This study explores the mutual constitution of materiality and action in smart infrastructure context by focusing on technicians' IT-enabled work with complex, distributed, and inherently unreliable smart power grid. Past research suggests infrastructures form a context and a topic unlike the dyadic interaction of humans and computers, and have provided accounts of the ways in which the smart infrastructures shape technicians' work. This study develops a view of agency in smart infrastructure context in order to increase understanding on materiality of action. A concept of infra-acting is brought forth that situates action as part of (the material constitution of) infrastructure. Infra-acting posits that performing actions as part of infrastructures are (1) conditioned by material history; (2) dependent on mobilizing actors; (3) shaped by invisible and dynamic actors; and (4) riddled by vagaries. An ethnographic research provides an empirical illustration to foreground technicians' actions corollary to the materiality of infrastructure.