Although increasingly understood and appreciated, thiol-mediated uptake (TMU) remains underused because practical traceless tags that solve daily delivery problems are not yet available. The most popular cell-penetrating poly(disulfide)s (CPDs) were initially introduced as traceless tags that could be grafted from any thiol-containing substrate of interest (SOI) in situ and would depolymerize in the cytosol right after uptake. This approach was operational but not ideal for solving practical problems because the concentrations of SOIs >30 µM needed in neutral water are above those acceptable in most biological studies. Here, we report that CPD grafting-to SOIs, rather than grafting-from, provides access to dynamic covalent cysteine bioconjugation with protein concentrations down to 50 nM, which is more than 600 times below standard grafting-from CPD chemistry. With rate constants up to 1500 M^-1 s^-1 , CPD grafting-to is as fast as the record covalent cysteine bioconjugation (heteroaromatic sulfones), in the range of the best bioorthogonal reaction (IEDDA), and 3000 times faster than cystine grafting-to. Experimental evidence for CPD grafting to probes, peptides and proteins with one, two, or several proximal thiols, efficient TMU of their conjugates, and the cytosolic release of functional SOIs, such as fluorescent antibodies against the nuclear pore complex, supports the discovery of operational traceless TMU tags, at last.