Saluca LLC The Hash-Chain Context Transfer Protocol (HCTP) is a payload format and synchronization protocol for incrementally transferring an ordered, append-only sequence of conversational context between two endpoints over a bandwidth-constrained channel. Acknowledged history is represented by a single fixed-size rolling hash commitment (the "static root"); only not-yet-acknowledged context blocks (the "dynamic window") are transmitted. As a result, the per-message wire overhead attributable to history is constant and independent of the total number of previously acknowledged turns. HCTP is transport- agnostic and carries no confidentiality or peer authentication of its own; it is intended to run over a secure transport. This document specifies the HCTP data model, wire format, rolling-root computation, and synchronization state machine.