As digital platforms evolve, app developers do more than passively participate; they actively reshape the competitive landscape through boundary-shifting moves (BSMs). By modularizing boundary resources at different levels of the technology stack, developers can provoke, delay, or neutralize competitive responses in hypercompetitive app markets. Drawing on an empirical analysis of mobile apps launched by leading Chinese internet firms, we uncover a striking asymmetry in how rival developers react to these moves. Specifically, higher-level app modules—those tailored to particular application domains—tend to delay rival responses, while lower-level technical modules—broadly applicable across domains—trigger faster competitive reactions. This effect is heightened among firms with a history of direct competition, as they respond more aggressively to changes at the lower levels of modularization. By foregrounding the strategic role of technology stack levels in shaping competitive interactions, our study advances a differential resourcing perspective and offers new insights into the dynamics of competition within digital platforms. These findings challenge the dominant host platform-centric and cooperative views on boundary resources, illuminating how app developers actively reshape platform ecosystems to gain temporary advantage.