This research aimed to describe the actual condition of deep learning lesson plans for writing observation report texts used by teachers, to design a cooperative-based model, to develop the model through expert validation and revision, and to disseminate it for tenth-grade students of SMA Negeri 3 Sigi. Using Research and Development with the Four-D model (Define, Design, Develop, Disseminate), data were collected through document analysis, semi-structured interviews with four teachers and thirty-two students, classroom observation, needs analysis questionnaires, and expert validation sheets. Qualitative data were analyzed through reduction, display, and conclusion drawing, while validation data were analyzed using descriptive percentages, with trustworthiness maintained through triangulation, member checking, and audit trails. Findings revealed disconnections among learning objectives, activities, and assessment, ineffective group work, insufficient time allocation, and non-formative feedback. The designed model comprises learning identification, instructional design, learning experience, and assessment, integrating Jigsaw, Numbered Heads Together, and STAD with diagnostic, formative, and summative assessments. Expert validation placed the model in the very feasible category (95.44%). This study uniquely integrates cooperative learning strategies with deep learning principles in a lesson plan model for writing observation report texts, an underexplored combination in Indonesian secondary education. The model offers teachers a structured, validated framework for designing cooperative-based deep learning lesson plans. This research contributes a feasible, practical lesson plan prototype, offering a replicable model for curriculum developers and educators seeking to enhance writing instruction through cooperative and deep learning approaches.