The research proposes that the Viable System Modelâs (VSM) strategy of managing complexity may be inflexible and costly for responders to disasters. The traditional VSM does not offer solutions on how to address a large amount of emerging complexity by a system that is rapidly changing. In these systems, the autonomy of operations can be very important for resilience. Applying the VSM during a response to a disaster may cause systemsâ rigidity when agility is required. The VSM perceives external complexity as a threat to viability. As such, it instructs systems to attenuate (reduce) external complexity to maintain viability. Such worldview promotes atomism. Further, it can deprive organisations of utilising critical resources that reside in the environment. Consequently, adopting a traditional VSM strategy can hinder achieving the resilience potential that is crucial to stay viable during disasters. Accordingly, three gaps in the VSM were identified. First, the research argues that the notion of variety as a measure of complexity is not practical nor sufficient to address disasters complexity. Variety does not distinguish between potential and actual complexity. Second, the VSM does not offer a complexity classification that facilitates rapid decision-making and operations autonomy. Third, the VSM does not provide a model that helps the system to efficiently address complexity drivers. To close the gaps, novel conceptual propositions and models to define, classify, and manage complexity and its generators are proposed. In addition, the role of the notions of systemsâ boundaries and identity in achieving higher resilience and viability beyond survival is discussed. The research addresses the operational complexity that is associated with spontaneous volunteers (SVs) during disasters response. The data were collected in two UK counties that encountered SVs during their response to major disasters. 23 semi-structured interviews were conducted with representatives of county councils, blue light agencies, British Red Cross, and volunteers. Further, two live exercises that were designed to test a new SV policy were observed. The data were analysed thematically through open coding and a focused coding using the VSM and the proposed conceptual models. This research contributes to the VSM, systems thinking, and disaster literatures. It opens the door for further research to develop the proposed propositions and models. Further, the research informs policymakers and practitioners in the field of disasters and beyond.