Cuproptosis represents a promising strategy for cancer therapy. Generating copper diethyldithiocarbamate (Cu(DTC)₂) directly at tumor sites may enhance copper-dependent antitumor activity; however, inefficient precursor loading and premature release may limit local Cu(DTC)₂ formation and increase off-target toxicity. To overcome these challenges, a reactive oxygen species (ROS)-responsive, thioketal-linked dopamine dimer (DTD) and an H₂O₂-responsive DTC prodrug (DQ) were co-assembled and subsequently coated with a copper–tannic acid (Cu@TA) network to form CDD nanoparticles (CDD NPs). The DQ-loading capacity and stimulus responsiveness of CDD NPs were characterized, and their in vitro anticancer activity was evaluated in 4T1 cells. DTD promoted DQ loading and linked the core to the copper network through catechol coordination. DTD-free CD NPs contained only 57.3% of the DQ in CDD NPs and had a lower Cu(I) fraction. Copper release from the Cu@TA network was limited under physiological conditions but increased in an acidic environment, whereas H₂O₂ triggered DQ cleavage, supporting the proposed in situ formation of Cu(DTC)₂. In 4T1 cells, CDD NPs induced oxidative stress, glutathione depletion, and apoptosis. Intracellular copper accumulation, partial rescue by tetrathiomolybdate, and changes in ferredoxin 1 (FDX1) and dihydrolipoamide S-acetyltransferase (DLAT) suggested possible cuproptosis involvement. CDD NPs improved prodrug loading and controlled precursor release, inhibiting tumor cells in vitro.