the 2026-07-28 era. Avoid assuming a library implementing one protocol release supports a newer feature or authentication mode. Record negotiated protocol version and SDK version in deployment inventory.

A practical selection rule

Use a direct, narrow API when the application already knows the exact operation. Use MCP when a consistent tool-access interface offers meaningful integration value. Use an agent-to-agent protocol only when autonomous services truly need to exchange work or status across an organizational boundary.

Testing checklist

Exercise unauthorized calls, mismatched versions, malformed schemas, timeout and cancellation, unexpected file contents and attempts to relay permissions. Verify that the receiving side can decline a task safely and that no hidden automatic side effect occurs simply because another agent requested it.

Security architecture exercise

Sketch three systems: a user-facing assistant, a document-search tool and a specialist agent operated by another organization. The assistant may speak MCP to the search service for bounded retrieval and use an agent-to-agent task protocol for delegation. Authentication for the tool is not transferable to the specialist: the remote party needs a separate trust agreement, narrowly scoped task payload and its own authorization. Identify who stores task messages, who can cancel the task and how the user can see which party performed the work.

Verify interoperability before procurement

Protocol names do not guarantee compatible authentication, streaming, notifications or lifecycle handling between products. Freeze spec versions and test the exact SDKs and transports at both ends. Include expired tokens, oversized attachments, mismatched versions and a receiving system that declines a task. If the job is simply to fetch one record from an API, adding an agent-to-agent layer is unnecessary complexity. Plan message retention and incident reporting across organizational boundaries rather than assuming a shared audit log exists.