Inclusion Criteria:
* 1\. Written informed consent must be signed before any trial-related procedures are performed.
* 2\. Male or female, aged ≥18 years and ≤75 years.
* 3\. Histologically or pathologically confirmed rectal adenocarcinoma, with confirmed proficient mismatch repair (pMMR) or microsatellite stable (MSS) status; diagnosed as locally advanced rectal cancer according to AJCC 9th edition, with cTNM staging as cT2-4N1-3M0 based on contrast-enhanced CT/MRI, and the lesion assessed as resectable by the investigator.
* 4\. No prior systemic therapy for the current disease, including surgery, anti-tumor radiotherapy/chemotherapy, immunotherapy, etc.
* 5\. ECOG performance status 0-1.
* 6\. Expected survival \>6 months.
* 7\. Adequate organ function, meeting the following laboratory criteria:
1. Absolute neutrophil count (ANC) ≥1.5×10\^9/L without granulocyte colony-stimulating factor support within the past 14 days;
2. Platelet count ≥100×10\^9/L without blood transfusion within the past 14 days;
3. Hemoglobin \>9 g/dL without blood transfusion or erythropoietin use within the past 14 days;
4. Total bilirubin ≤1.5× upper limit of normal (ULN);
5. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5× ULN;
6. Serum creatinine ≤1.5× ULN and creatinine clearance (calculated using the Cockcroft-Gault formula) ≥60 mL/min;
7. Adequate coagulation function, defined as international normalized ratio (INR) or prothrombin time (PT) ≤1.5× ULN;
8. Normal thyroid function, defined as thyroid-stimulating hormone (TSH) within the normal range. If baseline TSH is outside the normal range, subjects with total T3 (or free T3) and free T4 within the normal range may also be enrolled;
9. Myocardial enzyme levels within the normal range (isolated laboratory abnormalities deemed clinically insignificant by the investigator's comprehensive judgment are also allowed for enrollment).
* 8\. For female patients who are not postmenopausal or surgically sterile: abstinence or use of effective contraceptive methods during the treatment period and for at least 7 months after the last dose of study treatment.
Exclusion Criteria:
* 1.Known squamous cell carcinoma, undifferentiated carcinoma, or other histologic types of colorectal cancer, or adenocarcinoma mixed with other histologic types.
* 2.Known deficient mismatch repair (dMMR) or microsatellite instability-high (MSI-H) status.
* 3.Presence of unresectable factors, including unresectability due to tumor-related reasons, presence of surgical contraindications, or subject refusal to undergo surgery.
* 4.Diagnosis of another malignancy other than colorectal cancer within 5 years prior to the first dose (excluding adequately treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin, and/or radically resected carcinoma in situ).
* 5.Current participation in interventional clinical research treatment, or receipt of another investigational drug or use of an investigational device within 4 weeks prior to the first dose.
* 6.Prior receipt of immunotherapy, including immune checkpoint inhibitors (e.g., anti-PD-1/L1 antibodies, anti-CTLA-4 antibodies, anti-TIGIT antibodies, anti-LAG3 antibodies, etc.), immune checkpoint agonists (e.g., ICOS, CD40, CD137, GITR, OX40 antibodies, etc.), immune cell therapy, or any other treatment directed at tumor immuno-mechanisms.
* 7.Severe cardiac disease or conditions, including but not limited to: a confirmed history of heart failure or systolic dysfunction (LVEF \<50%); uncontrolled high-risk arrhythmias, such as atrial tachycardia, resting heart rate \>100 bpm, significant ventricular arrhythmias (e.g., ventricular tachycardia), or high-grade atrioventricular block (i.e., Mobitz II second-degree AV block or third-degree AV block).
* 8.Poorly controlled hypertension (systolic blood pressure \>180 mmHg and/or diastolic blood pressure \>100 mmHg).
* 9.Inability to swallow, intestinal obstruction, or other factors affecting drug administration and absorption.
* 10.Known hypersensitivity to any component of the study drug regimen; history of immunodeficiency, including positive HIV test, or other acquired or congenital immunodeficiency diseases, or history of organ transplantation.
* 11.Female patients who are pregnant, breastfeeding, or of childbearing potential with a positive baseline pregnancy test, or women of childbearing potential who are unwilling to use effective contraceptive methods throughout the trial and for at least 7 months after the last dose of study treatment.
* 12.Presence of serious concomitant disease or other confounding conditions that would interfere with planned treatment, or any other condition that, in the investigator's opinion, makes the subject unsuitable for study participation.
