AsiaInfo Technologies (China) Inc. China Telecom AsiaInfo Technologies (China) Inc. AsiaInfo Technologies (China) Inc. AsiaInfo Technologies (China) Inc. Cross-domain and policy-controlled agent collaboration can require mediation decisions that are not always suitable for an agent client or an agent server alone. A collaboration request may need to combine a stated goal, capability profiles, tenant or domain policy, trust evidence, disclosure limits, operational constraints, and handoff context before a concrete interaction mechanism is used. This document identifies gateway-side interaction mediation as a distinct interoperability gap in many AI Agent Gateway deployments. It does not claim that every agent collaboration requires a gateway. Rather, it explains why many deployments need a mediation point where request goals, capability profiles, policy, trust, and handoff constraints can be evaluated before interaction proceeds. The document describes a Gateway Mediation Layer as a decision- support and interface layer between application-level goals and concrete agent interaction mechanisms. It focuses on the role, inputs, outputs, and narrow set of interoperable artifacts that may require further work, such as Mediation Requests, Mediation Responses, Handoff Contexts, Failure Records, and Evidence References. It does not propose standardizing internal matching algorithms, domain ontologies, prompt engineering, or model-specific decision logic.