Protocol Stack — MCP, A2A, ACP, UCP

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This article will cover the four protocols that achieved concurrent significant adoption by the end of Q1 2026: MCP (Model Context Protocol) for agent-to-tool connections; A2A (Agent-to-Agent) for inter-agent coordination across organizational boundaries; ACP (Agentic Commerce Protocol) for commercial negotiations; and UCP (Universal Commerce Protocol) for merchant ecosystem interactions.

The convergence was concurrent and largely uncoordinated. Each protocol addressed a different surface of the same underlying requirement: standardized machine-to-machine interaction without human intermediation. The resulting protocol stack was not designed as a stack. It became one.

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