Purpose This study analyzes supplier development investment decisions under a triadic setting (two buyers and a common supplier). In a triadic setting, the supplier development investment decision of one buyer can have a spillover effect of the benefits on other buyer. Therefore, it is utmost important for the investing buyer to understand the impact of benefit spillover on other competing buyers'. Therefore, one of the purposes of this study to analyze the supplier development investment decision of buyers under two scenarios. First, under cooperative development structure where both buyers jointly invest in supplier and share equal benefits. Second, non-cooperative investment structure where both buyers individually invest in supplier development and share unequal benefits. Design/methodology/approach In order to assess the impact of supplier development investment decisions on the profitability of buyers and the common supplier, the authors used game-theoretic approach. The authors design a Stackelberg leader-follower game where the supplier acts as Stackelberg leader and buyers follow the supplier's pricing decision to maximize their profit level. Additionally, both buyers decide either to cooperate or non-cooperate while investing in supplier development. Findings The results show that the cooperative investment is always an optimal strategy for buyers and supplier. Interestingly, the efficient buyer's share of investment level is lower under non-cooperative investment structure and he is better-off due to its capability of taking advantage from the other buyer's investment. However, the inefficient buyer, on the other hand, is worse-off under non-cooperative investment. Furthermore, comparative analysis between the two shows that initially, the buyer who extracts more profit because of the other buyers' development investment tends to prefer the non-cooperative development investment set up. However, after a certain point, the same buyer is better-off under cooperative development investment through cooperation, and sharing equal benefit of the supplier's development, as the supplier in turn, starts charging a higher wholesale price under non-cooperative investment case. Originality/value To the best of authors’ knowledge, extant literature on supplier development has mostly focused on. One supplier-one buyer; thus, the learning spillover effect has almost been unexplored. In real-life, different buyers often purchase from the shared supplier. Therefore, it is important to analyze the spillover of supplier development benefits due to investment of one buyer on other buyer and deriving the condition under which buyers would be incentivized to invest jointly or individually.