Multi-agent deployments of large language models (LLMs) are increasingly embedded in market, allocation, and governance workflows, yet covert coordination among agents can silently erode trust and social welfare. Existing audits are dominated by heuristics that lack theoretical guarantees, struggle to transfer across tasks, and seldom ship with the infrastructure needed for independent replication. We introduce \emph{Audit the Whisper}, a conference-grade research artifact that spans theory, benchmark design, detection, and reproducibility. Our contributions are: (i) a channel-capacity analysis showing how interventions such as paraphrase, rate limiting, and role permutation impose quantifiable capacity penalties -- operationalized via paired-run Kullback--Leibler diagnostics -- that tighten mutual-information thresholds with finite-sample guarantees; (ii) \textsc{ColludeBench}-v0, covering pricing, first-price auctions, and peer review with configurable covert schemes, deterministic manifests, and reward instrumentation; and (iii) a calibrated auditing pipeline that fuses cross-run mutual information, permutation invariance, watermark variance, and fairness-aware acceptance bias, each tuned to a \(10^{-3}\) false-positive budget. Across 600 audited runs spanning 12 intervention conditions, the union meta-test attains TPR~$=1$ with zero observed false alarms, while ablations surface the price-of-auditing trade-off and highlight fairness-driven colluders invisible to MI alone. We release regeneration scripts, seed-stamped manifests, and documentation so that external auditors can reproduce every figure and extend the framework with minimal effort.

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