Flexibility is a hallmark of gig-economy platforms, yet how it influences gig workers’ behaviors remains underexplored, especially in the healthcare industry. Leveraging a quasi-natural experiment on a gig healthcare platform, this paper investigates the impact of task flexibility on individuals’ behaviors and platform sustainability. Using the different-in-differences design, we find that the decline in task flexibility significantly decreases individuals’ willingness to engage in tasks and efforts in service delivery. We further uncover the underlying mechanism through an experiment. The results show that the negative effects are attributable to changes in individuals ' psychological perceptions, particularly feelings of unfairness, disrespect, and low economic expectations stemming from the reduced task flexibility. Our findings provide novel insights into the flexibility design for gig platforms.