This research demonstrates a new methodology—stack casting, which combines fabric formwork and robotic assembly. In the following research, a robot placed a textile mold on top of a precast environment to erect a significant architectural object. Through this method the robotic casting process obtained further contextual information to create highly customized assemblies. The natural form-fit boundary of the fabric strengthened the identification between global and local systems. As the digital method met the natural form-finding process, an interactive tectonic was generated—calibrating itself for any composition and accommodating deviations in geometry. Driven by the correspondence between digital and physical, the technique extended the building method of a dry-fitted cast in situ wall. To build a full-scale prototype, robotic stack casting collaborated with other versatile methods to invent new set-up strategies, which link to both on-and off-site construction. This research is part of a bigger picture rising in soft robotics. By interpreting unique forming into the computational design, precise and imprecise fabrication were redefined in a playful phenomenon. In traditional robotic stacking, the robot bridges the digital and physical world but its lack of adaptivity causes problem when placing imprecise objects. In this research’s process, the use of inexact fabric molds fixed the tolerance and found centers of gravity through relative reactions. They adjusted to the unique personal properties of their surroundings and compensated for discrepancies in material. As an alternative to the application of sensor feedback for accommodating tolerance in material, the use of stack casting digital formwork instead adjusts and reshapes the non-ideal masonry from the real-time architectural context. This playful process opens the door to further imaginations from design to build.