<p>In business-to-business (B2B) contexts, companies increasingly rely on dyadic cross-boundary collaboration—pairing an on-site boundary spanner with a one-to-one client—to manage partnerships effectively and achieve joint payoffs. Yet, actionable strategies for individual boundary spanners to cultivate client trust remain underexplored. To address this gap, we draw on the active trust management perspective to explore trust-building tactics that boundary spanners can employ to foster successful collaboration. We identify benefit-enhancing initiative—prosocial, self-initiated behaviors by boundary spanners to promote client benefits and welfare—as a key driver of client trust, which subsequently enhances joint performance and client satisfaction with the company. We further propose cooperation length (longer vs. shorter) as a critical boundary condition. Through a multi-source, three-wave field study of 447 sales representative–client dyads, we confirmed the positive indirect effects of benefit-enhancing initiative on joint performance and client satisfaction with the company through client trust. These indirect effects were stronger when cooperation length was shorter than when it was longer. Our research advances understanding of how and when boundary spanners can strategically navigate the inherent challenges in dyadic cross-boundary collaboration to achieve both economic and relational collaborative outcomes.</p>