面试题库

5 天上下文工程

D3 工具结果与检索的上下文管理：按需加载、摘要与裁剪、结构化返回

• Agent 挂了三十个工具，明显吃不消了。你会怎么裁？依据是什么？An agent has thirty tools mounted and it is clearly struggling. How do you cut the list, and on what basis?

  国内高频海外高频进阶#tool-design#tool-budget

  分析过程 · 先想清楚再作答

  1. 这题在考你能不能区分两类完全不同的代价。只答「工具定义占 token」的人只看到了一半，面试官真正在意的是另一半——选择成本。
  2. 怎么拆：先分两类问题。预算问题是工具定义每一轮都重发，3 个工具约 308 token、8 个约 731，挂三十个就是两千多，每轮都在花。选择问题是工具越多模型越容易选错，判据很硬：如果一个人类工程师都说不清什么时候该用哪个工具，那模型也做不到。
  3. 给四条可执行的裁剪判据，按顺序问：职责有没有重叠（有就合并或把边界写进描述）、过去一百次真实会话里被调用过几次（零次直接摘掉）、任务类型能不能提前判断（能就按任务动态挂载）、能不能把几个查询合并成一个带参数的工具。
  4. 结论：第三条通常收益最大。多数 Agent 的工具清单是静态的，启动时挂全集挂到会话结束，而一次会话往往只属于两三类任务中的一类，按任务大类动态挂载一步就能砍掉一半。
  5. 可预期的追问：动态挂载有什么副作用？工具定义排在缓存前缀最前面，改它会让整段前缀失效，所以只能按任务大类切几档，不能每轮重算——会话开始时定一次，中途除非任务类型真变了否则不动。

  How to reason about it · think before answering

  1. The question separates people who see only the token bill from people who also see the selection cost. Answering with token count alone covers half the problem.
  2. Two distinct costs. Budget: tool definitions are resent every turn, roughly 308 tokens for three tools and 731 for eight, so thirty tools burn a couple thousand tokens per turn. Selection: more tools means more wrong choices, and the sharp test is that if a human engineer cannot say which tool applies, the model cannot either.
  3. Give four ordered criteria: overlapping responsibility (merge, or write the boundary into the description), call frequency across the last hundred real sessions (zero calls means remove), whether the task type can be determined up front (if so, mount per task), and whether several query tools can collapse into one parameterized tool.
  4. Conclusion: the third usually wins biggest. Most agents mount the full set at startup and keep it for the whole session, while any given session belongs to only two or three task classes, so per-task mounting typically halves the definitions immediately.
  5. Expect the follow-up on side effects: tool definitions sit at the very front of the cache prefix, so changing them invalidates everything after. Mount by coarse task class once at session start rather than recomputing every turn.

  答题要点

  • 两类代价：预算（工具定义每轮重发，随数量线性增长）与选择（重叠工具让模型在决策点上摇摆）。
  • 判据：人类说不清该用哪个，模型也做不到。
  • 四条裁剪顺序：合并职责重叠的、摘掉零调用的、按任务类型动态挂载、把多个查询合并成带参数的一个。
  • 动态挂载收益最大，但会打掉缓存前缀，所以按任务大类切档、会话内不再变。

  Key points

  • Two costs: budget (definitions resent every turn, growing with count) and selection (overlapping tools make the model waver at decision points).
  • Test: if a human cannot say which tool applies, neither can the model.
  • Four cuts in order: merge overlaps, drop never-called tools, mount per task type, collapse several queries into one parameterized tool.
  • Per-task mounting pays most but invalidates the cache prefix, so switch by coarse task class once per session.