When observing an action, how do people intuitively determine if it has communicative intent, such as in the case of pantomimes? We focus on two alternative theories: one suggests that instrumental intention competes with communicative intention, where the weaker the former is, the stronger the latter is; the other proposes that instrumental intention is nested within communicative intention, where the presence of the former facilitates the latter. To test these theories, we introduced the concept of contextual affordance, which manipulates the degree to which the instrumental components of an action are revealed. Through behavioral experimentation, we found a non-linear relationship between contextual affordance and communicativeness rating: partial affordance–providing implicit cues to an action’s instrumental component without being fully rational–elicited the strongest perception of communicative intention, whereas full affordance or no affordance resulted in a weaker perception of communicative intention. Our study provides a novel definition of communicative intention and reveals that recognizing the instrumental goal and perceiving the suboptimality in achieving it work together to create a strong communicative signal. This work represents a step toward developing an integrated theory of pantomimes, specifying how the rationality principle can be applied to serve multiple purposes simultaneously.