This paper introduces a novel regression framework for modeling response variables restricted to the unit interval by proposing unit-Lindley additive partially linear models (UL-APLMs). This model class combines parsimony and interpretability of one-parameter unit-Lindley distribution with the flexibility of additive partial linear structures, enabling the coexistence of linear and smooth covariate effects. Additive terms are modeled using B-spline basis under a penalized likelihood framework to ensure smoothness. Estimation is carried out by maximizing the penalized log-likelihood function. The goodness-of-fit of the models is assessed through residual analysis, whereas the robustness of the parameter estimates and the detection of influential data points are evaluated using the local influence approach, which incorporates curvature diagnostics under case-weight and response perturbation schemes. The sensitivity and penalized observed information matrices are derived explicitly for the proposed model. Simulation studies demonstrate the accuracy of the estimation procedure under various scenarios. Real data on the assessment of the psychological profile of patients with hypopituitarism illustrate the applicability of the model, highlighting the diagnostic importance.

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