Why it matters

Caching and compaction are what make agents practical. Without caching, the Chapter 4 cost curve makes any non-trivial session painfully expensive. Without compaction, the agent simply hits a wall mid-task and cannot continue. Together they turn the agent loop from a neat demo into something you can run all day. They are also where a lot of the "why is my agent slow or expensive?" answers live, so understanding them pays off the moment you operate a real agent.