Interview Bank

Context Engineering in 5 Days

D2 System Prompts and the Instruction Hierarchy: the Right Altitude, Persistent Instruction Files, Progressive Disclosure, Less Is More

• What belongs in the system prompt, what belongs in a persistent instruction file, and what should be loaded on demand?哪些内容该进系统提示，哪些该进持久指令文件，哪些该按需加载？

  Common in ChinaCommon overseasIntermediate#instruction-hierarchy#progressive-disclosure

  How to reason about it · think before answering

  1. This tests layering. Answering that frequently used content goes in the system prompt is circular, since defining frequently used is the actual question. Give an ordered decision procedure instead.
  2. Three questions, first yes wins. Would the model get it wrong without this rule? If not, delete. Is it needed for every class of task? If not, push it into a reference file and leave a one-line index. Whatever remains stays, rewritten as a mechanically checkable sentence.
  3. Add the category people miss: runtime facts that change over time, such as whether it is currently night hours, whether holiday shipping delays apply, or this session's order id. They look like rules but start lying to users hours later. They belong in tool output or in the user message.
  4. Conclusion: sort by stability and frequency. The more stable, the earlier; the rarer, the later; anything that changes every call never enters the system prompt. Bonus: this ordering is what makes a cache prefix hittable, so editing a task variable does not invalidate everything.
  5. Expect the follow-up: is pushing content down safer than deleting it? No. Reference files still have to be maintained and still consume context when read, just later and less often. Keeping things just in case is the main cause of prompt bloat.

  分析过程 · 先想清楚再作答

  1. 这题在考分层意识。只回答「常用的放系统提示」是循环论证——问题恰恰是怎么定义常用。面试官想听的是一条排序明确的判定流程。
  2. 怎么拆：给三问，第一个答是就落定。第一问，模型不看这条会做错吗，不会就删除；第二问，是不是每一类任务都用得上，不是就下沉到引用文件、主提示里只留一句索引；剩下的保留，并重写成能被机械核对的一句话。
  3. 补一层常被漏掉的划分：还有一类内容根本不属于以上三者——随时间变化的运行时事实，比如当前是不是夜间、是不是节假日延迟期、本次会话的订单号。它们看着像规则，写死在系统提示里就会在半天之后开始骗用户，应该由工具返回或每次拼进用户消息。
  4. 结论：判据是稳定程度加使用频率。越稳定越靠前，越少用越靠后；而每次都变的东西根本不进系统提示。附带一个工程收益——按稳定程度排序之后，缓存前缀才有机会命中，改一条任务变量不会打掉整段缓存。
  5. 可预期的追问：下沉是不是比删除安全？不是。搬进引用文件的内容仍然要维护、仍然会在需要时占上下文，只是晚一点少一点。真正没用的条目要删，「先留着以防万一」正是提示词膨胀的主因。

  Key points

  • Three questions decide placement: would the model err without it (no means delete), is it needed by every task class (no means push down), and the rest stays as a checkable sentence.
  • A fourth category is runtime fact (time of day, holiday delays, this session's order id); it belongs in tool output or the user message, not the system prompt.
  • Order layers by stability so the cache prefix stays hittable.
  • Pushing down is not free: reference files still cost maintenance and context, so genuinely useless rules should be deleted.

  答题要点

  • 三问定去处：模型不看会做错吗（不会就删）、每类任务都用得上吗（不是就下沉）、剩下的保留并重写成可核对的一句话。
  • 第四类是运行时事实（时间、节假日、本次订单号），不属于系统提示，应由工具返回或拼进用户消息。
  • 分层顺序按稳定程度排，稳定的在前，这样缓存前缀才有机会命中。
  • 下沉不是免罪符：引用文件仍要维护、仍会占上下文，没用的要删掉。