Phenotypic diversity and contingent cooperation coevolve in a mathematical model where individuals can switch between different traits. Having a variety of traits helps cooperators work together more effectively. It is beneficial for cooperators to have the ability to switch between traits, making it harder for defectors to disrupt cooperation. However, high levels of phenotypic diversity can sometimes collapse if defectors take over. This study shows that cooperation and diversity can strengthen each other, leading to better understanding of how different traits and cooperation evolve together.