Know the difference between compaction and agentic memory, and use both.

Compaction trims what's already in the window (Anthropic productized a server-side compaction API on Opus 4.6 that auto-summarizes older turns mid-session). Agentic memory moves information entirely outside context as structured notes the agent writes and pulls back on demand. Conflate them and your long-horizon agent either loses critical state to summarization or drowns in context it should have offloaded (source).