Does Your Agent's Memory Survive a Model Upgrade? A Controlled Study of Memory Portability
Upgrading the model behind an agent is routine; migrating its memory is not, so the store stays and the new model is simply pointed at it. Holding 48 identical histories fixed and varying only how they are stored — full transcript, retrieved chunks, model-written notes, or subject–predicate–object facts under a fixed schema — this study swaps the model that reads them. Notes are the fragile case: accuracy moves −13.28 points in one migration direction and +9.91 in the other, so the same upgrade is a loss or a gain depending on who wrote and who reads, while the fixed-schema store shifts by 0.0004 and re-embedding half a vector index captures only 4.96 of the 11.90 points a full re-embed gives. Notes also cannot be repaired from the notes — store-only repair missed the target in all 48 cases while a retained raw history recovered 34 — so keep source transcripts and test each upgrade direction on its own.