The Protocol Stack Nobody Audited: Four Agent Protocols and the Cross-Protocol Audit Gap

The four-protocol enterprise agent stack — MCP, A2A, ACP, and UCP — was designed by four teams with four threat models. MCP lists structured audit trails as roadmap work; A2A obtains credentials out-of-band, outside its own scope; ACP documents no audit standard; UCP signs only single transactions. Reading the four design documents together exposes a cross-protocol accountability void no single document owns. This analytical white paper traces a single request across all four layers, maps the regulatory exposure to NIST AI RMF, SR 26-2 (the 2026 successor to SR 11-7), the EU AI Act, ISO 42001, HIPAA, and SOC 2, and sketches what a cross-protocol audit surface would require — with an explicit Honest Limits section, because no breach has been attributed to a cross-protocol failure and the core argument is inference, not incident.