Gist compression silently deletes dates: temporal expressions survive at 3.05%, and one prompt line restores them to 62.39%
The Sleeping Agent (arXiv:2608.11775, Aug 12) traces a specific failure in gist-based context compression: the abstraction prompt preserves relational and event structure while discarding dates and times, so temporal expression retention collapses to 3.05%. Rewriting the gist prompt to explicitly retain temporal markers lifts retention to 62.39% and recovers +0.314 judge accuracy on temporal questions (95% CI 0.254-0.375), while named-entity and event preservation barely move (x1.02 and x1.11) — a surgically targeted fix, not a general quality bump. Measured over 1,935 matched questions across ten LoCoMo conversations. Anyone summarizing conversation history into long-term memory should audit their compression prompt for the same omission; the loss is invisible until a user asks 'when'.
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