The main purpose of the present study is to propose a framework for implementing group dynamic assessment (DA) using students' smartphones for improving and assessing EFL learners' ability to produce well-formed and appropriate requests. This study focuses on five learner reciprocity moves during DA interactions to get deeper insights into the learners' development. During three treatment sessions, the participants of the DA style instruction were asked to make requests in response to a number of scenarios during mobile-mediated group interactions via WhatsApp. The DA group participants were prompted to revise and self-correct their incorrect requests by a sequence of incrementally-ordered hints from most implicit to most explicit based on the sociocultural framework. The analysis of learners' reciprocity moves during the three DA sessions revealed that learners became progressively more responsive to mediation for formulating correct requests and achieved a more agentive role to interact with the mediator. Analyzing learners' scores obtained from three testing occasions also revealed that mobile-mediated DA was significantly effective to promote the learners' knowledge of request strategies.