• Female patients aged 18-70 years;

• Eastern Cooperative Oncology Group performance status of 0 or 1;

• Clinical stage at initial diagnosis (per the 8th edition of the American Joint Committee on Cancer Staging Manual): cT1-4/N1-3/M0 (cT1mi/N0 or cT1a-b/N0 are eligible), meeting the criteria for neoadjuvant therapy as per NCCN 2022 guidelines;

• Histologically confirmed hormone receptor-positive (estrogen receptor ≥1% and/or progesterone receptor ≥1%) and HER2-positive (immunohistochemistry 3+ or fluorescence in situ hybridization-positive) invasive breast cancer;

• Completion of ≥4 cycles of neoadjuvant therapy with trastuzumab and pertuzumab, without recurrence or metastatic disease prior to adjuvant treatment; residual cancer burden class 0 or I after neoadjuvant therapy; time from initial surgery to randomization ≤12 weeks;

• Adequate organ function within 2 weeks prior to screening (without transfusion or use of growth factors):

‣ Absolute neutrophil count ≥1.5 × 10⁹/L;

⁃ Platelet count ≥90 × 10⁹/L;

⁃ Hemoglobin ≥90 g/L;

⁃ Total bilirubin ≤1.5 × upper limit of normal (ULN);

⁃ Alanine aminotransferase and aspartate aminotransferase ≤1.5 × ULN;

• Post-neoadjuvant therapy echocardiography showing left ventricular ejection fraction (LVEF) ≥50% during screening, with an absolute decrease of ≤15% compared to pre-chemotherapy values; if no pre-chemotherapy LVEF assessment is available, LVEF must be ≥55% during screening;

• Life expectancy ≥6 months;

• For premenopausal or non-sterilized female patients: agreement to abstain from sexual activity or use effective non-hormonal contraception during study treatment and for 8 weeks after the last dose;

⁃ Willingness to participate voluntarily, provide signed informed consent, demonstrate good compliance, and cooperate with follow-up.