Mixed-halide perovskites segregate into iodide- and bromide-rich phases under light and have limited long-term reliability. We report for the first time on mechanical instability originating from illumination, which is monitored through thin-film stress measurements, both ex situ and in situ. We show that low-cost polymer additives, namely, corn starch and polyvinylpyrrolidone, not only induce a desirable compressive intrinsic stress in the thin film but also suppress the photoactivated phenomenon. The additives show no significant changes in stress and photoluminescence (PL) responses when the perovskite films are illuminated under ambient conditions. The controlled real-time in situ monitoring of the relative changes in stress quantifies mechanical durability under 1 sun illumination cycles. We therefore demonstrate a facile route to a photostable mixed-halide perovskite thin film using a scalable deposition technique with a quench-free processing route using nontoxic additives.