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From Frontend Engineer to Agent Engineer in 30 Days

D23 MCP and Skills: the Protocol, Server/Client, How It Differs From Function Calling; a Tour of the Claude Agent SDK

• What roles do the MCP server and client play, what can a server expose, and which transports exist?MCP 里 server 和 client 分别承担什么角色？server 能暴露哪几类东西，传输方式有哪些？

  Common in ChinaCommon overseasIntermediate#mcp#protocol#transport

  How to reason about it · think before answering

  1. This looks like recall, but it discriminates on two small things: whether you separate host from client, and whether you know there are primitives beyond tools. 'Server provides tools, client calls them' is below the bar.
  2. Lay out three roles: the server is the capability provider and its own process; the client is the piece inside the host that talks to exactly one server; the host is your agent application, holding several clients at once. People who conflate host and client fall apart the moment you ask how they would connect to three servers.
  3. Cover all three server-side primitives and say who chooses each: tools are executable actions chosen by the model; resources are read-only data addressed by URI; prompts are reusable templates — the latter two are normally chosen by the user or host. That 'who chooses' framing shows you actually read the spec: modelling a large document as a resource rather than a tool moves the decision to spend those tokens from the model back to a human.
  4. The client side declares capabilities too, letting the server call back into the host: sampling asks the host to run a model completion, roots tells the server which directories are visible, elicitation asks the host to collect user input. Naming them without elaborating is the right level of detail.
  5. Two transports: stdio for a local subprocess, Streamable HTTP for remote. The dated detail worth knowing is that the older two-endpoint HTTP+SSE transport is now legacy, kept only for backwards compatibility — presenting it as current signals you read last year's blog posts.
  6. Expect the follow-up: anything special about stdio servers? Stdout is reserved for JSON-RPC, so every log line must go to stderr or the client receives unparseable messages; and the host owns the subprocess lifecycle, so it must reap the child on exit or leave orphans behind.

  分析过程 · 先想清楚再作答

  1. 这题看着是背概念，实际区分度在两个小地方：一是能不能把宿主和 client 分开说，二是知不知道 tools 之外还有别的原语。只答「server 提供工具、client 调用工具」是及格线以下。
  2. 先把三个角色摆清楚：server 是能力提供方，一个独立进程；client 是宿主里负责跟某一个 server 说话的那一小块，一个 client 只连一个 server；宿主是你的 Agent 应用，它同时持有多个 client。很多人把宿主和 client 当成一个东西，一问「连三个 server 怎么办」就露馅。
  3. server 侧三种原语要一起说，并且要说清谁来选：tools 是可执行的动作，由模型来挑；resources 是按 URI 读的只读数据；prompts 是可复用的提示词模板，后两者通常由用户或宿主来挑。这句「谁来选」比原语名字本身更能体现你真读过协议——把一份大文档做成 resource 而不是 tool，等于把花不花这笔 token 的决定权从模型手里收回给人。
  4. client 侧也能声明能力让 server 反过来请求宿主：sampling 是让宿主跑一次模型补全，roots 是告诉 server 哪些目录可见，elicitation 是请宿主向用户要一条输入。知道有这三样、不展开，分寸刚好。
  5. 传输两种：stdio 用于本地子进程，Streamable HTTP 用于远程。这里有个时间戳式的加分点——旧的 HTTP 加 SSE 双端点传输已经被标为 legacy，只为兼容老客户端保留；把它当现行方案讲，等于告诉对方你看的是去年的文章。
  6. 可以预期的追问：stdio server 有什么特别要注意的？答 stdout 被 JSON-RPC 独占，所有日志必须走 stderr，否则 client 会收到解析不了的消息；另外子进程的生命周期归宿主管，退出时要杀掉，不然留一堆孤儿进程。

  Key points

  • The server is the capability provider in its own process; a client connects to exactly one server; the host holds many clients
  • Three server-side primitives: tools chosen by the model, resources as URI-addressed read-only data, prompts as reusable templates — the latter two usually chosen by a human
  • Clients can declare sampling, roots and elicitation so the server can call back into the host
  • Two transports: stdio for local subprocesses and Streamable HTTP for remote; the old HTTP+SSE transport is legacy
  • On stdio, stdout belongs to JSON-RPC so logs must go to stderr, and the host must reap the child process

  答题要点

  • server 是能力提供方（独立进程），client 是宿主里连接单个 server 的那一块，宿主可以同时持有多个 client
  • server 侧三种原语：tools 由模型挑，resources 是按 URI 读的只读数据，prompts 是可复用模板，后两者通常由人来挑
  • client 侧还能声明 sampling、roots、elicitation，让 server 反过来请求宿主做事
  • 传输两种：stdio（本地子进程）与 Streamable HTTP（远程）；旧的 HTTP 加 SSE 已是 legacy，不要当现行方案讲
  • stdio server 的 stdout 被 JSON-RPC 独占，日志必须走 stderr；子进程生命周期由宿主负责回收