Managing behavioral interactions and conflicts across organizational boundaries remains a persistent challenge in project management. Inter-organizational relationship quality is closely related to the sustainable development of future cooperation, yet prior studies have reported contradictory findings. Rooted in the field of project behavior and conflict management, this study examines how relational behavior and task conflict jointly shape inter-organizational relationship quality. This study employs fuzzy-set qualitative comparative analysis (fsQCA) to uncover multiple configurational pathways leading to different levels of inter-organizational relationship quality. It examines how situational factors (task conflict and time delay) interact with behavioral factors (the relational behavior of both parties) to shape these outcomes. Based on survey data from 212 questionnaires, this study concludes that a key factor driving high relationship quality is when the three dimensions of relational behavior of both parties are aligned or when one collaborator more closely meets the expectations of both parties. Conversely, low relationship quality occurs when both task conflict and time delay are in an unfavorable state, especially when the relational behavior of the two parties is not equal. The findings contribute to conflict management literature by advancing a configurational understanding of how behavioral reciprocity and situational stressors shape relationship outcomes. These configurations show high consistency and coverage, indicating robust and empirically relevant patterns in inter-organizational relationship quality.