diff --git a/src/google/adk/tools/_automatic_function_calling_util.py b/src/google/adk/tools/_automatic_function_calling_util.py
index aef4424a49..0ad0c90a87 100644
--- a/src/google/adk/tools/_automatic_function_calling_util.py
+++ b/src/google/adk/tools/_automatic_function_calling_util.py
@@ -37,6 +37,7 @@
 from ..features import FeatureName
 from ..features import is_feature_enabled
 from ..utils.variant_utils import GoogleLLMVariant
+from ._gemini_schema_util import _sanitize_schema_formats_for_gemini
 
 _py_type_2_schema_type = {
     'str': types.Type.STRING,
@@ -366,7 +367,7 @@ def from_function_with_options(
           )
 
           parameters_json_schema[name] = types.Schema.model_validate(
-              json_schema_dict
+              _sanitize_schema_formats_for_gemini(json_schema_dict)
           )
           if param.default is not inspect.Parameter.empty:
             if param.default is not None:
diff --git a/tests/unittests/tools/test_from_function_with_options.py b/tests/unittests/tools/test_from_function_with_options.py
index 537094da39..6c20b62724 100644
--- a/tests/unittests/tools/test_from_function_with_options.py
+++ b/tests/unittests/tools/test_from_function_with_options.py
@@ -361,3 +361,29 @@ def complex_tool(
           ),
       },
   )
+
+
+def test_sanitized_in_json_schema_fallback():
+  """Test schema is sanitzed for complex union type."""
+
+  def complex_tool(
+      query: str,
+      mode: str = 'default',
+      tags: dict[str, str] | None = None,
+  ) -> str:
+    return query
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      complex_tool, GoogleLLMVariant.GEMINI_API
+  )
+
+  assert declaration.parameters.properties['tags'] == types.Schema(
+      any_of=[
+          types.Schema(
+              # should not contain `additional_properties={'type': 'string'} from pydantic
+              type=types.Type.OBJECT,
+          ),
+          types.Schema(type=types.Type.NULL),
+      ],
+      nullable=True,
+  )