diff --git a/api/core/tools/tool/tool.py b/api/core/tools/tool/tool.py
index 55f94d7619635b..4094207beb5c9f 100644
--- a/api/core/tools/tool/tool.py
+++ b/api/core/tools/tool/tool.py
@@ -222,6 +222,12 @@ def invoke(self, user_id: str, tool_parameters: Mapping[str, Any]) -> list[ToolI
         if not isinstance(result, list):
             result = [result]
 
+        if not all(isinstance(message, ToolInvokeMessage) for message in result):
+            raise ValueError(
+                f"Invalid return type from {self.__class__.__name__}._invoke method. "
+                "Expected ToolInvokeMessage or list of ToolInvokeMessage."
+            )
+
         return result
 
     def _transform_tool_parameters_type(self, tool_parameters: Mapping[str, Any]) -> dict[str, Any]: