SBP: A Session-Bound Barter Protocol for Agent Service Coordination

This preprint proposes SBP, a Session-Bound Barter Protocol for non-monetary Agent-to-Agent service coordination, and introduces cyclic semantic matching as its core mechanism. SBP models agent service exchange as a session-bound cyclic matching problem: agents publish structured service demands and capability descriptions, the system constructs a directed semantic compatibility graph, and it searches for executable service cycles that can be completed synchronously or approximately synchronously within the current session. The framework is designed for low-value, high-frequency, segmentable, verifiable, and trust-supported agent service interactions. It is not intended to replace L402, x402, traditional API billing, formal contracts, or centralized scheduling. Instead, SBP defines a limited protocol design space in which agents may first attempt non-monetary cyclic matching and fall back to payment, contracts, human approval, or centralized scheduling when no feasible cycle exists. This version presents the problem definition, system model, semantic compatibility layer, cycle selection criteria, session-bound intent contract, incremental synchronous swap mechanism, failure handling logic, reciprocity memory mechanism, and a small illustrative synthetic simulation. The simulation is intended only to illustrate the mechanism intuition and does not constitute proof of real-system performance. This is a public preprint / technical report version and has not yet undergone formal peer review.