Accumulation Is Maintenance: Anti-Preferential Placement and Online Structural Curation for Hierarchical Agent Memory

Long-term memory systems for LLM agents have converged on online insertion: each new observation is routed to a location in a hierarchy or graph, and the structure is then left alone; at best, a single top layer is periodically repartitioned. We show that this neglect is not benign. On a multi-topic accumulation workload derived from real usage, undisciplined insertion produces structural pathologies: a single early node absorbs 64% of all records through preferential attachment, topics fragment across near-duplicate siblings, and the hierarchy stays flat. Recall-centric benchmarks such as LoCoMo and LongMemEval miss these pathologies, because recall holds as long as an answer is somewhere in memory. We present Gardener, a hierarchical memory system built on the principle that accumulation is maintenance, not placement. Gardener combines anti-preferential placement, which deterministically removes the volume advantage behind rich-get-richer growth; an online curation loop that merges, splits, and relocates nodes under strict guards (original records are never modified, and every write is executed by deterministic code); and a three-way ingestion triage that moves abstention from recall time to accumulation time. We also propose an accumulation-quality evaluation protocol pairing structural-health metrics with writer attribution (the oracle-to-e2e gap). In same-condition comparisons on an English-translated workload against online tree, graph, and extraction baselines (MemTree, A-MEM, Mem0), Gardener matches embedding-based recall while its non-destructive supersede updates eliminate stale-value citations on knowledge-update probes: 0% in every run, against roughly 50% for every baseline. Because the guards are deterministic code, their bound survives a backbone change that model behavior does not: unguarded accumulation ranges from the 64% collapse on the original backbone to moderate drift on a newer one, while guarded accumulation stays within the health gate in both regimes, curation repairing what the newer backbone's intake degrades.