In the growing literature exploring the links between immigrant diversity and worker productivity, recent evidence strongly suggests that diversity generates productivity improvements. However, even the most careful extant empirical work remains at some remove from the mechanisms that theory says underlie this relationship: interpersonal interaction in the service of complex problem solving. This paper aims to `stress-test' these theoretical foundations, by observing how the relationship between diversity and productivity varies across workers differently engaged in complex problem solving and interaction. Using a uniquely comprehensive matched employer-employee dataset for the United States between 1991 and 2008, this paper shows that growing immigrant diversity inside cities and workplaces offers much stronger benefits for workers intensively engaged in various forms of complex problem solving, including tasks involving high levels of innovation, creativity, and STEM. Moreover, such effects are considerably stronger for those whose work requires high levels of both problem solving and interaction.