面试题库

5 天上下文工程

D1 上下文是最稀缺的资源：窗口、注意力衰减与成本，从提示词工程走到上下文工程

• 窗口越来越大了，为什么不能把所有可能有用的资料都塞进去？Context windows keep growing. Why not just put everything potentially relevant into the prompt?

  国内高频海外高频进阶#context-rot#attention-budget#cost

  分析过程 · 先想清楚再作答

  1. 这题的题眼是「你知不知道窗口是容量、注意力是预算」。只回答「太贵了」的人会被判成没做过工程，因为成本是三笔账里最容易想到、也最不致命的一笔。
  2. 怎么拆：分成能力和代价两条线。能力这条线要点出上下文腐烂——随着上下文变长，模型准确回忆其中信息的能力会下降，根源在于 Transformer 里 n 个 token 有 n 平方级别的两两关系，注意力被摊薄；而且训练语料里长序列本来就少，处理长距离依赖的参数不够多。
  3. 关键是要强调它是一条缓坡不是一道悬崖：没有哪个长度会突然崩掉，每多塞一千个不相干的 token，正确率就低一点点。这个措辞能立刻区分读过一手材料的人。
  4. 代价这条线给三笔账：钱（模型无状态，每轮全量重发，总输入是累加值不是最后那次的值）、延迟（首字返回变慢）、正确率（无关内容稀释注意力）。第三笔最贵，因为它不会报错，只会给出看起来合理但违反了约束的回答。
  5. 可预期的追问：那你怎么判断某段内容该不该加？给一条可执行的判据——说不出它会改变模型哪一个具体决定，就不该加。

  How to reason about it · think before answering

  1. The hinge is whether you treat the window as capacity or attention as a budget. Answering only with cost reads as inexperience, since cost is the easiest and least dangerous of the three bills.
  2. Split into capability and cost. On capability, name context rot: recall accuracy degrades as context grows, rooted in the n-squared pairwise relationships a transformer maintains over n tokens, plus the fact that long-range parameters are underrepresented in training.
  3. Stress that this is a gradient, not a cliff. No specific length breaks; every thousand irrelevant tokens shaves a little accuracy. That phrasing distinguishes people who read primary sources.
  4. On cost, give three bills: money (the model is stateless, so every turn resends everything and the total is cumulative, not the last call), latency (slower time to first token), and accuracy (irrelevant content dilutes attention). The third is worst because it never raises an error, it just returns a plausible answer that violates a stated constraint.
  5. Expect the follow-up: how do you decide whether a given chunk earns its place? Give an operational test: if you cannot name the specific decision it changes, it does not go in.

  答题要点

  • 窗口是容量，注意力是预算；容量够不代表模型用得好。
  • 上下文腐烂：上下文越长，准确回忆的能力越差，是渐进的性能梯度而不是一道悬崖。
  • 三笔账：钱（每轮全量重发，成本是累加值）、延迟、正确率。
  • 正确率那一笔最危险，因为它不报错，只会给出看似合理却违反约束的回答。
  • 判据：说不出这段内容会改变哪一个具体决定，就不该放进去。

  Key points

  • The window is capacity; attention is the budget. Fitting is not the same as being used well.
  • Context rot: recall degrades as context grows, as a gradient rather than a hard cliff.
  • Three bills: money (stateless models resend everything each turn, so cost is cumulative), latency, and accuracy.
  • Accuracy is the dangerous one because it fails silently with plausible answers that break stated constraints.
  • Test: if you cannot name the specific decision a chunk changes, leave it out.