Student agency in the form of students' active involvement in developing self-regulated learning skills by setting goals, monitoring, and adjusting their own learning process, is increasingly recognised as a key component of classroom self-assessment among researchers. The purpose of this article is to offer a conceptual and practical framework for scaffolding students' agentic engagement in formative self-assessment as a co-regulatory, three-phase process centred around students' competence development within the Zone of Proximate Development (ZPD). Agentic engagement in learning occurs when students make proactive, intentional, and constructive contributions to a learning activity, by offering input and making suggestions. This article explores process, product, and competence dimensions of formative self-assessment. It draws upon data from 256 Australian primary students involved in a one-setting practitioner study, conducted as a writing project in which students used a planning template. The findings show that when the planning template was used to scaffold teacher-learner transactions, a range of both direct and indirect teacher-learner transactions occurred, which were prompted by students' agentic engagement in their learning. The direct teacher-learner transactions included joint, two-way transactions focused on co-regulation, which were either initiated by the learner or by the teacher. The findings also included examples of one-way teacher-learner transactions that involved interactions in which the transaction was aimed at addressing a specific learning need or challenge. These findings imply that using a self-assessment planning template to foster learning through co-regulation enables both instruction and feedback to occur at the point of need, in a task-specific context within the student's ZPD.