With the rapid diffusion of artificial intelligence (AI) technologies in organizational settings, AI tools have become increasingly embedded in employees' daily work processes. While prior studies have emphasized the productivity-enhancing potential of AI, empirical findings regarding its impact on employees' work experiences remain mixed. Drawing on the Job Demands–Resources (JD-R) model and Conservation of Resources (COR) theory, this study examines the dual effects of AI tool use on employees' work efficiency by focusing on two parallel mediating mechanisms: capability enhancement and perceived technostress. Using survey data collected from 421 employees who actively use AI tools in their work, structural equation modeling (SEM) and bootstrapping analyses were employed to test the proposed hypotheses. The results indicate that AI tool use positively influences employees' work efficiency through capability enhancement, while simultaneously exerting a negative indirect effect via increased perceived technostress. These findings reveal the coexistence of resource-enhancing and stress-inducing effects of AI tool use in the workplace. This study contributes to the literature by empirically demonstrating a dual-path mechanism through which AI tools affect employee performance outcomes. The findings also provide practical implications for organizations seeking to leverage AI technologies to improve work efficiency while mitigating employees' technostress.