A2A vs MCP vs OpenAPI: when to use which protocol

Three protocols, three different problems:\n\n**OpenAPI** — designed for humans building integrations. REST + JSON Schema. Great for stable, well-documented services that developers consume.\n\n**MCP (Model Context Protocol)** — designed for LLMs consuming tools. Structured tool definitions, resource URIs, sampling. Optimized for the LLM-to-tool boundary.\n\n**A2A** — designed for agents calling agents. JSON-RPC 2.0, task lifecycle, agent cards. Optimized for autonomous agent-to-agent workflows.\n\nThey're complementary, not competing. A single service might expose all three. The A2A layer sits above the others — an agent that needs to call a tool uses MCP internally, but exposes A2A to other agents.\n\nThe confusion comes from trying to pick one. Pick all three for the appropriate boundary.