Donors currently allocate the majority of their aid to countries affected by civil war and political violence (OECD 2018). Scholarship on international aid has historically argued that donor aid allocation is motivated by exogenous strategic interests, not the endogenous needs of the recipient country (Alesina and Dollar 2000). Using an original survey-embedded experiment completed by over 1,130 aid experts, we find that donors do, in fact, respond to changing dynamics within the conflict-affected country, albeit in predictable ways. Donor aid modalities, like all organizational routines, provide them with limited allocation options, which they use to reward post-conflict governments for implementing peace agreements and help populations if violence returns (March 1999). These findings, which hold across aid donors and recipient countries, challenge core assumptions within the international aid and peacebuilding literatures, demonstrating the emergence of a previously unobserved set of rules that govern aid allocation to post-conflict countries.