**IBI343 Study** aims to test how safe and effective a new drug, IBI343, is when used with other treatments for serious tumors. Tumors are clumps of abnormal cells that can grow and spread in the body. This study is for adults 18 years or older with a specific type of stomach or pancreatic cancer that tests positive for a marker called CLDN18.2. This marker helps doctors choose the right patients for the trial.

**Eligibility:** Participants must sign a consent form and be able to attend all visits. They need to have a good performance score, meaning they can do daily activities with little help. Women who can have babies and men whose partners can have babies must use birth control during and after the study.

• Participants will not receive other cancer treatments during the study.
• Patients must have had no major surgery or live vaccines just before starting the study.
• The study requires a commitment to visits and procedures outlined by the researchers.