This paper investigates the chemical composition and cloud properties of the hot Jupiter WASP-43b, focusing on its nightside clouds and disequilibrium chemistry. Using the JWST's Mid-Infrared Instrument, the authors measure a phase-resolved emission spectrum from 5 μm to 12 μm, revealing a significant day-night temperature contrast (1,524 ± 35 K and 863 ± 23 K, respectively) and evidence of water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models suggest the presence of nightside clouds that become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to equilibrium chemistry predictions, methane is not detected on the nightside, with upper limits of 1–6 ppm depending on model assumptions. These findings indicate that the atmosphere of WASP-43b is shaped by disequilibrium processes, providing new insights into the properties of its nightside clouds. However, discrepancies between observations and models highlight the need for further exploration of cloud and disequilibrium chemistry in numerical simulations.This paper investigates the chemical composition and cloud properties of the hot Jupiter WASP-43b, focusing on its nightside clouds and disequilibrium chemistry. Using the JWST's Mid-Infrared Instrument, the authors measure a phase-resolved emission spectrum from 5 μm to 12 μm, revealing a significant day-night temperature contrast (1,524 ± 35 K and 863 ± 23 K, respectively) and evidence of water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models suggest the presence of nightside clouds that become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to equilibrium chemistry predictions, methane is not detected on the nightside, with upper limits of 1–6 ppm depending on model assumptions. These findings indicate that the atmosphere of WASP-43b is shaped by disequilibrium processes, providing new insights into the properties of its nightside clouds. However, discrepancies between observations and models highlight the need for further exploration of cloud and disequilibrium chemistry in numerical simulations.