Compaction does not summarize everything behind the system prompt. It leaves the most recent rounds alone. An agent that summarized the round it was in the middle of would lose the thread it is holding, so the summary stops short and the live end of the conversation stays verbatim. That leaves the window in three parts: the prompt, the summary of what is old, and the rounds still in their own words.
Paid members: the worksheet and its printable PDF are below ↓