This article reports on a study of writing and assessment in upper primary school, focusing on how teachers develop assessment competence through professional discussions in interdisciplinary groups. The empirical data consist of dialogues between teachers assessing and giving feedback on students' texts from different subjects. To support their evaluations, the teachers used and referred to a defined construct of writing and norms for expected writing proficiency. The analyses reveal a complex picture of the teachers' use of the assessment resources and their processes of appropriating a broader understanding of writing, including acquiring an extended meta-language. Three thematic categories are crystallised: an instrumental approach and a flexible and functional approach, appearing as two ideal typical points on a continuum, linked together by an overarching category labelled learning in progress. The article argues for an analytical reading as a basis for formative assessment in general writing education, and points to the need for knowledge on writing, text and linguistics -traditionally seen as domains for L1 teachers. The findings are discussed in light of the L1 teachers' responsibility for both the specific subject discourses of literacy and Bildung, and for providing students with general knowledge on language and text to support their overall writing competence. Critical implications of this double responsibility for the L1 subject are presented and reflected upon.