In the second language (L2) assessment realm, cognitive diagnostic assessment (CDA) emerges as an exceptionally practicable methodology, enabling a meticulous analysis of linguistic competencies and furnishing granular insights into learners’ proficiencies and deficiencies, thereby charting precise remedial pathways. The primary objective of this research was to develop a cognitive model of attributes underlying an International English Language Testing System (IELTS) assessment descriptor, namely grammatical range and accuracy. The model sought to enable the development of a diagnostic instrument informed by CDA, which would investigate candidates’ grammatical strengths and weaknesses to improve their performance in the IELTS. Through a multi-stage process involving qualitative data collection, interpretation, and synthesis, a comprehensive scheme emerged, categorizing cognitive attributes into two main areas: (a) knowledge of grammatical forms, including pronouns, determiners/quantifiers, adjectives, adverbials, nouns/noun phrases, verbs/tenses, and prepositions, and (b) familiarity with structural nuances, including punctuation and structural sophistication. These nine micro-level attributes comprised several sub-components aligned with three proficiency classes: A1-A2, B1-B2, and C1-C2. The model laid the groundwork for developing a three-booklet multiple-choice test. Alongside conducting pilot testing and item analysis, the study employed a saturated psychometric model to validate the CDA-informed instrument. The results confirmed the instruments’ internal consistency, validity, satisfactory fit, and effectiveness in classifying examinees based on attribute-specific mastery levels. The findings underscored severe weaknesses in punctuation, structural complexity, and verb tense usage, pinpointing crucial areas demanding targeted instructional enhancement. The theoretical implications highlight a refined understanding of grammatical competencies, while pedagogically, the results advocate for targeted teaching strategies.