Introduction <p>First-year undergraduate medical students are rarely positioned as active participants in the design of the curricula through which they are learning. This marginalisation of student voice is particularly consequential when students are simultaneously forming their professional identities and adapting to the norms and structures of medical education. The Change Laboratory (CL) is a structured intervention grounded in Cultural-Historical Activity Theory (CHAT) that creates deliberate conditions for participatory curriculum inquiry. This study investigated how engagement in a CL intervention affected first-year medical students’ and educators’ experiences of the undergraduate learning environment, with particular attention to how the CL functioned as a mechanism for curriculum diagnosis, the structural contradictions it surfaced, and the changes it produced in participants’ relationship to the curriculum.</p> Methods <p>A qualitative study was carried out within a first-year undergraduate medical programme. A total of fourteen participants — comprising first-year students (<i>n</i> = 7), academic educators (<i>n</i> = 5) and programme administrators (<i>n</i> = 2) — took part in seven structured Change Laboratory sessions focused on identifying and collaboratively addressing curriculum tensions in the first year. Semi-structured interviews were subsequently conducted to elicit participants’ accounts of the CL process and its effects on their relationship to the curriculum. Data were analysed using reflexive thematic analysis.</p> Results <p>Although the themes are constructed from participant accounts, they describe a process of curriculum diagnosis with programme-level consequences: the structural contradictions identified through the CL informed governance discussions and contributed to changes affecting the entire first-year cohort. Three themes were constructed from the data: (1) Reclaiming voice — describing how the CL created conditions in which students’ perspectives were legitimised and their contributions actively sought; (2) The diagnostic power of shared dialogue — capturing how structured conversation enabled participants to identify the underlying sources of curriculum tensions that had previously resisted resolution; and (3) Building durable investment — reflecting how co-design processes generated sustained personal and professional commitment to curriculum quality that extended well beyond the sessions themselves.</p> Conclusion <p>CL participation generated a qualitative shift in how first-year students and educators understood their respective roles in relation to the curriculum. Crucially, the CL functioned not as a student participation event but as a curriculum diagnosis mechanism: it surfaced structural contradictions affecting the entire first-year programme that conventional feedback mechanisms had consistently failed to detect. The findings suggest that the CL can address student marginalisation in undergraduate medical education not only through relational inclusion but through generating actionable curriculum knowledge that neither students nor educators could produce alone — a contribution with implications for any undergraduate programme where student voice is structurally excluded.</p>