Inclusion Criteria:
* Voluntarily provide written informed consent prior to screening.
* Male or female subjects aged ≥18 years.
* At least one measurable lesion per RECIST 1.1 criteria.
* Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.
* Histologically or pathologically confirmed locally advanced rectal adenocarcinoma, cT3/cT4N+M0 stage per AJCC/UICC 8th edition, with distance from anal verge ≤10 cm. Diagnosis is confirmed by contrast-enhanced CT/MRI, supplemented by colonoscopy and diagnostic laparoscopy if necessary; no prior anti-tumor treatment.
* Scheduled for surgical resection following neoadjuvant therapy based on clinical staging.
* Expected survival of more than 3 months.
* No emergency indications such as bowel obstruction, bleeding or perforation.
* Adequate major organ function as follows (no blood transfusion, granulocyte-colony stimulating factor (G-CSF) or other hematopoietic growth factor support within 14 days prior to screening):
1. Hematology: Absolute neutrophil count ≥1.5×10⁹/L, platelet count ≥100×10⁹/L, hemoglobin ≥90 g/L, white blood cell count ≥3.5×10⁹/L.
2. Liver function: ALT and AST ≤2.5×ULN; total bilirubin ≤1.5×ULN (≤3×ULN for patients with Gilbert syndrome).
3. Renal function: Serum creatinine ≤1.5×ULN or creatinine clearance ≥60 mL/min.
4. Coagulation function: APTT, INR, PT ≤1.5×ULN.
Exclusion Criteria:
* Prior anti-tumor therapy including chemotherapy, radiotherapy, hormonal therapy or molecular-targeted therapy.
* Prior treatment with anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, anti-CTLA-4 antibodies, or other agents targeting T-cell co-stimulatory or immune-checkpoint pathways.
* History of other malignant tumors within 5 years or concurrent other malignancies, except for cured carcinoma in situ of cervix, non-melanoma skin cancer, or other malignancies with ≥5 years disease-free survival after curative treatment.
* Peripheral neuropathy ≥Grade 2 per NCI-CTCAE v5.0.
* Known active central nervous system metastases and/or carcinomatous meningitis.
* History of severe hypersensitivity reaction (NCI-CTCAE v5.0 ≥Grade 3) to anti-PD-1 monoclonal antibodies, other monoclonal antibodies, oxaliplatin, S-1 or related compounds.
* History of hereditary bleeding disorders or coagulopathy with high bleeding risk.
* Major surgical procedure within 4 weeks prior to screening.
* Not recovered from prior surgical complications with residual toxicity \>Grade 1 (NCI-CTCAE v5.0), excluding alopecia and fatigue.
* Requirement for immunosuppressive medication within 2 weeks before screening or during study treatment, except for:
1. Intranasal, inhaled, topical or intra-articular corticosteroids.
2. Physiological-dose systemic corticosteroids (≤10 mg/day prednisone or equivalent).
3. Short-term (≤7 days) corticosteroids for prophylaxis or treatment of non-autoimmune allergic conditions.
* Active or history of recurrent autoimmune disease.
* History of interstitial lung disease or non-infectious pneumonitis.
* Known active tuberculosis (Mycobacterium tuberculosis) infection.
* Human immunodeficiency virus (HIV) positive, other acquired or congenital immunodeficiency, history of organ transplantation or stem-cell transplantation.
* Hepatitis B or C virology findings at screening meeting any of the following:
1. HBsAg-positive with HBV-DNA ≥10⁴ copies/mL or ≥2000 IU/mL (antiviral therapy may be administered for HBV carriers at investigator's discretion).
2. Active hepatitis C: HCV-antibody-positive with detectable HCV-RNA above assay lower limit.
* Active or uncontrolled infection requiring systemic therapy within 2 weeks prior to screening.
* Receipt of live-virus vaccine within 4 weeks prior to screening.
* Bowel obstruction, or history of inflammatory bowel disease, extensive bowel resection with chronic diarrhea, Crohn's disease, ulcerative colitis or chronic diarrhea.
* Lactating females, or females planning pregnancy during study treatment or within 6 months after treatment completion.
* Subjects (fertile males, females and their male partners) unwilling to use effective contraception during study treatment and for 6 months after treatment completion.
* Any other condition that, in the investigator's opinion, would compromise study compliance or outcome assessment, making the subject unsuitable for study participation.
