This study is a prospective, randomized, two-cohort trial designed to evaluate the efficacy and safety of two regimens in the treatment of gastric or gastroesophageal junction adenocarcinoma with peritoneal metastasis: SHR-1701 combined with intravenous and intraperitoneal paclitaxel plus S-1, or SHR-1701 combined with CAPOX.

Inclusion Criteria:

1. Age ≥ 18 years old.
2. Pathologically confirmed gastric or gastroesophageal junction adenocarcinoma (Siewert typeⅡ and type Ⅲ gastroesophageal junction adenocarcinoma are eligible for inclusion).
3. Definitively diagnosed with peritoneal metastasis by exploratory laparoscopy and/peritoneal lavage cytology (CY0P1/CY1P0/CY1P1 are eligible, ovarian metastasis is permitted), and without gastric outlet obstruction or intestinal obstruction.
4. No prior anti-tumor treatment history (including radiotherapy, chemotherapy, targeted therapy, immunotherapy, etc.).
5. Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1.
6. Normal function of major organs.

Exclusion Criteria:

1. Known HER2-positive.
2. Previous history of gastric cancer surgery.
3. Contraindications to surgical treatment or chemotherapy, or poor physical status and organ function precluding major abdominal surgery.
4. Presence of distant metastases other than peritoneal metastases (e.g., liver, lung, ovarian, para-aortic lymph node, brain metastases, etc.).
5. Active autoimmune disease within 4 weeks prior to enrollment or a history of autoimmune disease.
6. Significant cardiovascular disease within 6 months prior to enrollment, including unstable angina or myocardial infarction.