Informal Digital Learning of English (IDLE) is associated with willingness to communicate in a second language (L2 WTC). Although affective mediators (e.g. enjoyment and anxiety) have been found to influence the relationship between IDLE and L2 WTC, it is unclear whether cognitive factors influence IDLE's effects on L2 WTC. This study aims to fill this gap by determining whether self-efficacy beliefs mediate the relationship between IDLE and L2 WTC among 246 Kazakhstani university EFL students. In contrast to previous research in other Asian contexts, our structural equation modelling results revealed that IDLE was not directly related to L2 WTC. Instead, self-efficacy beliefs acted as a full mediator between IDLE constructs and L2 WTC. These findings suggest that EFL students who participate in IDLE activities more frequently have a stronger belief in their own ability to perform various tasks in English, which leads to a greater willingness to communicate. This is the first research of its kind in Kazakhstani EFL context. This is also the first study to show that IDLE and L2 WTC are linked "via" a cognitive factor. Our research offers a context-specific pedagogical IDLE model to teachers and teacher educators in Kazakhstan and other similar EFL contexts.