Despite growing interest in diagnostic assessment tools in second language writing, limited empirical research has addressed their development for EFL contexts. Responding to this need, this study aimed to develop and validate a diagnostic-oriented rating scale designed to deliver targeted feedback on Iranian EFL learners’ academic writing. Using a mixed-methods approach, essential descriptors reflecting core writing skills were identified through think-aloud protocols and expert feedback, followed by quantitative analyses to ensure reliability and validity. The findings indicate that the 21 empirically derived descriptors capture essential aspects of academic writing—content fulfillment, organizational knowledge, and language use—enabling instructors to assess learner proficiency with greater precision. The scale’s validation process, including inter-rater reliability, content validity, and criterion-related validity checks, supports its effectiveness as a diagnostic tool closely aligned with expert evaluations. This tool is a valuable resource for both large-scale assessments and classroom applications, supporting a learner-centered approach and empowering students to address specific writing challenges.
