Interview Bank

Context Engineering in 5 Days

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

• When should you compact the context, and when should you just move to a larger context window?什么时候该压缩上下文，什么时候该直接换一个更大的窗口？

  Common in ChinaCommon overseasIntermediate#compaction#context-rot

  How to reason about it · think before answering

  1. This checks whether you treat the window as capacity and attention as a budget. Answering only that you compact when it does not fit misses half the cases, since plenty of sessions should be compacted while the window is still mostly empty.
  2. Separate the two problems. Not fitting is capacity, and a bigger window fixes it. But a bigger window does not fix context rot: recall degrades as context grows, on a gradient, so a large nominal window is not a promise of stable behavior at that length. Fitting and being used well are different.
  3. Give the test: watch two signals, not one. High window occupancy means compact for capacity. Low occupancy with a low share of actually-useful tokens also means compact, for attention. The second is the one people miss because nothing looks urgent.
  4. Then order the tactics. Compaction is not the first move. Clear stale tool results first, since that needs no model call, is deterministic, and is reversible. Only then summarize history. Doing it the other way costs an extra call and risks losing information while usually treating the smaller bucket.
  5. Expect the follow-up: when is compaction itself not enough? When the state is an accumulating ledger rather than a summarizable conclusion, such as exact tallies, maps, or a long-term plan. Those belong in files outside the window.

  分析过程 · 先想清楚再作答

  1. 这题在考你有没有把窗口当容量、把注意力当预算。只回答「窗口不够就压缩」的人漏掉了一半——很多时候窗口还很空，但已经该压了。
  2. 怎么拆：先把两个问题分开。窗口不够是容量问题，换大窗口确实能解决；但换大窗口解决不了上下文腐烂——上下文越长模型准确回忆的能力越差，这是一条缓坡，标称窗口大不等于在那个长度上表现稳定。所以「装得下」和「用得好」是两件事。
  3. 给判据：看两个指标而不是一个。窗口占用率高就该压（容量问题）；占用率不高但有效信息占比很低，也该压（注意力问题）——后者最容易被忽略，因为看起来毫无压力。
  4. 再给顺序上的结论：压缩不是第一手段。先清掉旧的工具结果（不用调模型、确定、可逆），不够再摘要历史。反过来做的人很多，因为摘要听起来更高级，但摘要要多花一次调用、要承担丢信息的风险，而它治的往往不是大头。
  5. 可预期的追问：那什么时候压缩也不够？当状态是渐进积累的账本而不是可总结的结论时——比如精确计数、地图、长期计划。这类东西要写到窗口外面的文件里，不能靠摘要保住。

  Key points

  • Not fitting is capacity and a larger window solves it; context rot is attention and a larger window does not.
  • Two triggers: high window occupancy, or low occupancy with a low share of useful tokens.
  • Clear stale tool results first (no model call, deterministic, reversible), then summarize history.
  • If the state is an accumulating ledger such as tallies, maps, or a plan, use external files instead of compaction.

  答题要点

  • 窗口不够是容量问题，换大窗口能解决；上下文腐烂是注意力问题，换大窗口解决不了。
  • 两个触发信号：窗口占用率高，或占用率不高但有效信息占比很低。
  • 顺序上先清旧工具结果（不调模型、确定、可逆），不够再摘要历史。
  • 如果状态是渐进积累的账本（计数、地图、长期计划），压缩救不了，要写到窗口外的文件里。