<p>Ant–plant associations are well known for their mutual benefits; ants defend plants from herbivores, while plants provide food and shelter. This study examines the shift from mutualism to parasitism by investigating how the interaction of the weaver ant (<i>Oecophylla smaragdina</i>) influences the biochemical and physiological responses of <i>Cassia fistula</i> L. Leaves were taken from healthy plants and from plants with mild to severe ant infestations, then tested for photosynthetic efficiency and antioxidant activity. During the initial phase of infestation, mutualism between ants and plants was observed, while severe ant infestations strongly reduced photosynthesis, with a major decline in the plant’s ability to use light energy. While mildly infested leaves showed an initial boost in protective antioxidant enzymes, these defences collapsed under severe infestation, leading to oxidative stress and cellular damage. Overall, the study reveals that while a small presence of ants may stimulate plant defences, large ant colonies harm the plant leaves by overwhelming its protective systems. The results clearly highlight that mutualism-parasitism shifts completely rely on the infestation intensity of <i>Oecophylla smaragdina</i> in <i>C. fistula</i>.</p>