Persistent memory that survives across sessions. Hipocampus gives your agent a 3-tier memory system (hot/warm/cold) with a self-compressing 5-level compaction tree. Every session, the agent auto-loads a ~3K token topic index (ROOT.md) so it knows what it knows — then retrieves specific details on demand via tree traversal or hybrid search (BM25 + vector via qmd). All memory writes happen through subagents to keep your main session clean. Zero dependencies, zero infrastructure — just markdown files.