Interfacial　interactions　among　thiourea　(TU),　gelatin　(GEL),　and　Cl−　critically　influence　copper　electrodeposition.　Using　electrochemical　methods　and　shell-isolated　nanoparticle-enhanced　Raman　spectroscopy　(SHINERS),　we　reveal　their　competitive　adsorption　effects.　Linear　sweep　voltammetry　(LSV)　showed　that　low　TU　concentrations　(≤4　ppm)　enhance　Cu2+　reduction,　with　a　distinct　[Cu(TU)n]+　reduction　peak　emerging　at　6　ppm.　However,　at　concentrations　≥30　ppm,　strong　chemisorption　led　to　a　pronounced　negative　resistance　effect　without　altering　the　nucleation　mechanism.　In　the　presence　of　6　ppm　TU,　Cl−　exhibited　a　concentration-dependent　dual　effect.　At　20–60　ppm,　it　enhanced　Cu2+　reduction　while　suppressing　[Cu(TU)n]+　reduction,　reducing　the　Rct　from　7.74　Ω　to　6.90　Ω.　GEL　acted　primarily　as　a　suppressor;　at　concentrations　≥30　ppm,　it　formed　a　dense　adsorption　film,　increasing　Rsf　from　2.18　Ω　to　5.35　Ω.　Shell-isolated　nanoparticles　(55　nm　Au　core,　∼2　nm　SiO2　shell)　enabled　SHINERS-based　in　situ　detection　of　additive　adsorption.　Cl−　displaced　SO42−　and　interfered　with　TU　adsorption,　while　high　GEL　levels　suppressed　TU　adsorption　and　attenuated　SERS　intensity.　This　work　uniquely　reveals　the　dual　role　of　Cl−　in　both　promoting　Cu2+　reduction　and　suppressing　[Cu(TU)n]+　adsorption,　a　behavior　not　previously　reported.　These　findings　provide　insights　into　the　rational　design　of　advanced　copper　electrodeposition　formulations.　©　2025