Self-Cleaning and Captured Anyway: A Measured Primitive for Error in a Store an Agent Writes to Itself

Agents increasingly write their own conclusions into a store they later retrieve from, closing a loop in which an error can be read back as evidence for itself. That contamination has been reported as a one-way process whose severity decays, and it is summarised by per-model averages. Against the store deployed systems actually use — one that only ever appends — the first description is wrong and the second has no referent. We take the loop to its infinite-tenure limit against an append-only store, prove that because writing never deletes the reachable state space has a hard upper edge at (n-1)/n, and measure the process with a single primitive carrying no fitted parameter: the copy function, the probability a model repeats what its store says. The outcome is then a choice between two edges rather than a decay: at a false-record fraction of 0.9 the interval between the two modes holds 3.6% of 220 runs where a uniform spread would put 20.6%, and is strictly empty on the first 15. The primitive orders every model's capture rate to within one seed and survives the move to real facts, its sign against the critical value predicting the direction of drift on 353 of 360 real-fact runs, and scale does not rescue the store. Of four interventions with criteria frozen first, timing dominates fraction at matched budget, a consistency gate helps the false majority, and provenance marking moves nothing unless the mark correlates with truth. Two consequences travel past this apparatus: a falling exact-match score on such a store measures loss of determinism, not of information, at a measured mismeasurement of +0.383; and the resampling unit is the seed, at a design effect of 3.75 under which one of our own rank-correlation claims falls from Spearman +0.98 at three seeds to between +0.31 and +0.80 at forty-four. An append-only store's fate is therefore readable in advance from one parameter-free measurement, at a seed budget the field currently underspends.