The CP -violating weak phase <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>s</mi></mrow></msub></math> and the decay width difference <math><mi>Δ</mi><msub><mrow><mi>Γ</mi></mrow><mrow><mi>s</mi></mrow></msub></math> between the light and heavy <math><msubsup><mrow><mi>B</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>0</mn></mrow></msubsup></math> mass eigenstates are measured with the CMS detector at the LHC in a sample of <math><mn>48</mn><mspace width="0.2em"></mspace><mn>500</mn></math>reconstructed <math><msubsup><mrow><mi>B</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>0</mn></mrow></msubsup><mo>→</mo><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow><mspace width="0.2em"></mspace><mi>ϕ</mi><mo>(</mo><mn>1020</mn><mo>)</mo><mo>→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><mspace width="0.2em"></mspace><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>K</mi></mrow><mrow><mo>−</mo></mrow></msup></math> events. The measurement is based on a data sample corresponding to an integrated luminosity of <math><mn>96.4</mn><mrow><mspace width="0.2em"></mspace><msup><mrow><mtext>fb</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>, collected in proton-proton collisions at <math><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13</mn><mspace width="0.2em"></mspace><mtext>TeV</mtext></math> in 2017–2018. To extract the values of <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>s</mi></mrow></msub></math> and <math><mi>Δ</mi><msub><mrow><mi>Γ</mi></mrow><mrow><mi>s</mi></mrow></msub></math>, a time-dependent and flavor-tagged angular analysis of the <math><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>K</mi></mrow><mrow><mo>−</mo></mrow></msup></math> 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 <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>=</mo><mo>−</mo><mn>11</mn><mo>±</mo><mn>50</mn><mspace width="0.2em"></mspace><mtext>(stat)</mtext><mo>±</mo><mn>10</mn><mspace width="0.2em"></mspace><mtext>(syst)</mtext><mrow><mspace width="0.20em"></mspace><mtext>mrad</mtext></mrow></math> and <math><mi>Δ</mi><msub><mrow><mi>Γ</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>=</mo><mn>0.114</mn><mo>±</mo><mn>0.014</mn><mspace width="0.2em"></mspace><mtext>(stat)</mtext><mo>±</mo><mn>0.007</mn><mspace width="0.2em"></mspace><mtext>(syst)</mtext><msup><mrow><mspace width="0.20em"></mspace><mtext>ps</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>, in agreement with the standard model predictions. When combined with the previous CMS measurement at <math><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>8</mn><mspace width="0.2em"></mspace><mtext>TeV</mtext></math>, the following values are obtained: <math><msub><mrow><mi>ϕ</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>=</mo><mo>−</mo><mn>21</mn><mo>±</mo><mn>44</mn><mspace width="0.2em"></mspace><mtext>(stat)</mtext><mo>±</mo><mn>10</mn><mspace width="0.2em"></mspace><mtext>(syst)</mtext><mrow><mspace width="0.20em"></mspace><mtext>mrad</mtext></mrow></math>, <math><mi>Δ</mi><msub><mrow><mi>Γ</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>=</mo><mn>0.1032</mn><mo>±</mo><mn>0.0095</mn><mspace width="0.2em"></mspace><mtext>(stat)</mtext><mo>±</mo><mn>0.0048</mn><mspace width="0.2em"></mspace><mtext>(syst)</mtext><msup><mrow><mspace width="0.20em"></mspace><mtext>ps</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>, a significant improvement over the 8 TeV result.