The CP-violating weak phase φs and the decay width difference ∆Γs between the light and heavy B 0 s mass eigenstates are measured with the CMS detector at the LHC in a sample of 48500 reconstructed B 0 s → J/ψφ → µ +µ−K +K− events. The measurement is based on a data sample corresponding to an integrated luminosity of 96.4 fb−1 , collected in proton-proton collisions at √ s = 13 TeV in 20172018. To extract the values of φs and ∆Γs , a time-dependent and flavor-tagged angular analysis of the µ +µ−K +K− final state is performed. The analysis employs a dedicated tagging trigger and a novel opposite-side muon flavor tagger based on machine learning techniques. The measurement yields φs = −11 ± 50 (stat) ± 10 (syst) mrad and ∆Γs = 0.114 ± 0.014 (stat) ± 0.007 (syst) ps−1 , in agreement with the standard model predictions. When combined with the previous CMS measurement at √ s = 8 TeV, the following values are obtained: φs = −21 ± 45 (stat) mrad, ∆Γs = 0.1073 ± 0.0097 (stat) ps−1 , a significant improvement over the 8 TeV result.