<p>Field observations indicate that the propagation of hydraulic fractures (HFs) in composite formations, particularly those consisting coal and rock layers, is influenced by complex interactions arising from material properties and stress sensitivity. This study investigates the asymmetric behavior of HFs in coal–rock composites through a series of true triaxial hydraulic fracturing experiments, complemented by a numerical model that incorporates dynamic leak-off effects. Our results demonstrate that the high stress sensitivity (&gt; 0.3&#xa0;MPa⁻¹) of soft-fragmentized coal significantly restricts HF propagation while fracturing, with maximum fracture lengths observed ranging from 26 to 41% of the height of the coal seam. The increasing leak-off coefficient associated with fracture extension accelerates the transition of the HF expansion pattern toward leak-off-dominated region. Notably, the findings reveal a unique propagation pattern in which the overlying rock layers continue to expand, while HF growth within the coal layer is limited. Additionally, employing higher fluid viscosity substantially reduces leak-off, maintaining HF propagation in the viscosity-dominated region, leading to over a 150% increase in HF lengths compared to traditional methods that rely solely on high injection rates. This research advocates for an injection strategy of high-rate initiation, followed by low–medium viscosity to optimize fracture length while managing height in coal seams. These insights contribute to a deeper understanding of hydraulic fracturing in complex geological settings and provide a foundation for enhancing field application strategies.</p>