• The patients are able to understand and voluntarily sign the written informed consent, which must be signed prior to the implementation of the designated research procedures required by the study.

• The age at the time of signing the informed consent form (ICF) is ≥ 18 years old, both male and female.

• Locally advanced or metastatic gastric / gastroesophageal junction adenocarcinoma (clinical stage ≥ T2N0M0) and pMMR/MSS(tumor biopsy immunohistochemical identified pMMR or next generation sequencing identified MSS) locally advanced rectal adenocarcinoma (clinical stage T3-4N0M0 or T1-4N+M0 ) were diagnosed by comprehensive evaluation.

• The patients are willing to provide fresh tissue for biomarker analysis, and the tissue samples provided are of sufficient quality to evaluate the status of biomarkers. If sufficient tissue is not provided, repeated sampling may be required.

• The patient has an Eastern Cooperative Oncology Group performance status (ECOG PS) score of 0 or 1.

• The expected survival time was ≥ 3 months.

• The patient has adequate organs function

‣ The patient has adequate hematologic function, as evidenced by an absolute neutrophil count (ANC) ≥1.5\*10\^9/L, hemoglobin ≥90g/L (5.58 mmol/L), and platelets ≥100\*10\^9/L.

⁃ The patient has adequate renal function as defined by a serum creatinine ≤1.5 times the ULN, or creatinine clearance (measured via 24-hour urine collection) ≥50 mL/minute (that is, if serum creatinine is \>1.5 times the ULN, a 24-hour urine collection to calculate creatinine clearance must be performed).

⁃ The patient has adequate hepatic function as defined by a total bilirubin ≤1.5 mg/dL (25.65 μmol/L), and aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 2.5 times the upper limit of normal (ULN; or 5.0 times the ULN in the setting of liver metastases).

⁃ The patient must have adequate coagulation function as defined by international normalized ratio (INR) ≤1.5

• Within 7 days before the first administration, women of childbearing age must confirm that the serum pregnancy test is negative and agree to use effective contraceptives during the study period and within 180 days after the last administration. In this program, women of childbearing age are defined as sexually mature women:

‣ No hysterectomy or bilateral ovariectomy

⁃ Natural menopause does not last for 24 months (amenorrhea after cancer treatment does not rule out fertility) (that is, menstruation occurs at any time in the previous 24 months).

∙ For male patients whose sexual partners are women of childbearing age, they must agree to use effective contraception during the study drug use and within 180 days after the last administration.