A long agent run or chat keeps growing until it threatens the context window. Compaction
is summarising the older turns into a compact note so the work can continue — but summarise carelessly and
you lose the one fact you needed. The exam's sharpest angle: progressive summarisation can quietly drop
precise, transactional data.
compactionprogressive summarisationlossy vs losslessexternal memorystructured notes
Explain like I'm 10
Your desk is filling up with pages, so you write a short summary and throw the old pages away to make
room. Smart! But if your summary says "we talked about the order" and forgets the exact order number
#4471, you've thrown away the one thing you actually needed. Good summarising keeps the important exact
details and only blurs the chit-chat.
Watch a summary lose something important
An agent has been handling a refund. The conversation is getting long, so it compacts. Choose how it
summarises and see what survives — and what silently vanishes:
before (full history)
after compaction
Strategies, from cheap to robust
Approach
How
Risk
Truncation
Drop the oldest turns.
Loses early context entirely (the goal!).
Progressive summarisation
Replace old turns with an LLM summary.
Lossy — precise/transactional facts (IDs, amounts) get blurred away.
Structured memory note
Maintain a running, schema'd scratchpad (decisions, IDs, open tasks) that survives compaction verbatim.
Needs discipline, but keeps the critical bits exact.
External memory / retrieval
Store facts outside the window; retrieve on demand.
Adds a retrieval step; must fetch the right thing.
Exam trap:progressive summarisation is lossy — never rely on it to carry
exact, transactional data (order numbers, dollar amounts, approval flags). The right design keeps those in a
structured note or external store that compaction preserves verbatim, and summarises only the
narrative. "Just summarise everything" is the wrong answer whenever precise values matter.
Takeaways: long runs need compaction, but summarisation is lossy —
it can silently drop precise transactional data. Protect critical facts (IDs, amounts, decisions, open tasks)
in a structured memory note or external store that survives verbatim; summarise only the
conversational narrative. Match the strategy to how exact the data must stay.