September 2026·Engineering·AGA-16
MCP, A2A, ACP, AP2: the agent protocol stack
Four protocols keep getting written about as rivals. They are layers. MCP gives an agent tools, A2A lets agents talk to each other, AP2 proves a human authorised a payment, and ACP completes the purchase. What each one actually does, who governs it now, and what a business has to implement to be sellable to an agent.
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