WHO Decides What? Rethinking the Law-Fact Divide in WTO and UNCITRAL’s ISDS Appellate Mechanism Review

Defining the boundary between questions of law and fact is a critical challenge in designing credible international appellate mechanisms. This paper examines how this structural safeguard shapes who decides what on appeal in two contrasting systems: the World Trade Organization’s (WTO’s) now-paralysed Appellate Body (AB) and United Nations Commission on International Trade Law’s (UNCITRAL’s) ongoing efforts to create an investor-state dispute settlement (ISDS) appellate mechanism. While the WTO’s dispute settlement rules initially drew a clear doctrinal line between law and fact, its AB blurred this boundary through expansive reinterpretations and its procedural reliance on the ‘objective assessment’ safeguard, contributing to institutional breakdown. In contrast, UNCITRAL’s Draft Statute begins with a vague ‘manifest error’ standard for factual review, offering no clear threshold or procedural filter to prevent excessive factual relitigation. Recent Working Group III (WG III) discussions, including a possible revision to require errors to be ‘apparent on their face’, highlight that textual drafting alone cannot ensure consistency or finality. Through a comparative doctrinal and procedural analysis, the paper argues that lessons from the WTO’s trajectory demonstrate that even precise treaty text is insufficient unless reinforced by practical safeguards, such as procedural filters and clear interpretive guidance. In practice, sustaining the law-fact divide is essential to preserve legitimacy, efficiency, and predictability for states, investors, and the broader dispute settlement system.