The dangers of a single story in current U.S. large-scale writing assessment are that assessment practice does not align with theory and this practice has negative effects on instruction and students. In this article, I analyze the connections or lack of connections among writing theory, writing assessment, and writing instruction, critique the construct and consequential validity of direct writing assessment and portfolio assessment, and reframe reliability as local consensus among experts. A new vision of large-scale sociocultural writing portfolios in K-12 education is offered that builds on the practices of past large-scale portfolio assessment but also encourages students to write in multiple languages/dialects and modes for multiple purposes. Another key feature of sociocultural portfolios is that students are encouraged to write for impact. These additional components will mitigate the dangers of a single story by ensuring that (a) assessment practice matches sociocultural writing theory, resulting in high construct validity, and (b) teachers are encouraged to utilize culturally sustaining pedagogy, resulting in high consequential validity.