Three-tier hierarchical memory (Letta / MemGPT pattern) — core / recall /
archival. Core holds a small, fast, frequently-accessed working set (e.g.
core_limit=2000 tokens). Recall holds raw interaction history for "what
did we talk about yesterday" questions. Archival holds effectively
unlimited overflow, still searchable. Make forgetting and archiving
explicit design choices, not afterthoughts. Use when an agent must
feel consistent across long sessions and the context window is the
scarcest resource. NOT for one-shot agents (no persistence needed),
NOT for event logs (use append-only kafka-style), NOT when every
fact is equally important (then a flat store is correct).
