A2A Basics: Agent Cards, Discovery, and Your First SendMessage
Once a teammate is another team's black box, you cannot see its memory, its tools, or its reasoning. All you get is the card it publishes, the task status it reports, and the artifacts it hands back. Today you hand-build an A2A v1.0 JSON-RPC server and client, discover a peer through its card, complete your first synchronous task, and walk through every breaking change from v0.3 to v1.0 so you can reject the old-shaped requests that outdated tutorials still send.
Today's Goals
- Can explain which part of an agent system A2A covers compared with function calling, MCP, and Skills, and answer why A2A is still needed now that MCP has a long-running task extension
- Can hand-build an A2A v1.0 server and client that complete card discovery, SendMessage, and GetTask, and return the error codes the specification requires
- Can list at least five breaking changes from v0.3 to v1.0, and explain why a server must treat a request without a version header as version 0.3
For two days the teammates lived in the same process. From today they are other teams' services: you cannot change their code or see their context. When you finish, come back to the top and check off the three goals.
Plain-Language Walkthrough
From one office to a partnership between companies
Four people in one department plan the offsite in a single room with a whiteboard on the wall: who booked which venue, who is talking to caterers, which quote is the newest. Anyone can look up and see. If someone misses something, a glance at the board fixes it.
Now the offsite is run jointly by four companies. An events company handles the venue, a hotel group handles catering, a travel agency handles transport, and your own finance team handles the budget. You cannot see the events company's whiteboard or how the hotel group schedules its staff. You get three things: their business card (who they are, what they do, how to reach them), the progress they report to you (accepted, working, done, cannot do it), and the documents they finally hand over (quotes, plans).
That is the change from Days 1 and 2 to today. In centralized orchestration, the orchestrator could put anything it wanted in front of a worker, and the worker's context was fully visible to it. From today, teammates are another team's black box, and the A2A (Agent2Agent) protocol standardizes exactly those three things: the card, the progress, and the documents.
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Interview questions
What is the difference between MCP and A2A? Which part of an agent system does each cover?MCP 和 A2A 有什么区别？各自管 Agent 系统里的哪一段？
Common in ChinaCommon overseasIntermediate#a2a#mcp#protocol🔒 Unlock this chapter to see the answer keys.
MCP already has an extension for long-running tasks. Why is A2A still needed?MCP 已经有长任务的扩展了，为什么还需要 A2A？
Common in ChinaCommon overseasDeep dive#a2a#mcp#long-running🔒 Unlock this chapter to see the answer keys.
Your A2A server receives a message/send request, or a request with no version header. How should it respond?你的 A2A 服务端收到一条 message/send 请求，或者一条没带版本头的请求，该怎么回？
Common in ChinaCommon overseasIntermediate#a2a#versioning#compatibility🔒 Unlock this chapter to see the answer keys.
What belongs in an Agent Card and what does not? Where is the line between the public card and the extended card?Agent Card 里该放什么、不该放什么？公开名片与扩展名片怎么分界？
Common in ChinaCommon overseasIntermediate#a2a#agent-card#security🔒 Unlock this chapter to see the answer keys.