Interview Bank

MCP in 7 Days: Wire Tools Into Any Agent

D1 Why a Protocol: the Host/Client/Server Triangle, JSON-RPC Messages, and Three Primitives

• The 2026-07-28 revision made MCP stateless and removed the initialize handshake. What does that cost, and how should a server that still needs state handle it?2026-07-28 这一版把 MCP 改成了无状态协议，删掉了 initialize 握手。这么改的代价是什么？服务端还想保存状态该怎么办？

  Common in ChinaCommon overseasDeep dive#protocol-versions#statelessness

  How to reason about it · think before answering

  1. The discriminator is whether you know what this revision changed. Anyone answering from memory about handshakes, session IDs, or stream resumption exposes themselves — all three were removed.
  2. State the change first: no initialize and no notifications/initialized; every request carries its protocol version and client capabilities in _meta, and a new server/discover method, which servers MUST implement, returns versions, capabilities, and identity in one call.
  3. Weigh it as saved versus paid. You pay in payload size, repeating the version and capability block on every request. You save three things: any replica can serve any request so scaling needs no sticky routing, unrelated requests can interleave on one connection, and after a restart in-flight requests simply get resent.
  4. Conclusion: it trades bandwidth for scalability — near-invisible on local stdio, valuable for multi-replica remote deployments.
  5. For state, use explicit handles: a creation tool returns a server-minted id, and later calls pass it back as an ordinary argument while the server keys its own storage on it and documents the lifetime in the tool description.
  6. Likely follow-up: is a handle safe? Say it unprompted — a handle is a name, not a credential. Re-authorize the caller on every call, generate handles with a secure random source, bind them to the authenticated principal, and expire them.

  分析过程 · 先想清楚再作答

  1. 这题的区分度在「知不知道这一版改了什么」。凭旧记忆答握手、会话标识、断流续传的人会当场暴露，因为这三样在这一版全被删了。
  2. 先说改了什么：没有 initialize 与 notifications/initialized，每条请求在 _meta 里自带协议版本与客户端能力；新增 server/discover 供客户端一次性取回版本、能力与身份，服务端必须实现它。
  3. 拆代价的角度是「省了什么、贵了什么」。贵的是报文：每条请求都要重复带版本与能力块。省的是三件事——任意副本都能处理请求所以扩容不用粘性路由、一条连接可以穿插无关请求、进程重启后在途请求重发即可。
  4. 结论：这是一次拿带宽换可伸缩性的交易，对本机 stdio 几乎无感，对多副本的远程部署收益很大。
  5. 状态怎么办：显式句柄。创建工具返回一个服务端铸造的 id，后续调用把它当普通参数传回来；服务端把状态按这个 key 存在自己的库里，并在工具描述里写清有效期。
  6. 可预期的追问：句柄安全吗？必须补一句——句柄是名字不是凭证，服务端每次都要重新校验调用者身份，句柄要用安全随机数生成、绑定到已认证的主体、并设过期时间。

  Key points

  • The revision removed the initialize handshake, protocol-level sessions, the GET stream, and stream resumption; each request now carries version and capabilities
  • It added server/discover, which servers must implement, letting clients fetch versions, capabilities, and identity up front
  • The cost is larger payloads; the payoff is sticky-free horizontal scaling, interleaved unrelated requests, and cheap retry after restarts
  • Cross-call state moves to server-minted explicit handles passed as ordinary tool arguments, and a handle is never authentication

  答题要点

  • 这一版删掉了 initialize 握手、协议级会话、GET 长连接与断流续传，改为每条请求自带版本与能力
  • 新增 server/discover，服务端必须实现，客户端可在任何请求前一次性取回版本、能力与身份
  • 代价是报文变胖，收益是无粘性路由的横向扩容、连接上可穿插无关请求、重启后重发即可
  • 跨调用状态改用服务端铸造的显式句柄，作为普通工具参数传递，并且句柄不等于身份认证