MCP or A2A? Two protocols for AI agents compared
The Model Context Protocol and the Agent2Agent protocol are often presented as competitors. In fact, they solve different problems and complement each other. For security, both leave the same question open.
Leutrim Miftaraj, Founder, Innopulse Consulting GmbH
Published
Key points
- MCP connects an agent with tools and data; A2A connects agents with each other.
- Many architectures use both: MCP inward, A2A between agents of different vendors.
- Neither protocol proves whether an action was allowed on someone's behalf; that requires mandate and decision.
What this is about
The Model Context Protocol connects an agent with tools and data sources: file systems, databases, ticketing systems, interfaces. An MCP server provides tools with descriptions, and the agent calls them. Authorization is based on OAuth 2.1.
The Agent2Agent protocol connects agents with each other. Agents describe their capabilities in an Agent Card, and other agents hand them tasks, track the status and receive results.
The comparison
| Feature | MCP | A2A |
|---|---|---|
| Connects | Agent with tools and data | Agent with agent |
| Origin | Anthropic, open project | Google, Linux Foundation |
| Description | Tools with parameters | Agent Card with capabilities |
| Typical flow | Tool call and response | Task, status, result |
| Authorization | Based on OAuth 2.1 | Various methods per agent |
| Typical risk | Tool poisoning, overly broad rights | Widening authority when passing on |
When to use what
- MCP when an agent is to access its own or third-party systems: read data, carry out actions in tools.
- A2A when tasks are distributed between specialized agents from different vendors.
- Both when an assistant hands tasks to specialist agents that in turn access systems through MCP.
How the two work together
A travel assistant hands the hotel booking to a hotel agent via A2A. The hotel agent uses a hotel's booking interface through MCP. Authority is passed on at two points: from the assistant to the hotel agent and from the hotel agent to the tool. At both points, the scope must be preserved and may only narrow. A delegation chain makes this verifiable.
Common misconceptions
- “A2A replaces MCP.” A2A connects agents, MCP tools. An agent that takes on tasks still needs access to tools.
- “The protocols govern authority.” They govern communication and authentication. What is allowed on whose behalf is not in the protocol messages.
- “A tool with a good description is safe.” Descriptions can contain hidden instructions (tool poisoning).
The role of KYA
KYA verifies neither MCP nor A2A messages. KYA verifies the evidence that should carry actions in both worlds: the principal's mandate, the delegation along the chain, the decision before execution and the receipt afterwards. The article Securing AI agents in MCP describes how MCP servers are secured.
Decision guide in three questions
- Should the agent access systems or data? Then MCP is the fitting protocol.
- Should tasks move between agents of different vendors? Then A2A is added.
- Is authority passed on? Then every level must be verifiable as delegation, regardless of the protocol.
Frequently asked questions
What is the difference between MCP and A2A?
MCP connects an agent with tools and data sources. A2A connects agents with each other so they can hand over tasks and exchange results.
Do I need both protocols?
Not necessarily. Whoever only connects one agent with systems manages with MCP. As soon as tasks move between agents of different vendors, A2A is added.
Which protocol is more secure?
Both have their own risks: with MCP, above all tool poisoning and overly broad rights; with A2A, widening authority when passing it on. Security comes from limits and checks, not from the choice of protocol.
Who is behind MCP and A2A?
MCP was introduced by Anthropic and is developed as an open project. A2A was introduced by Google and handed to the Linux Foundation.
How do I check actions over MCP and A2A?
With a gateway that checks every action with consequences against the mandate, and with evidence of delegation, decision and execution that KYA can verify.
Can an MCP server also be an agent?
Technically, an agent can itself sit behind an MCP tool. For security, what counts then, as with A2A, is that passing on the instruction is limited and traceable.
Are there alternatives to MCP and A2A?
There are further approaches and vendor-specific interfaces. MCP and A2A are currently the most widespread open protocols for their respective tasks.
How do I protect my own MCP server?
With authentication following OAuth 2.1, least privilege per tool, clear descriptions without hidden instructions and a check of every action with consequences before execution.
Does KYA check tool calls in real time?
No. KYA verifies evidence such as mandate, decision and receipt. Real-time checking is done by a gateway with a decision service before execution.
Sources
- Model Context Protocol: Spezifikation: https://modelcontextprotocol.io/specification
- Agent2Agent-Protokoll (A2A), Repository: https://github.com/a2aproject/A2A