Precise construction of asymmetric bimetallic coordination sites is important for developing efficient Fenton-like catalysts, but stabilizing such structures on nitrogen-rich carbon supports remains challenging. Herein, an ultrasmall CoSn particle-supported C₂N catalyst (CoSn/C₂N) was prepared through a supramolecular assembly-assisted pyrolysis strategy using melamine, cyanuric acid, CoPc, SnPc, and oxalic acid as precursors. The obtained CoSn/C₂N exhibited a flower-like architecture assembled from ultrathin wrinkled nanosheets, with highly dispersed CoSn particles mainly distributed at 1.0–1.2 nm. XANES, EXAFS fitting, and WT-EXAFS analyses revealed the partially oxidized states of Co and Sn, confirmed Co–Sn coordination, and demonstrated strong coupling between the bimetallic particles and the C₂N. Benefiting from this unique local structure, CoSn/C₂N efficiently activated peroxymonosulfate for sulfachloropyridazine degradation, achieving complete removal within 10 min with an apparent rate constant of 0.25 min^-1, higher than those of Co/C₂N and Sn/C₂N. The CoSn/C₂N/PMS system maintained high activity over a broad pH range, resisted interference from common ions and humic acid, and retained complete removal after five cycles. Quenching experiments and EPR spectra identified singlet oxygen as the dominant reactive species, indicating a ¹O₂-mediated nonradical oxidation pathway. This work provides a feasible strategy for designing C₂N-confined bimetallic catalysts for antibiotic wastewater treatment.