LLM agents increasingly decide from evidence assembled by upstream systems: retrievers choose documents, recommenders choose posts, and memory systems choose prior events. Existing evaluations usually hold this evidence fixed, missing failures in which individually ordinary items form a systematically one-sided context. We introduce a counterfactual evidence audit: expose an agent to two mirrored sets of five documents, measure the difference in six downstream decisions, and use that contrast to predict its response to disjoint 45-document contexts. The protocol was frozen before testing three held-out open-weight model families. Across 18 held-out model-task cells, five-document effects predict full-context effects with Spearman rho=.855 (p<.001), reduce mean absolute prediction error by 62% relative to a zero-effect predictor, and recover the direction of 12 of 13 material effects. A reviewer-requested post-hoc task-mean baseline is also substantially weaker (MAE .369 versus .167). Matched controls show that selecting one-sided ordinary items, rather than merely reordering identical items, causes the shift in a susceptible model. Across seven open-weight families, susceptibility transfers from an interactive feed to a static RAG dossier (rho=.750, exact p=.033), while a provenance warning does not reliably mitigate it. A separate study of three deployed Codex agent tiers finds strong audit-to-full ranking (rho=.951, p<.001) but no individually significant full-context effect after correction. Within this single synthetic remote-work domain, the result supports a domain-specific triage procedure, not a universal steering claim: evidence selection must be evaluated as part of the composed agent system.

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