This study explores ChatGPT affordances, students' use of prompts, and their perceptions of using the tool in EFL writing. With screen capture technology to observe students' interactions with ChatGPT during writing process and semi-structured interviews, the data were analysed using a collaborative reflective thematic analysis. The results revealed that students' use of ChatGPT has the potential to improve their essay quality in terms of lexical resources, grammatical range and accuracy, and coherence and cohesion. ChatGPT also provides such affordances as brainstorming, essay outlining, idea development, and feedback on hedging devices. Overall, students considered ChatGPT a valuable personal learning tutor that may stimulate student autonomy by offering immediate feedback in a less anxiety-provoking learning environment. However, some experienced challenges in producing effective prompts and understanding ChatGPT's feedback. The study concludes that while the use of ChatGPT can enhance L2 writing development, teachers' guidance is necessary during students' interaction with the tool.