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7 天 MCP：把工具接进任何 Agent

D2 写第一个 MCP server：stdio 传输、官方 SDK、参数 schema、工具注解与 Inspector 调试

• 什么时候该返回 JSON-RPC 的 error，什么时候该返回 isError 为真的工具结果？给我一个判据。When should a tool return a JSON-RPC error versus a result with isError set to true? Give me a decision rule.

  国内高频海外高频进阶#error-handling#tool-design

  分析过程 · 先想清楚再作答

  1. 这题几乎是 MCP 服务端的入门分水岭。能背出「两类错误」只算及格，区分度在于能不能给出一条可执行的判据，以及知不知道错误文案是写给谁的。
  2. 拆法：问「谁能修好这个错」。请求本身不合法——工具名不存在、参数不满足调用工具的 schema、服务端内部异常——模型再怎么改参数都没用，这类走 JSON-RPC 的 error，典型是 -32602。工具跑了但业务没成——下游 API 失败、日期格式不对、金额越界——模型换个参数就可能成功，这类走 result 里的 isError。
  3. 判据一句话：**模型换个参数有没有可能成功？有就用 isError，没有就用 error。** 注意 isError 仍然是一个成功的 JSON-RPC 响应，resultType 照样是 complete。
  4. 结论要带上文案要求：规范说客户端应当把执行错误交给模型自我纠正，所以文案是写给模型看的，要列出可选值、正确格式、边界条件。写「参数错误」等于让模型瞎猜。
  5. 生产视角的坑：最危险的不是分错类，而是**两类都不返回**——不做校验，让非法输入算出 NaN 或空结果静默返回。模型会把错误答案当正确答案用下去，且不留痕迹。靠输出 schema 校验去兜底也不算处理，因为模型拿到的是一段 schema 堆栈。
  6. 可预期的追问：客户端要不要把协议错误也喂给模型？规范说可以，但基本没用，因为模型改不了；更该做的是记日志报警，那是你的 bug 不是模型的。

  How to reason about it · think before answering

  1. This is close to a pass/fail line for MCP server work. Reciting 'two kinds of errors' is baseline; the discriminator is producing an actionable rule and knowing who the error text is written for.
  2. Ask who can fix it. If the request itself is invalid — unknown tool, arguments failing the call-tool schema, an internal server fault — no amount of parameter tweaking helps, so return a JSON-RPC error, typically -32602. If the tool ran but the business case failed — downstream API error, bad date format, amount out of range — a different argument might work, so return isError in the result.
  3. The rule in one line: could the model succeed by changing an argument? If yes use isError, if no use error. Note that isError is still a successful JSON-RPC response with resultType complete.
  4. Carry the text requirement into the conclusion: the spec says clients should hand execution errors to the model for self-correction, so the message is written for the model. List allowed values, the correct format, the boundary. 'Invalid parameter' just makes it guess.
  5. The production trap is not misclassifying but returning neither — skipping validation so an illegal input yields NaN or an empty result that is silently returned. The model then uses a wrong answer with no trace. Leaning on output-schema validation is not handling it either, since the model receives a schema stack trace.
  6. Likely follow-up: should clients feed protocol errors to the model too? The spec permits it but it rarely helps, because the model cannot fix them. Log and alert instead — that one is your bug.

  答题要点

  • 协议错误走 JSON-RPC 的 error：未知工具、请求不满足 schema、服务端内部错，模型改参数也无济于事
  • 执行错误走结果里的 isError 为真：下游失败、业务校验不过，它仍是成功的 JSON-RPC 响应
  • 判据是模型换个参数有没有可能成功，有就 isError，没有就 error
  • 执行错误的文案写给模型看，要列出可选值与正确格式；最危险的是两类都不返回、静默给出错误结果

  Key points

  • Protocol errors use the JSON-RPC error field: unknown tool, schema-invalid request, internal fault — unfixable by the model
  • Execution errors use isError true in the result and remain a successful JSON-RPC response
  • The rule: if a different argument could succeed, use isError; otherwise use error
  • Write execution-error text for the model with allowed values and formats; the worst case is neither, silently returning a wrong result