Short answer: A2A (Agent-to-Agent) is the open protocol for making independent AI agents interoperate — discover each other, delegate tasks, and stream results — across vendors and frameworks. Use it when two or more agents built by different teams or products need to work together; keep using MCP for an agent talking to its own tools.
What A2A actually is
A2A was announced by Google in April 2025 with 50+ launch partners and donated to the Linux Foundation in June 2025, which is what made it a neutral standard rather than one vendor's SDK. Mechanically it is JSON-RPC 2.0 over HTTP(S):
- Agent Cards — a JSON document (served at a well-known URL) advertising an agent's identity, skills, endpoint, and auth requirements. This is the discovery layer: a client agent fetches the card to learn what a remote agent can do before sending it anything.
- Tasks — the unit of work. A client agent opens a task with a remote agent; the task carries a lifecycle (submitted → working → input-required → completed/failed) so long-running work is first-class, not a hack on request/response.
- Messages and artifacts — turns within a task, and the durable outputs a task produces.
- Streaming + push — Server-Sent Events for live progress, webhooks for very long tasks, so a delegating agent isn't forced to poll.
A2A vs MCP (the confusion to kill)
They are complements, not competitors. MCP standardizes the vertical connection — one agent to its tools, files, and data sources. A2A standardizes the horizontal connection — peer agents negotiating work with each other. A realistic stack uses both: your agent reaches its own tools over MCP, and hands a subtask to a partner company's agent over A2A. If everything runs inside one framework and one trust boundary, you don't need A2A at all — a plain subagent call is simpler and faster.
Why it matters right now
Adoption crossed from spec to shipping product in 2026: v1.0 of the protocol stabilized, major agent runtimes bundle A2A support (Nous Research's Hermes Agent shipped an A2A v1.0 plugin in its August 2026 release, closing one of its oldest feature requests), and enterprise platforms (Google ADK, Azure AI Foundry, SAP, Salesforce) expose A2A endpoints. Interop is becoming a checklist item the way OpenAPI once did.
Engineering cautions
- Trust boundary, not just transport. A remote agent is an untrusted principal: validate artifacts, scope credentials per task, and never let a remote agent's output flow into privileged actions without your own approval gates.
- Agent Cards are claims, not proof. Treat advertised skills like an unverified résumé; probe with a low-stakes task first.
- Lifecycle discipline. Model the input-required state explicitly, or delegated tasks will silently stall.
- Observability. Log task IDs end-to-end; cross-agent traces are your only debugging tool when the other side is a black box.