From System Symptom to Agent-History Diagnosis: A Three-Coordinate Protocol for LLM Population Drift

Behavioural change in multi-agent LLM populations may reflect shared time, heterogeneous agent histories, or visible interaction evidence. We introduce DriftClock, a three-coordinate protocol separating shared time, private exposure, and visible history. In naming games, round and exposure correlate at $0.91$; exact same-round contrasts instead localize the signal to agent histories: switching after disagreement declines with exposure, future exposure is null, a 64-population Mistral study replicates exposure-indexed dynamics, and long-horizon decline transfers across models and objectives. Matched $H\in \{10,30,60\}$ memory-window interventions reveal a finite-context breakpoint: response falls as histories fill and partially rebounds when old evidence leaves longer windows. Discovered in Qwen, this breakpoint passes a frozen Mistral $H30-H60$ test and an independent-seed Qwen replication; social-update rules separately show that exposure signatures need not imply autonomous ageing. Standardized challenges preserve eventual adaptation, while post-hoc common-support analyses locate the raw speed gap in measured visible evidence. Paired history branches separate local source effects from propagation: history differences reach other agents while action differences remain sparse and model-dependent. DriftClock moves from population symptom to history localization, formation, functional consequence, and transmission, establishing finite-history turnover as a reproducible boundary condition on cumulative-age explanations.