During their most recent “100-Day-Challenges”, teams from France, the Netherlands, and Germany collaborated to co-develop sovereign public sector IT. The Netherlands is using components from La Suite Numérique and openDesk to build MijnBureau, while French and German teams continue to work together on their workspace solutions. This session explores how these cross-border collaborations tackle technical and organisational challenges and enable interoperability between different national workspaces. By sharing experiences and lessons learned, we highlight the potential of coordinated European initiatives to strengthen digital sovereignty and co-develop resilient, citizen-focused public sector software across borders.