diff --git a/api/core/tools/workflow_as_tool/tool.py b/api/core/tools/workflow_as_tool/tool.py
index dfcdcffca8..58686b08ce 100644
--- a/api/core/tools/workflow_as_tool/tool.py
+++ b/api/core/tools/workflow_as_tool/tool.py
@@ -363,11 +363,24 @@ class WorkflowTool(Tool):
                             files.append(file_dict)
                     except Exception:
                         logger.exception("Failed to transform file %s", file)
+            elif parameter.type == ToolParameter.ToolParameterType.FILES:
+                value = tool_parameters.get(parameter.name)
+                if not parameter.required and self._is_empty_files_parameter_value(value):
+                    value = []
+                parameters_result[parameter.name] = value
             else:
                 parameters_result[parameter.name] = tool_parameters.get(parameter.name)
 
         return parameters_result, files
 
+    @staticmethod
+    def _is_empty_files_parameter_value(value: Any) -> bool:
+        """Identify empty optional file-list placeholders before workflow input validation."""
+
+        if value is None or value == "":
+            return True
+        return isinstance(value, list) and all(item is None or item == "" for item in value)
+
     def _extract_files(self, outputs: dict[str, Any]) -> tuple[dict[str, Any], list[File]]:
         """
         extract files from the result
diff --git a/api/tests/unit_tests/core/tools/workflow_as_tool/test_tool.py b/api/tests/unit_tests/core/tools/workflow_as_tool/test_tool.py
index b35df9239c..c9e8a1aaf1 100644
--- a/api/tests/unit_tests/core/tools/workflow_as_tool/test_tool.py
+++ b/api/tests/unit_tests/core/tools/workflow_as_tool/test_tool.py
@@ -696,6 +696,53 @@ def test_transform_args_invalid_files(monkeypatch: pytest.MonkeyPatch):
     assert invalid_files == []
 
 
+@pytest.mark.parametrize("empty_value", [None, "", [], [None], [""]])
+def test_transform_args_normalizes_optional_files_parameter(
+    monkeypatch: pytest.MonkeyPatch,
+    empty_value: Any,
+):
+    """Pass optional workflow file-list inputs as an empty list when no files were provided."""
+    tool = _build_tool()
+    images_param = ToolParameter.get_simple_instance(
+        name="images",
+        llm_description="images",
+        typ=ToolParameter.ToolParameterType.FILES,
+        required=False,
+    )
+    images_param.form = ToolParameter.ToolParameterForm.FORM
+    monkeypatch.setattr(tool, "get_merged_runtime_parameters", lambda: [images_param])
+
+    params, files = tool._transform_args({"images": empty_value})
+
+    assert params == {"images": []}
+    assert files == []
+
+
+def test_workflow_tool_invocation_normalizes_optional_files_parameter(monkeypatch: pytest.MonkeyPatch):
+    """Ensure casted empty FILES values do not reach workflow input validation as [None]."""
+    tool = _build_tool()
+    images_param = ToolParameter.get_simple_instance(
+        name="images",
+        llm_description="images",
+        typ=ToolParameter.ToolParameterType.FILES,
+        required=False,
+    )
+    images_param.form = ToolParameter.ToolParameterForm.FORM
+    tool.entity.parameters = [images_param]
+
+    monkeypatch.setattr(tool, "_get_app", lambda *args, **kwargs: None)
+    monkeypatch.setattr(tool, "_get_workflow", lambda *args, **kwargs: None)
+    monkeypatch.setattr(tool, "_resolve_user", lambda *args, **kwargs: Mock())
+
+    generate_mock = MagicMock(return_value={"data": {}})
+    monkeypatch.setattr("core.app.apps.workflow.app_generator.WorkflowAppGenerator.generate", generate_mock)
+
+    list(tool.invoke("test_user", {"images": None}))
+
+    call_kwargs = generate_mock.call_args.kwargs
+    assert call_kwargs["args"]["inputs"]["images"] == []
+
+
 def test_extract_files():
     """Extract file outputs into result and file list."""
     tool = _build_tool()