International courts and tribunals have converged, across 2024 and 2025, on a standard of due diligence that is ‘informed by the best available science’. Commentary reads this convergence as an epistemic ratchet: as the evidence base improves, the obligation tightens. This article argues that the ratchet operates principally on the content of an assessment and only marginally on its frame, and that the difference is decisive for circular water and waste infrastructure. Since Pulp Mills, the International Court of Justice has held that the specific content of an environmental impact assessment is for each State to determine, and no subsequent decision has retracted that holding. What international law therefore incorporates is an assessment whose system boundary, functional unit, allocation rule, baseline, and time horizon are fixed by the party whose conduct is under review, and whose frame of review reaches only where an omission is manifest. The term proposed here for that residual and largely unreviewed power is the scoping prerogative. Circularity is where the prerogative bites hardest, because a closed loop is a claim about a boundary rather than a property of a material: a wastewater reuse scheme closes a loop at the district scale and opens one at the basin scale; a plastics export closes a loop in the exporting State’s material flow account and opens one in the importing State’s environment; a biosolids scheme closes a nutrient loop and opens a contaminant loop that a nutrient-indexed assessment does not measure. Three cases, selected on explicit criteria for variation in how far the assessment method is externally fixed, support a finding that the article calls the relocation of scoping discretion: fixing the method does not cure the prerogative but moves it to a less visible parameter. The article closes by reconstructing the obligation as a duty of scoping fidelity comprising four constraints, by distinguishing those already latent in the law of prevention from those that remain proposals, by converting the duty into a scoping questionnaire, and by stating what that duty cannot achieve.