Introduction The aim of this study was to design an approach to improving care for frail older patients in hospital services where Comprehensive Geriatric Assessment (CGA) was not part of the clinical tradition. Methods The intervention was based on the principles of CGA, using quality improvement methodology to embed care processes. Qualitative methods and coproduction were used to inform development of the intervention, which was directed towards the health care professionals involved in peri-operative/surgical cancer care pathways in two large UK teaching hospitals. A formative, qualitative evaluation was undertaken; data collection and analysis were guided by Normalisation Process Theory. Results The clinicians involved agreed to use the toolkit, identifying potential benefits including improved surgical decision making and delivery of interventions pre-operatively. However, sites concluded that pre-operative assessment was not the best place for CGA, and at the end of the 12-month trial, implementation was still nascent. Efforts competed against the dominance of national time-limited targets, and concerns relating to patients’ immediate treatment and recovery. Some participants involved in the peri-operative pathway felt that CGA required ongoing specialist input from geriatricians, but it was not clear that this was sustainable. Conclusions Clinical toolkits designed to empower non-geriatric teams to deliver CGA were received with initial enthusiasm, but did not fully achieve their stated aims due to the need for an extended period of service development with geriatrician support, competing priorities, and divergent views about appropriate professional domains.