Writing has become relevant in an era in which written texts permeate all areas and social practices. This is the result of a complex cognitive and metacognitive process that demands significant efforts from those who engage in it. In this context, it is necessary to reconsider pedagogical practices, so that educators may guide the process of writing production, using instructions as mediating tools for the mental construction of the texts students are asked to write. This article illustrates the design and validation of a rubric to assess the writing prompts developed by teachers. An instrumental study was conducted, starting with the justification of the research and concluding with reliability and validity tests on the instrument. To collect information, the research team performed a pilot study using 12 prompts, and then 16 experts evaluated the relevance, formulation, and coherence of the rubric. A mixed approach was employed, considering both expert observations and statistical tests on the evaluations. The pilot study resulted in a Cronbach's alpha reliability level of 0.979. Regarding the relevance of the instrument's items, Aiken's V coefficient resulted in a range of 0.917-1. It is concluded that the designed rubric constitutes a valid instrument for evaluating writing prompts and ensuring that necessary instructions are included to guide students during the writing process.