Covers Mem0, Letta, Cognee, Graphiti, Hindsight, EverMemOS, Tacnode, and Hyperspell. Built a small prototype with each and traced open-source implementations end-to-end from API through storage through retrieval. Tacnode is closed-source; Hyperspell is a managed platform — both analyzed from documentation and open-source client code.

  A few threads in there:                                                                                                                                                                      
                                                                                                                                                                                               
  - The design spectrum from minimal structure (Mem0: two LLM calls, no schema) to rich structure (Graphiti: bi-temporal edges, two-phase entity dedup, per-edge contradiction detection)    
  - Hindsight running 4-way parallel retrieval with cross-encoder reranking on a single PostgreSQL database
  - Hyperspell prioritizing data access over knowledge construction — 43 OAuth integrations, zero extraction. Not a different approach to the same problem, a bet that the bottleneck is upstream.
  - Structure without selection pressure is art. Many of these systems build elaborate relationship schemas with no mechanism to decide what's worth remembering.
  - Tacnode thinking from the infrastructure layer up — ACID, time travel, multi-modal storage. Nobody else is really working from that depth. Both layers matter.
  - The article also asks what memory actually is for agents that need to plan and adapt, not just recall. Most systems converge on extract-store-retrieve. Some are hinting at something deeper.

  All systems at pinned versions. Point-in-time, not a ranking.