The proliferation of 'behind-the-border' issues has become a major challenge for the global trading system. Why are some trade-related issues integrated in the strongly judicialized WTO with its dispute settlement system, while others are governed in isolation from the trade regime? Contributing to the literature on WTO issue boundaries, the paper highlights the role of uncertainty and demand for transgovernmental regulatory cooperation as drivers of regime outcomes. I argue that high technical and political uncertainty delays GATT/WTO integration of trade-related issues in favour of a soft-law trade pathway. A further-reaching separation from the trade regime occurs when uncertainty remains high and national regulatory authorities exhibit a demand for trade-separate cooperation with foreign counterparts. Case studies on government procurement and competition policy substantiate the argument. Government procurement has been integrated into the GATT/WTO trade regime under a market access agenda pursued by EU and US trade actors, but only after uncertainty had been reduced in technical soft-law trade talks at the OECD. Conversely, in competition policy - despite repeated efforts at GATT/WTO integration - cooperation has thrived in less judicialized transgovernmental networks such as the International Competition Network, driven by competition agencies eager to solve regulatory cooperation problems independently from the trade regime.