The Qosh Tepa Canal, under construction in northern Afghanistan since 2022, represents the most significant unilateral change to the water balance of the Amu Darya basin since the Soviet-era expansion of irrigation that precipitated the desiccation of the Aral Sea. Published estimates of the canal's diversion capacity vary widely, but the upper range would more than double Afghan withdrawal from the river and would reduce availability substantially for downstream Uzbekistan and Turkmenistan. Afghanistan, however, stands outside the institutional architecture of the basin. It is not a party to the regional agreements of 1992 and 1993, nor to the 1997 United Nations Watercourses Convention or the 1992 Helsinki Convention, and the absence of broad diplomatic recognition of the government in Kabul rules out treaty-based solutions in the medium term. This article analyses the resulting institutional vacuum along four dimensions: legal, informational, verificational and diplomatic. It then assesses the semi-official engagement formats that have emerged since 2024 and develops three scenarios for the period 2026 - 2030, each defined by specific triggers and observable indicators. The article concludes with a phased roadmap of confidence-building measures organised along informational, verificational and restraining dimensions. Its central practical claim is that the window for establishing agreed procedures before the canal is fully commissioned is limited to roughly two to three years, after which downstream states will confront an operational diversion facility as an accomplished fact.