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30 天从前端工程师到 Agent 工程师

D23 MCP 与 Skills：协议、server/client、与 function calling 区别；Claude Agent SDK 一览

• MCP 协议解决了什么问题？它和 function calling 有什么区别？What problem does MCP solve, and how is it different from function calling?

  国内高频海外高频基础#mcp#tool-calling#protocol

  分析过程 · 先想清楚再作答

  1. 这题有一个标准的错误答案，面试官就是靠它筛人：把 MCP 说成「function calling 的升级版」「以后不用写 function calling 了」。说出这句，后面讲得再多也已经扣完分了。
  2. 把两者放回各自的链路上就不会混：function calling 是「模型 ↔ 你的程序」之间的约定，MCP 是「你的程序 ↔ 能力提供方」之间的约定。它们不在同一段线上，所以是上下游，不是替代。
  3. 给一个能一句话验证的证据：MCP server 通过 tools/list 返回的每个工具，它的 inputSchema 本身就是 JSON Schema，你要做的只是把它搬进 function calling 的 parameters 字段发给模型。模型自始至终不知道 MCP 存在。接了 MCP 之后 function calling 那段代码一行都不会少。
  4. 再答「解决了什么问题」：接入成本从乘法变加法。N 个宿主乘 M 个能力等于 N 乘 M 份接入代码，有了协议就变成 N 加 M；顺带把责任边界划清楚了，第三方能力出问题不用先在你的服务里复现。
  5. 顺手把 Skills 也区分掉，这是很自然的追问：MCP 扩展的是「能做什么」（新增可调用的动作），Skills 扩展的是「怎么做得好」（一组提示词、脚本和参考资料打成的按需加载包）。一个给能力，一个给方法论。
  6. 可以预期的追问：那 MCP 的价值到底在哪？答案是它把「能力的发现与调用」标准化了，所以能力可以由别人维护、被多个宿主复用、不改代码就增删——但发给模型的那一段，永远还是 function calling。

  How to reason about it · think before answering

  1. This question has a canonical wrong answer that interviewers screen on: calling MCP 'function calling v2' or saying you no longer need function calling. Say that and the rest of your answer cannot recover the points.
  2. Put each one back on its own hop and the confusion disappears: function calling is the contract between the model and your program; MCP is the contract between your program and a capability provider. Different hops, so they stack — they do not replace each other.
  3. Offer a one-line proof: every tool returned by an MCP server's tools/list carries an inputSchema that is already plain JSON Schema, and all you do is copy it into the parameters field of a function-calling tool definition. The model never learns MCP exists, and adopting MCP removes not a single line of your function-calling code.
  4. Then answer what it actually solves: integration cost goes from multiplication to addition. N hosts times M capabilities means N times M integrations; a shared protocol makes it N plus M. It also draws a responsibility boundary — a third-party capability failing is no longer something you must first reproduce inside your own service.
  5. Volunteer the Skills distinction, since it is the natural follow-up: MCP extends what the agent can do (new callable actions), Skills extend how well it does it (a bundle of prompt, scripts and reference material, loaded on demand). One adds capability, the other adds method.
  6. Expect the follow-up: then where is MCP's value? In standardizing discovery and invocation, so capabilities can be owned by another team, reused by several hosts, and added or removed without a code change — while the hop to the model stays function calling.

  答题要点

  • function calling 是「模型和你的程序」之间的约定，MCP 是「你的程序和能力提供方」之间的约定，两者是上下游不是替代
  • MCP server 列出的每个工具最终仍要翻译成 function calling 的 JSON Schema 发给模型，模型不知道 MCP 存在
  • 它解决的是接入成本：N 个宿主乘 M 个能力的乘法，变成 N 加 M 的加法，同时把责任边界划到进程边界上
  • 把 MCP 说成 function calling 的升级版是最常见的错误答案，主动点破这一点会加分
  • 顺带区分 Skills：MCP 扩展「能做什么」，Skills 扩展「怎么做得好」

  Key points

  • Function calling is the model-to-your-program contract; MCP is the your-program-to-provider contract — they stack rather than replace
  • Every MCP tool still gets translated into a function-calling JSON Schema before it reaches the model, which never learns MCP exists
  • It solves integration cost: N hosts times M capabilities becomes N plus M, and the process boundary becomes the ownership boundary
  • Calling MCP an upgraded function calling is the classic wrong answer — naming that yourself scores points
  • Distinguish Skills too: MCP extends what the agent can do, Skills extend how well it does it