This article is twofold: (a) it explored the impacts of mediation strategies on promoting cognitive modifiability in a dynamic assessment (DA) writing exam among ESP learners, and (b) it compared test-takers' performance in DA vs. static assessment (SA). The analysis drew on qualitative and quantitative data of a dynamic writing exam of classroom interactions of an experimental group (n = 25) and a Many-Faceted Rasch Measurement (MFRM) analysis of SA of writing test scores of a control group (n = 107). Results showed that, unlike previous studies, DA did not advance test-takers' current level of thinking into a more developed one nor did it predict better cognitive modifiability. Also, analyses of test scores were not conducive to significant differences in the two groups' ability. Recommendations called for implementing DA as an additional form of assessment in this EFL context. Pedagogical, research and methodological implications were also highlighted.