diff --git a/src/agents/mcp/util.py b/src/agents/mcp/util.py
index bbfe1885c..5a963bc01 100644
--- a/src/agents/mcp/util.py
+++ b/src/agents/mcp/util.py
@@ -116,7 +116,7 @@ async def invoke_mcp_tool(
         if len(result.content) == 1:
             tool_output = result.content[0].model_dump_json()
         elif len(result.content) > 1:
-            tool_output = json.dumps([item.model_dump() for item in result.content])
+            tool_output = json.dumps([item.model_dump(mode="json") for item in result.content])
         else:
             logger.error(f"Errored MCP tool result: {result}")
             tool_output = "Error running tool."