Fiber-based composite materials are becoming increasingly popular in automotive, aerospace, and structural applications due to their lightweight nature and improved mechanical performance. The present study assesses luffa cylindrica-snake grass (LCSG) fiber reinforced composites with coconut shell powder (CSP) as a filler, produced using compression molding. CSP was added at weight fractions of 2.5, 5.0, 7.5, and 10.0 wt.%, and the effects on tensile, flexural, impact, and water absorption characteristics were investigated. The LCSG/CSP4 composite (7.5 wt.% CSP) showed the best performance, with a 33.06% increase in tensile strength, a 28.31% improvement in flexural strength, and a 23.12% increase in impact resistance compared to neat LCSG composites. However, over 7.5 wt.% CSP inclusion, mechanical property degradation occurred due to filler aggregation, poor dispersion, and interfacial debonding. Microstructural research shows that lower concentrations of fiber/matrix CSP particles allow for effective stress transmission, whereas high filler content leads to fiber breakage and matrix damage, creating voids and stress concentration zones. CSP inclusion reduced moisture uptake, with LCSG/CSP4 showing a 7.47% decrease compared to neat composites, enhancing dimensional stability and durability in humid conditions. CSP-filled LCSG composites show a potentially strong alternative for structural, automotive, and industrial applications due to their improved mechanical properties and moisture resistance. © 2025 Elsevier B.V., All rights reserved.