In 2016, in Switzerland, we implemented transitional interprofessional and interinstitutional shared decision-making processes (IIPs) between a short-stay inpatient care unit (SSU) and primary care professionals. Between 2018 and 2019, we evaluated this intervention using a realist design to answer the following questions: for whom, with whom, in which context and how have IIPs been implemented? Our initial theory was tested via interviews with patients, primary care professionals and staff from the SSU. Results showed that a patient's stay at the SSU, with actors committed to facilitating IIPs, reinforced the perceived appropriateness and implementation of those IIPs. However, this appropriateness varied according to different contextual elements, such as the complexity of needs, preexisting collaborative practices and the purpose of the inpatient stay. Since IIPs occurred in a context of fragmented practices, proactive and sustained efforts are required of the actors implementing them and the organizations supporting them.