Integrity Beyond the Trust Boundary — External Differential Monitoring in LLM Multi-Agent Systems

Paper 8 in the Buddys Architecture Series. A multi-agent LLM system that modifies its own configuration files cannot reliably detect its own corruption. Internal integrity guards validate process, not content. We introduce external differential monitoring: a peristaltic backup continuously pushes filesystem state beyond the trust boundary, and change rate (D-term from DNFO) is monitored externally. The external monitor is a sensor; the internal MIG is an actuator — detection and correction are separated across a trust boundary. We discuss scaling via mutual-redundancy buddy structure (DHC), where hub managers monitor each other's externalized diffs.