Transparent and precise endpoint definitions are a crucial aspect of clinical trial conduct and reporting, and are used to communicate the benefit of an intervention. Previous studies have identified inconsistencies in endpoint definitions across oncological clinical trials. Here, the Head and Neck Cancer International Group assessed endpoint definitions from phase 3 trials or trials considered practice-changing for patients with recurrent or metastatic mucosal head and neck squamous cell carcinoma, published between 2008 and 2021. We identify considerable and global heterogeneity in endpoint definitions, which undermines the interpretation of results and development of future studies. We show how fundamental components of even incontrovertible endpoints such as overall survival vary widely, highlighting an urgent need for increased rigour in reporting and harmonisation of endpoints.