Interview Bank

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。

How to reason about it · think before answering

1. The real question is the second half. 'It helps gateways route' is half an answer; the interviewer is waiting for a concrete attack, which separates having read the spec from having understood it.
2. Why mirror: intermediaries should not parse the body to make decisions. A load balancer routing by method, a rate limiter capping tools/call, an observability probe tagging spans — all can read a header instead of deserializing tens of kilobytes. The same applies to Mcp-Name (from params.name or params.uri) and MCP-Protocol-Version.
3. Then derive the risk: if intermediaries decide on the header and the server executes on the body, there are two sources of truth. Concretely, a gateway configured as 'tools/list is unauthenticated, tools/call is authenticated' is bypassed by sending the header as tools/list and the body as tools/call. The same trick evades rate limits, audit tagging, and per-parameter regional isolation.
4. Conclusion: the spec therefore requires any server that processes the body to validate the match and reject with 400 plus -32020 (HeaderMismatch). It is not pedantry — it collapses two sources of truth back into one.
5. Volunteer the implementation trap: header values are visible ASCII only, so non-ASCII tool names or resource URIs use the =?base64?...?= sentinel, and the server must decode before comparing or its own check will reject valid requests. Integer values should be compared numerically, not as strings.
6. Likely follow-up: should intermediaries validate too? The spec advises that any intermediary enforcing policy from mirrored headers first confirm MCP-Protocol-Version names a revision that mandates header-body validation, and otherwise reject rather than trust unvalidated headers.

分析过程 · 先想清楚再作答

1. 这题的题眼在后半句。只答「方便网关路由」是答了一半，面试官等的是「不一致会怎样」——能不能自己举出攻击场景，是区分「读过规范」和「理解规范」的地方。
2. 先说为什么镜像：中间层不该为了做决策去解析请求体。负载均衡想按方法分流、限流器想给 tools/call 单独设阈值、可观测探针想打标签，只看头就够了，不用把几十 KB 的 body 反序列化一遍。同理还有 Mcp-Name（取自 params.name 或 params.uri）和 MCP-Protocol-Version。
3. 再推风险：既然中间层按头决策、服务端按体执行，两个事实来源就分叉了。举个具体的：网关配了「tools/list 免鉴权、tools/call 要鉴权」，攻击者把头写成 tools/list、体写成 tools/call，鉴权就被绕过去了。同样的套路可以绕限流、绕审计、绕按参数值做的地域隔离。
4. 结论：所以规范规定处理请求体的服务端必须校验头体一致，不一致必须回 400 加 -32020（HeaderMismatch）。这不是格式洁癖，是把「两个事实来源」重新合并成一个。
5. 实现上有个坑值得主动说：头值只能是可见 ASCII，非 ASCII 的工具名或资源 URI 要用 =?base64?...?= 哨兵格式编码，服务端必须先解码再比对，否则自己的校验会把正常请求判成不一致。整数值应当按数值比较而不是按字符串比较。
6. 可预期的追问：中间层自己要不要校验？规范建议按头做策略的中间层先确认 MCP-Protocol-Version 指向的是一个要求头体校验的版本，版本更老或头缺失时应当直接拒绝，而不是信任未经校验的头值。