Researchers have engineered a novel M6L4 octahedral molecular cage by integrating photoactive cyclometalated platinum(II) units, creating a visible-light-responsive "nanoreactor" that drives highly efficient photochemical reactions through precise molecular confinement. This innovative design overcomes the limitations of previous hosts, achieving perfect stereo- and site-selective cross-[2 + 2] cycloaddition reactions, and most notably, enabling catalytic cross-[2 + 2] cycloaddition of chemically inert substrates using this supramolecular confinement approach.