<![CDATA[Diagnostic assessment can play a dual role in pre-service teacher education by identifying prospective teachers’ learning needs while strengthening their capacity to respond to language difficulties encountered in school settings. This study examined the use of diagnostic assessment in the language development of 30 second-semester students enrolled in the Indonesian Language and Literature Education Study Program at Universitas Negeri Surabaya, supported by an error analysis of 30 descriptive texts written by seventh-grade students at a junior high school in Surabaya. A descriptive qualitative design with purposive sampling was employed. Data were collected through diagnostic tests covering listening, speaking, reading, and writing, as well as classroom observation, interviews, and documentation, and were analyzed using the interactive model of Miles, Huberman, and Saldana. The findings identified four major areas of difficulty among the prospective teachers: standard spelling and grammar (43.3%), academic vocabulary (36.7%), paragraph coherence (50.0%), and speaking confidence (40.0%). Supplementary school-level analysis recorded 505 language errors, 87.9% of which involved spelling, with capitalization as the most frequent category. Diagnostic findings informed differentiated instruction, scaffolding, and formative feedback during a twelve-week instructional period. The mean post-test score increased by 18.4 points, while qualitative evidence indicated greater learning autonomy, speaking confidence, and metalinguistic awareness. These findings suggest that diagnostic assessment can support responsive instruction in pre-service language teacher education while strengthening prospective teachers’ assessment literacy for addressing language-learning needs in secondary-school contexts.]]>
