Developing disciplinary literacy is an emerging priority for secondary teachers as they prepare students for college, career, and civic life. One way to develop and assess disciplinary literacy in history is through source-based argument writing (SBAW) with multiple sources. SBAW requires students to synthesize information across texts and use disciplinary reasoning to evaluate sources and answer historical questions. Some research in history education has examined how writing and historical reasoning can be developed and measured, but the understanding of how specific tasks differentially elicit key thinking and writing skills is nascent in its development. To address this gap, we analyzed the writing of 30 grade 8 students in history who wrote argument essays in response to two different writing prompts. We scored their writing for multiple discrete components of writing quality and used MANOVA to compare writing across prompts. We found that 1) key disciplinary thinking skills are difficult for students to express in SBAW and 2) writing prompt characteristics significantly influenced students' expression of historical reasoning (i.e., sourcing, corroboration, and counterargumentation). Implications for the design of source-based writing tasks in history, as well as instruction and measurement, are discussed.