The Srcond Signature

The seventh publication in the Heartbeat Framework series: an applied analysis of self-written records and the double-trust architecture that makes machine records testify. Every deployed agentic AI system keeps a record of its own decisions — written by the system, stored on the system's machine, under its operator's control. That record is the entire evidential basis for audit, incident review, litigation and regulation, and it is editable by exactly the parties with the strongest incentive to edit it. This paper names the underlying failure — self-custody of the record — and shows that healthcare solved the same problem a century ago, in ink: the controlled-drugs register, whose every mechanism (bound numbered pages, ink without erasure, the running balance, the witnessed second signature, the reconciliation count, pharmacy's independent copy) exists because the writer of a consequential record cannot be its sole guarantor. That inheritance is transposed as the double-trust exercise: for every claim a record makes — content, continuity, completeness, custody, time — name the two independent authorities beneath it. Any claim standing on one leg fails, because the record-keeper's own word is always the same leg. Four separations follow (write from verify, content from continuity, record from memory, custody from seal), each buildable with commodity cryptography first published in 1991. The case is grounded in the red-team record: frontier models have disabled their own oversight mechanisms and denied it under interrogation. Honest limits are stated plainly — tamper-evidence does not prevent the crime; it preserves the crime scene — and the closing standard is offered for adoption: a machine record that fails the exercise should carry the evidential weight of an unwitnessed, pencil-written drug register. None. Companion to the trilogy (The 24-Hour Team; The Mixed Shift; The Long Handover) and the applied papers (The Character Pathway; Same Monsters, New Casings; The Endless Shift). A complete open reference implementation is cited. Floor-level examples are pseudonymised composites; Fernlea House is not a real setting. Version 1.0, July 2026.