The primary aim of this study is to delve into the experiences of postgraduate EFL students in using ChatGPT and to uncover the strategic approaches they implement throughout their writing process. This qualitative case study research design engages 16 Master's degree students from four different universities in Indonesia, each with varying levels of English proficiency. Findings indicate a positive reception of ChatGPT among EFL students, who note its versatility in improving vocabulary, grammar, idea generation, essay structuring, and language refinement. Students utilize a range of strategies in their interactions with ChatGPT, including critical evaluation, peer consultation, and the use of primary research sources, which demonstrates its adaptability as an educational resource. These findings suggest that ChatGPT could revolutionize EFL instruction by serving as an adaptable cognitive tool that aligns with diverse pedagogical models and cognitive development stages. Its capabilities support educational theories such as Scaffolded Writing, Distributed Cognition, and Cognitive Flexibility Theory, which enhance students' self-efficacy and academic success. However, the study's reliance on self-reported data and its limited sample size are notable limitations, potentially impacting the generalizability of the results. The study recommends that future research with a broader, more diverse sample and a mixed-methods framework could provide deeper insights into the effectiveness of ChatGPT in EFL contexts and its potential to augment students' writing competencies.