How to reach a creative agreement in negotiations when the Zone of Possible Agreement (ZOPA) does not apparently exist? To answer this question, we drew on the cognitive flexibility theory and proposed a model predicting that negotiators’ mental fatigue would engender fewer creative agreements, and their integrative complexity acted as an underlying mechanism. Across four studies, we measured (Study 1) and manipulated (Studies 2–4) mental fatigue to test our hypotheses. We found that negotiation dyads with higher mental fatigue were less likely to display integrative complexity and hence less likely to reach creative agreements in negotiations without an apparent ZOPA. We also demonstrated that in this kind of negotiation, simply identifying additional issues or proposing packaging offers were not enough; negotiators need to do both to construct creative agreements. This research contributes to the literature of negotiation, creative problem‐solving, and the cognitive flexibility theory.