## Definition The **Agent-to-Agent (A2A) protocol** is an open standard for agents — possibly built by different vendors or on different frameworks — to discover one another, delegate tasks, and exchange results. Where the [[Model Context Protocol]] connects an agent *down* to tools and data (vertical), A2A connects an agent *across* to **other agents** (horizontal). ## What it standardizes - **Discovery** — an "agent card" advertising an agent's identity, skills, and endpoint. - **Delegation** — send a task, stream its progress, and receive the result over a common message format. - **Interoperability** — agents built on different stacks cooperate without a bespoke integration for each pair. ## MCP vs A2A - [[Model Context Protocol|MCP]]: agent ↔ tools / data. - A2A: agent ↔ agent. They are complementary halves of an agent's connectivity. Together they let a [[Multi-Agent System]] span teams and organisations instead of living inside one process. ## Why it matters Without a shared protocol, every agent-to-agent link is a custom integration. A standard turns a closed [[Orchestrator-Subagent Pattern|orchestrator/subagent]] setup into an open ecosystem where specialised agents can be composed like services. ## Related - [[Model Context Protocol]] - [[Multi-Agent System]] - [[AI Agent]] - [[Orchestrator-Subagent Pattern]] ## Sources - A2A (Agent-to-Agent) open protocol specification (2025).