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Abstract: The new Advanced Photon Source storage ring reduces electron beam emittance by about two orders of magnitude relative to the original APS, increasing X-ray beam brightness by roughly a factor of 500. However, this smaller beam leads to increased Touschek scattering, which would otherwise reduce the storage ring lifetime to about one hour. To mitigate this, a team from ANL Physics Division and the APS developed and commissioned a passive, beam-driven superconducting higher-harmonic cavity bunch lengthening system (BLS) as part of the APS Upgrade project. Operating at 1.408 GHz, the fourth harmonic of the 352 MHz main RF system, the superconducting approach provides the required voltage within tight space constraints while offering a simple higher-order-mode damping scheme and excellent operational flexibility. The system is built around a single-cell niobium cavity cooled to 2 K by a dedicated cryogenic plant. Installed in the tunnel in 2023 and first operated with beam in 2024, the system has been successfully commissioned and is routinely operated with cavity voltages up to 1.1 MV. Studies for several operating conditions demonstrate substantial bunch lengthening, fulfilling design goals to mitigate Touschek scattering in a modern, low-emittance storage ring. Finally, prospects for a substantially simplified solution based on Nb3Sn (niobium-three-tin) technology are discussed.