This is a proof of principle clinical trial determining efficacy of single dose dualimmune checkpoint inhibitors to increase intra-tumoral T cells in esophageal, gastroesophageal junction, and gastric adenocarcinomas. These are subjects who have not previously been treated for their disease, who are willing to undergo biopsy procedures, who's disease has not spread to other parts of the body, who's tumors have ARID1A mutations.

Inclusion Criteria:

• Non metastatic GEC including locally advanced unresectable
• Treatment naïve
• Histologically proven adenocarcinoma of the esophagus or the stomach with ARID1a mutation either by liquid biopsy (ctDNA) or tissue NGS/WES
• MSI-Stable or pMMR
• Age ≥ 18 years
• Body weight > 66 pounds
• ECOG ≤ 2
• Repeat biopsy feasible
• No clinically significant autoimmune disease

Exclusion Criteria:

• Patients with known metastatic disease
• Prior systemic treatment for esophagus, GEJ, or the stomach adenocarcinoma
• Patients with uncontrolled autoimmune disease per investigator discretion
• Inability or refusal to undergo biopsy procedures to obtain tissue samples