Edge-labelled ultrasoft hydrogel microsphere stress gauges (eMSGs) are low-polymer-content polyacrylamide beads with a bright fluorescent nanoparticle shell that deform under cell-generated forces, enabling mechanical stress mapping inside 3D tissues at resolutions down to ∼10 Pa. These sensors have been validated in inducible breast cancer tumor models to reveal dynamic internal stress patterns, and their fluorescence remains stable for repeated imaging even in optically dense tissues over 100 μm thick