WTO adjudication faces a crisis of social legitimacy. I diagnose that crisis in terms of competing theories of interpretation, and thick and thin political moralities. WTO law’s dominant self-image is as voluntarist-positivist treaty law, reflecting an implicit thin international political morality. Interpretation is exclusively a matter of textual analysis complemented by originalist intent. Yet text is indeterminate and intent obscure, requiring a supplementary judicial gap-filling role. But that role is expressly excluded by the WTO DSU. The legitimacy problem is thus a function of the thin political morality. If instead we assume a thicker political morality, with economic duties to insiders and outsiders, the voluntarist-positivist image loses appeal, licensing an anti-positivist moral reading that can legitimise interpretation in hard cases without judicial legislation.