The Common Core State Standards emphasize students writing analytically in response to texts. Questions remain about the nature of instruction that develops students' text-based writing skills. In the present study, we examined the role that writing task quality plays in students' mastery of analytic text-based writing. Text-based writing tasks (N = 149) were collected from 27 fifth-grade teachers in an urban district, and teachers completed daily surveys (i.e., instructional logs) to assess the frequency of their reading and writing instruction (30-45 days total). Students (N = 793) completed a performance assessment of their text-based writing skills. Results showed that the large majority of writing tasks guided students to retrieve isolated facts or engage with surface-level features of texts in their writing (i.e., were of a low level of cognitive demand). The cognitive demand of text-based writing assignments predicted multiple features of students' writing performance, including students' ability to reason analytically about texts (effect size [ES] = 0.46), use evidence to support their claims (ES = 0.46), and organize their writing (ES = 0.35), even after controlling for other dimensions of literacy instruction. The quality (grist) of text to which students responded predicted one dimension of students' writing performance, use of evidence to support their claims (ES = 0.37). Designing cognitively demanding text-based tasks should be considered an essential part of writing instruction reform and professional development programs for teachers that aim to increase students' writing skills aligned to the Common Core.