In response to calls for a research and innovation system that is more open to public scrutiny, we have seen a growth of formal and informal public engagement activities in the past decades. Nevertheless, critiques of several persistent routines in public engagement continue to resurface, in particular the focus on expert knowledge, cognitive exchange, risk discourse, and understandings of public opinion as being static. In an attempt to break out of these routines, we experimented with an innovative engagement format that employs situated speculative prototyping to support citizens in contextualizing and discussing developments regarding—in this case—nanotechnology. This format invites participants to imagine and critically reflect on technological futures through collaborative prototyping and story-writing. In this paper, we outline five reconstructed contextualization patterns in which participants engaged during the format’s exercises and use these to assess the value of the format in the current engagement landscape. We show that situated speculative prototyping has potential in the realm of informal public engagement initiatives, taking an explorative approach, but also warn of ‘the designer fallacy’ as a prominent pitfall of prototyping that could reproduce techno-scientific framings and obstruct critical reflection on technological directions and implications if not treated with caution.