This case study of new English-language learners in an urban elementary U.S. classroom addresses the synergy between disciplinary language and science, technology, engineering, math (STEM) learning. Social semiotic theory frames the discourse demands related to engineering design and the active use of discursive resources in students' responses to these demands. Discourse analysis of students' interactions and writing provides a fine-grained view of "language in use" within an engineering design space where students demonstrated linguistic agency and productive engineering accomplishments. The case provides insights for educators who seek to optimize the integration of the language of STEM with culturally and linguistically responsive engineering education.