Firms who buy from suppliers often engage in supplier development to reduce the supplier's production cost. Being aware that their efforts may benefit a rival firm when there is a shared supplier, some buyers only invest in “specific supplier development,” that is, in those processes or technologies where spillover cannot occur. Other buyers willingly accept the spillover that arises from supplier development, and invest in “generic supplier development.” Our game-theoretic model captures a buyer's choice to invest in these distinct supplier development types as a way to endogenize spillovers. In contrast to the literature, this paper considers the benefits of investing in a combination (i.e., portfolio) of cost-reducing generic and specific supplier development. We demonstrate how supplier development affects a shared supplier's wholesale pricing decisions; whereas generic supplier development lowers wholesale prices equally across buyers, specific supplier development only lowers the wholesale price of the investing party. Our model shows that buyers should treat the spillovers from generic supplier development as an investment opportunity rather than a threat. In equilibrium, a buyer will always invest in a portfolio of both supplier development types, and having a better generic than specific investment capability may even make generic supplier development the most prevalent option for him, depending on the level of competition. Moreover, even if the buyers can commit to only investing in specific supplier investment, the resulting equilibrium gives lower buyer profits than a portfolio that includes generic investments. We also find that the presence of specific investments may raise generic supplier development, benefiting all supply chain actors. However, incorporating specific supplier development into a supplier development portfolio or a commitment to investment in only specific supplier development can lead to a prisoner's dilemma in terms of buyer profits. We show how investment capabilities and competitive intensity drive the buyers' investment decisions and supply chain actors' profits. The paper's main results also hold for asymmetric generic investment capabilities, though we highlight that the least capable buyer will free-ride on his rival's investments, consequently making him earn higher profits.