How does Model Context Protocol (MCP) differ from Agent-to-Agent (A2A) protocols?
TL;DR
MCP connects an agent to deterministic tools, databases, and APIs (vertical tool access). A2A coordinates task delegation, state handoffs, and capability discovery between autonomous peer agents (horizontal collaboration).
Production agent architectures separate tool invocation from peer delegation. MCP and A2A operate on orthogonal planes.
Model Context Protocol (MCP) — agent to tool
MCP standardizes how an LLM runtime discovers and calls deterministic external capabilities over stdio or Streamable HTTP:
- Querying Postgres or Supabase schemas
- Reading GitHub pull requests and CI checks
- Inspecting live DOM state through a headless browser hub
An MCP tool takes typed JSON inputs and returns deterministic structured outputs. It holds no autonomous goal loop of its own.
Agent-to-Agent (A2A) — agent to peer agent
A2A protocols govern how one autonomous agent delegates an open-ended objective to another specialist agent that owns its own planner, memory, credentials, and multi-step execution loop:
- Publishing an Agent Card declaring supported tasks and authentication scopes
- Negotiating long-running asynchronous tasks, streaming progress updates, and structured artifacts
- Enforcing cross-boundary permissions so Agent A never sees Agent B's raw database credentials
How they compose in one stack
Inside a multi-agent deployment, each specialist agent uses MCP to operate its private tools and databases while communicating across team boundaries via A2A handoff packets.