<p>This longitudinal study examined how pencil grip maturity and task demands jointly shape the development of handwriting fluency, defined as the integration of speed and accuracy, in a sample of 290 Spanish-speaking children assessed from early childhood education through the end of second grade. Across five measurement points, children completed two handwriting tasks: letter copying and name writing. Twelve grip types were identified and classified into three developmental levels: primitive, transitional, and mature. Generalized additive mixed models showed that children using mature grips consistently exhibited higher fluency and steeper growth trajectories than those with transitional or primitive grips did, particularly in the name-writing task. These differences became most pronounced during the transition from preschool to primary school. When learning-risk status was considered, at-risk children demonstrated lower overall fluency. However, within this group, those with mature grips performing meaningful tasks showed faster growth rates, suggesting that motor maturity may partially compensate for learning vulnerability under favorable task conditions. In contrast, the high perceptual and motor demands of the letter-copying task attenuated differences between grip types and risk groups, functioning as a performance equalizer. Overall, the findings indicate that handwriting fluency develops through the dynamic interplay of grip maturity, task-specific demands, and learning-risk status during the early years of schooling. Rather than serving merely as a developmental marker, pencil grip maturity emerges as a sensitive indicator of fluent transcription development, with meaningful and low-demand tasks providing optimal conditions for growth, particularly among children at risk of writing difficulties.</p>