Evidence Is Not Authority: A Causal Study of State Arbitration in Long-Horizon Agent Memory

Long-horizon agents can retrieve both historical and current user-state evidence yet still act on the historical state. Because existing memory benchmarks primarily score final answers, they often cannot distinguish a lost update from one that was visible but ignored. We call this choice among jointly visible states state arbitration and study it through an evidence-to-action audit and paired interventions. Across 240 paired histories and three model families, retaining an early explicit declaration while holding later behavior fixed lowers current-state accuracy by 33–39 points. Our primary intervention changes only the lifecycle relation between two records already in context. Marking the current record active and the old record superseded raises current-state actions from 24% to 53–55% on two 70B-class models. Natural validity/provenance fields also help, whereas timestamps alone do not. The contrast also transfers to structured action versions of STALE, LongMemEval, and MemoryAgentBench, but incorrect labels can reverse the result: marking a valid record superseded reduces one model's correct actions from 241 to 8 of 400. In native paths, managers often fail either to expose new evidence or to link it to the state it replaces. These findings identify a read-time failure distinct from retrieval and suggest preserving history while making the currently applicable record explicit and inspectable.