• Male or female patient ≥18 years of age at time of informed consent form signature.

• Patient with MSI-H/dMMR, HER2 negativeadvanced or metastatic gastric, gastro-oesophageal junction or oesophageal adenocarcinoma whose tumours express PD-L1 with a combined positive score (CPS) ≥ 5.

• Patient to be treated with a first line therapy for locally advanced/metastatic disease.

• No prior treatment with chemotherapy for locally advanced/metastatic disease.

• o Note - adjuvant or neoadjuvant chemotherapy is allowed providing that 6 months have relapsed between completion of adjuvant chemotherapy and recurrence.

• Measurable disease (outside any previous irradiated field within the past 6 months) defined as at least one unidimensional lesion that can be accurately measured as ≥ 10 mm with CT scan according to RECIST V1.1 (Appendix 01).

‣ Note: Lesions intended to be biopsied should not be defined as target lesions.

⁃ Note: previously irradiated lesions can be selected as target lesion only if recurrence/PD is documented after RT.

• Patient with PS ECOG 0 or 1 (Appendix 02).

• Adequate hematologic and end-organ function, defined by the following laboratory test results:

⁃ Absolute neutrophil count ≥ 1.5 109/L (without growth factor support within 14 d) Platelets ≥ 100 109/L (without transfusion for platelets within 7 d) Hemoglobin ≥ 9 g/dL (without transfusion within 7 d) Creatinine clearance according to CKD-EPI ≥ 30 mL/min/1.73 m2 Serum total bilirubin ≤ 1.5 x ULN (except for patients with Gilbert disease for whom a total serum bilirubin ≤ 3 x ULN is acceptable) ASAT and ALAT ≤ 3 x ULN (or up to 5 x ULN in case of liver metastasis or hepatic infiltration)

• Availability of a representative formalin-fixed paraffin-embedded (FFPE) sample of the primary or metastatic tumor tissue (resection or biopsy) with an associated pathology report must be available. This tumor sample must meet the following quality/quantity control criteria: ≥30 % of tumor cells and a tumor surface area ≥ 5mm2 or biopsiable disease (see next inclusion criteria).

• Tumor lesion visible by medical imaging and accessible to repeatable percutaneous or endoscopic sampling that permits core needle biopsy without unacceptable risk of a significant procedural complications, and suitable for retrieval of a minimum of 4 cores with a needle minimum diameter :16-gauge.

‣ Note 1: Fine needle aspirates, bone biopsies do not satisfy the requirement for tumor tissue.

⁃ Note 2: Tumor lesions used for biopsy should not be lesions used as RECIST 1.1 target lesions unless there are no other lesions suitable for biopsy. If a RECIST target lesion is used for biopsy, the lesion must be ≥ 2 cm in longest diameter.

• Women of childbearing potential (WOCBP) must have a negative serum pregnancy test at the Screening Visit (within 72 hours of first dose of study drugs) and must agree to use highly effective contraceptive measures starting with the Screening Visit through

‣ 9 months after the end of the treatment with oxaliplatin

⁃ 6 months after the end of the treatment with fluorouracil

⁃ 5 months after the end of the treatment with nivolumab or botensilimab or Balstilimab

⁃ 6 months for capecitabine

• Highly effective contraception is defined in Appendix 03.

⁃ Note Non-childbearing potential is defined as:

⁃ ≥ 50 years of age and has not had menses for greater than 1 year.

⁃ Amenorrheic for ≥ 2 years without a hysterectomy and bilateral oophorectomy and a follicle-stimulating hormone value in the postmenopausal range upon pre-study (screening) evaluation.

⁃ Status is post-hysterectomy, bilateral oophorectomy, or tubal ligation.

∙ Male patients with a female partner(s) of childbearing potential must agree to use highly effective contraceptive measures throughout the study starting with the screening visit through 6 months after the end of the treatment with oxaliplatine or 3 months after the last dose for other study treatments is received. Males with pregnant partners must agree to use a condom; no additional method of contraception is required for the pregnant partner.

‣ Patient should understand, sign, and date the written voluntary informed consent form prior to any protocol-specific procedures performed and should be able and willing to comply with study visits and procedures as per protocol.

‣ Patients must be covered by a medical insurance.