Interview Bank

Context Engineering in 5 Days

D4 Long-Running Sessions: Compression, Notes and Memory Files, Subagent Isolation and Handoff Summaries

• Why does a subagent return only a summary instead of its full transcript, and how should the lead agent specify the handoff?子代理为什么只回传摘要而不回传全过程？主代理该怎么写交接要求？

  Common in ChinaCommon overseasDeep dive#subagents#handoff#cost

  How to reason about it · think before answering

  1. This tests architectural intent. Saying it saves tokens is only half right, and the lesser half: multi-agent setups are more expensive overall, not cheaper.
  2. State the purpose. Subagent isolation buys a clean main context, not a smaller bill. A subagent can burn tens of thousands of tokens exploring in its own window while the main thread gains only a condensed result of one or two thousand tokens. It is separation of concerns applied to context.
  3. Put the cost on the table, which is where engineering experience shows: agentic applications use roughly four times the tokens of chat, and multi-agent systems roughly fifteen times. So the fit is narrow: heavy exploration with a condensable result, independent parallelizable subtasks, and a main thread that genuinely does not need the intermediate steps. Missing any one, fall back to compaction.
  4. Conclusion: specify a handoff contract of three sections (settled conclusions, open items, hard constraints), with every conclusion carrying a source identifier such as a file path, order id, or URL. Without identifiers the main thread can only trust everything or redo everything; with them it can spot-check the one claim it doubts. Require the constraints section even when empty, or the main thread cannot tell absent from forgotten.
  5. Expect the follow-up about the lead agent's own plan: write it outside the window too, since truncation or compaction late in a long task tends to eat the original plan first.

  分析过程 · 先想清楚再作答

  1. 这题在考架构意图。答「为了省 token」只对了一半，而且是次要的那一半——子代理架构整体上是更贵的，不是更省的。
  2. 怎么拆：先说清目的。子代理隔离买的不是省钱，是主线上下文的干净。子代理可以在自己独立的窗口里烧掉几万 token 反复探索，主线只多了一两千 token 的浓缩结论，中间过程一个字都没进主线。这是关注点分离在上下文层面的落地。
  3. 把代价摆出来，这是最能体现做过工程的地方：Agent 类应用本来就比聊天多用约 4 倍 token，多 Agent 系统约 15 倍。所以适用面很窄——探索量大但产出能浓缩、子任务彼此独立可并行、主线确实不需要看中间过程，三条缺一就该退回压缩。
  4. 结论给交接契约：三段式（已定结论、待办事项、硬约束），且每条结论必须带来源标识（文件路径、订单号、URL）。原因是没有标识的结论不可复查，主线只能全盘相信或全盘重做；带标识之后主线可以只对存疑的那条做定点核实。硬约束那一段没有也要写「无」，不能省略，否则主线分不清是没有还是忘了写。
  5. 可预期的追问：主代理自己的计划怎么办？也该写到窗口外面。长任务后期一旦触发截断或压缩，最先丢的往往就是最初那份计划，而它恰恰最不该丢。

  Key points

  • Isolation buys a clean main context, not savings; multi-agent is more expensive overall.
  • Magnitudes: agents use about four times chat tokens, multi-agent about fifteen times.
  • Fits when exploration is heavy but condensable, subtasks are independent and parallel, and the main thread does not need intermediate steps.
  • Handoff contract in three sections, every conclusion carrying a source identifier, and an explicit none when constraints are empty.
  • Persist the lead agent's plan outside the window, since it is the first casualty of truncation late in long tasks.

  答题要点

  • 隔离买的是主线上下文的干净，不是省钱；多 Agent 整体更贵。
  • 代价数量级：Agent 约为聊天的 4 倍 token，多 Agent 约 15 倍。
  • 适用三条：探索量大且产出可浓缩、子任务独立可并行、主线不需要中间过程；缺一就退回压缩。
  • 交接契约三段式，每条结论必须带来源标识，硬约束段即使为空也要显式写「无」。
  • 主代理自己的计划也要写到窗口外，长任务里它最容易被截断或压缩吃掉。