If international environmental agreements (IEAs) are to remain relevant over time, the institutional capacity to adjust them to changing circumstances, referred to here as adaptability, is an important asset. Yet, while some IEAs include various adaptability features, others do not. This paper develops the concept of affinity, defined as the varying sense of connection between negotiating countries, and argues affinity is a major driver of adaptability variation. Two rationales may explain why negotiators include adaptability provisions in IEAs. When affinity is low, negotiators likely use adaptability as a safeguard in case cooperation does not go according to plan or to gather more information about other signatories before further cooperation. By contrast, when affinity is high, they can use adaptability as a springboard for long-term expansive cooperation. I test both hypotheses on a sample of 1137 IEAs and find that adaptability is negatively associated with affinity, supporting the safeguard rationale.