Interview Bank

Prompt Engineering From Scratch in 5 Days

D3 Structured Output: JSON Schema, Templates and Variables, Multilingual Output

• Why should structured output be enforced with a schema instead of a 'please respond in JSON' instruction, and do you still need validation once the schema passes?结构化输出为什么要用 schema 约束，而不是在提示词里写「请输出 JSON」？schema 通过之后还需要校验吗？

  Common in ChinaCommon overseasBasic#structured-output#json-schema

  How to reason about it · think before answering

  1. This screens for whether the candidate has ever wired model output into code. People who only read output with their eyes think 'respond in JSON' is enough.
  2. List what that instruction cannot prevent: prose wrapped around the JSON, inconsistent key spelling, numbers as strings, extra keys, missing keys when an array is empty. Each maps to a schema keyword: required, enum, type, additionalProperties.
  3. Then the mechanism: the schema constrains generation itself, the model can only produce that shape, so the gain is qualitative rather than incremental.
  4. The second half is the differentiator: schemas constrain shape, not content — integer is not 4xx, string is not non-empty. Business rules still need code-level validation that returns an error list for retries.
  5. Follow-ups: strict-mode limits — every property in required, additionalProperties false, a supported subset of JSON schema, a first-use compile cost; and how to express optional fields — allow null in the type rather than dropping the key from required.

  分析过程 · 先想清楚再作答

  1. 这题在筛「有没有真的把模型输出接进过程序」。只在聊天窗口里用过模型的人会觉得「请输出 JSON」够了，因为他们是用眼睛读的。
  2. 拆法：列出「请输出 JSON」挡不住的几种踩空——外面包一段解释、字段名拼法不一致、数字变字符串、多出字段、空数组时省掉键。每一种都对应 schema 里的一个关键字：required、enum、type、additionalProperties。
  3. 再答原理：schema 在生成时约束形状，模型不是「生成完再检查」而是「只能生成这个形状」，所以稳定性是质变而不是量变。
  4. 后半句是区分度：schema 只能约束形状，不能约束内容——整数不等于 4xx，字符串不等于非空。业务规则必须在代码里再查一遍，校验函数返回错误列表供重试使用。
  5. 可预期的追问：严格模式有什么限制？所有字段都要进 required、要写 additionalProperties false、只支持 schema 子集、首次编译有开销；以及「可选字段怎么表达」——类型允许 null 而不是从 required 里去掉。

  Key points

  • 'Respond in JSON' only guarantees JSON, not which JSON: wrapper prose, key spelling, stringified numbers, extra or missing keys all slip through
  • A schema constrains generation itself; required, enum, type and additionalProperties each block one failure class
  • Validation is still needed after the schema passes because correct shape does not mean correct content
  • Strict mode needs every property in required and additionalProperties false; express optional fields by allowing null

  答题要点

  • 「请输出 JSON」只约束「是 JSON」，挡不住包解释文字、字段名不一致、数字变字符串、多字段、省键这几种踩空
  • schema 在生成时约束形状：required、enum、type、additionalProperties 各挡一种错误
  • schema 通过之后仍要校验业务规则，因为形状正确不等于内容正确
  • 严格模式要求所有字段进 required 且 additionalProperties 为 false；可选字段用允许 null 表达