Recent literature has underlined the interplay among climate mitigation, adaptation, and finance, as well as between climate action and other development agendas, including sustainable resource use, human development and equity, and environmental pressures. Such an interconnected policy environment requires an integrated ecosystem of disciplines, methods, and tools. Despite the significant evolution of integrated assessment models (IAMs) in the last decade, there remain several criticisms on their design, use, and adequacy to respond to unaddressed and emerging questions in the light of the Paris Agreement and net-zero ambition. These include openness, legitimacy, and ownership, as well as technical feasibility to represent demand-side and broader societal transformations, cross-sectoral interactions, physical impacts and adaptation, climate finance and labour dynamics, and other sustainability goals. DIAMOND will update, upgrade, and fully open six IAMs that are emblematic in scientific and policy processes, improving their sectoral and technological detail, spatiotemporal resolution, and geographic granularity. It will further enhance modelling capacity to assess the feasibility and desirability of Paris-compliant mitigation pathways, their interplay with adaptation, circular economy, and other SDGs, their distributional and equity effects, and their resilience to extremes, as well as robust risk management and investment strategies. This will be done via integration of tools and insights from psychology, finance research, behavioural and labour economics, operational research, and physical science. We will develop a transdisciplinary scientific approach to legitimise the implementation process and co-create research questions that stretch the frontiers of climate science, as well as establish vibrant communities of practice to transparently open model enhancements and to develop capacities, thereby lowering the entrance barriers to the established IAM community.