Purpose: This paper introduces the concept of posthuman co-production. It explores how processual and relational onto-epistemologies inform an artful, response-able (Barad 2007) feminist new materialist praxis that decentres the human and re-centres matter. Design/methodology/approach: Posthuman co-production gives prominence to crafting "dartaphacts" (Renold, 2018); creative research artefacts, carrying "what matters" and enacting change that can be mapped across time and multiple "problem spaces" (Lury, 2020), as an expansive, post-qualitative praxis of slow, co-production. Findings: The paper stories this praxis across three "fugal figurations" providing glimpses into the post-qualitative journeys of assembled dartaphacts in the policy and practice field of relationships and sexuality education (RSE) in Wales. Each fugue hints at the polytical, resourceful and living potential of dartaphacts in the making and their mattering over a period of six years. Collectively, they chart a rhizomatic journey that re-configures co-production as a response-able, becoming-with what matters. Originality/value: As more-than-human forces for change, dartaphacts continue to surface "the cries of what matters" (Stengers 2019) for children and young people well beyond the periods of funded research and engagement, giving new meaning to the sustainability and material legacies of research impact.