Protocol Selection: MCP Vs A2A
Pick the right wire protocol, MCP or A2A or both, by looking at the shape of each connection and its security needs, rather than defaulting to whatever is trending.
Description
When agents run in separate places, you need a protocol for them to talk over the wire. Pick it by looking at the shape of each connection. Use MCP when a host agent needs clean, typed access to tools and data sources. MCP stands for Model Context Protocol. Use A2A when separate agents work together as peers, especially across team or trust boundaries. A2A stands for Agent-to-Agent. Use both when one agent is at once a tool user and a peer. Make security the deciding factor, not which protocol you read about first. Look at what credentials are exposed, how the connection is encrypted, and how user identity is passed along.
When to apply
Use this when an agent has to talk to external tools, data, or other agents across a process boundary or an organisational boundary. Apply it before you open a port or publish a manifest. Don't apply it when everything runs in one process, because direct function calls are simpler and avoid the protocol overhead. One exception: a single-process system that expects to split soon may still gain from drawing the protocol seam early.
What it involves
- Map each integration to a protocol family
- Apply security considerations as a gate
- Compose MCP and A2A where the agent is both consumer and peer
- Pin protocol versions and capability manifests
- Record the rejected alternatives
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