Research suggests that teachers' and students' engagement in formative assessment contributes to students' writing development by influencing both what and how students learn. However, while research exists on how teachers and students engage in formative assessment in traditional classroom settings, little work has examined the nature of formative assessment when students write together online. In this multiple-case study that took place in two states, we investigate (1) the common and unique contextual factors that shape ELA teachers' formative assessment beliefs and practice, and (2) the challenges they face when engaging in ongoing assessment while students write together online in their classrooms. Data include pre- and post-study semi-structured teacher interviews and post-instruction debriefs, and field notes from 100 hours of classroom observations. Findings suggest that district-level and classroom-level factors shape teachers' perceptions of formative assessment and how they adjust their approach to writing instruction over time. This work illustrates the unique contextual factors that shape teachers' beliefs about and engagement with formative assessment, specifically for online collaborative writing.