What Can AI Offer as a Design Probe for Creativity? Reflections from Co-Designing a Creative Writing Tool

As generative AI tools enter creative domains, researchers increasingly use participatory design to align systems with user needs. However, this participation is routinely flattened into usability testing, obscuring fundamental ideological conflicts between system developers and artists. In this paper, we reposition the generative AI system from an end-product to be optimized into a diagnostic design probe. We situate this approach as the culmination of a four-tier evaluation toolkit for AI and creativity. We exemplify our diagnostic-based approach on the development and evaluation of Fabula (available on request at https://deepmind.google.com/frontiers/fabula), an interactive app for writeres that uses narrative plans informed by narratological theory, and that allows writers to structure stories hierarchically into scenes and beats that can be written and (re)generated and revised at script and story plan level. By iteratively deploying an AI writing assistant, we map the explicit sites of contestation between human and algorithmic agency, using the AI system as a probe to surface the negotiations of creativity artists are willing—and unwilling—to make. We argue that rather than attempting to ``solve'' these ideological frictions through optimization, the AI research community must design systems that explicitly expose and negotiate them, enabling practitioners to engage with the machine on their own terms.