The Missing Piece of the Agent Stack
MCP solved agent-to-tool communication. But agents also need to talk to each other. Before A2A, every multi-agent system used custom protocols — LangGraph, CrewAI, and AutoGen all had incompatible approaches. A2A defines a standard wire format for agent-to-agent communication, launched by Google in April 2025 and donated to the Linux Foundation.
How A2A Works
A2A is built on HTTP, JSON-RPC, and SSE. Agents publish Agent Cards in JSON format describing their capabilities. Other agents discover these at runtime and delegate tasks. The task lifecycle has defined states: submitted, working, completed, failed, canceled. Tasks can be short or long-running, supporting both polling and push notification patterns.
A2A vs MCP vs WebMCP vs AP2
MCP handles agent-to-tool (vertical). A2A handles agent-to-agent (horizontal). WebMCP handles agent-to-browser (session). AP2 rides on A2A for payment authorization. A complete enterprise stack uses all four. ADK 1.0 provides cross-language A2A support (Python, Java, Go, TypeScript) with declarative storage backends. A2A is the protocol that makes the 'agent workforce' vision real — when agents can discover and delegate across organizations, the network effects begin.
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