diff --git a/src/google/adk/tools/_automatic_function_calling_util.py b/src/google/adk/tools/_automatic_function_calling_util.py
index 392e256b33..aef4424a49 100644
--- a/src/google/adk/tools/_automatic_function_calling_util.py
+++ b/src/google/adk/tools/_automatic_function_calling_util.py
@@ -368,6 +368,11 @@ def from_function_with_options(
           parameters_json_schema[name] = types.Schema.model_validate(
               json_schema_dict
           )
+          if param.default is not inspect.Parameter.empty:
+            if param.default is not None:
+              parameters_json_schema[name].default = param.default
+            else:
+              parameters_json_schema[name].nullable = True
         except Exception as e:
           _function_parameter_parse_util._raise_for_unsupported_param(
               param, func.__name__, e
@@ -392,6 +397,11 @@ def from_function_with_options(
         type='OBJECT',
         properties=parameters_json_schema,
     )
+    declaration.parameters.required = (
+        _function_parameter_parse_util._get_required_fields(
+            declaration.parameters
+        )
+    )
 
   if variant == GoogleLLMVariant.GEMINI_API:
     return declaration
diff --git a/tests/unittests/tools/test_from_function_with_options.py b/tests/unittests/tools/test_from_function_with_options.py
index a3f68ee11c..2146dec7db 100644
--- a/tests/unittests/tools/test_from_function_with_options.py
+++ b/tests/unittests/tools/test_from_function_with_options.py
@@ -319,3 +319,36 @@ async def test_function(param: str) -> AsyncGenerator[Dict[str, str], None]:
   # VERTEX_AI should extract yield type (Dict[str, str]) from AsyncGenerator
   assert declaration.response is not None
   assert declaration.response.type == types.Type.OBJECT
+
+
+def test_required_fields_set_in_json_schema_fallback():
+  """Test that required fields are populated when the json_schema fallback path is used.
+
+  When a parameter has a complex union type (e.g. list[str] | None) that
+  _parse_schema_from_parameter can't handle, from_function_with_options falls
+  back to the parameters_json_schema branch. This test verifies that the
+  required fields are correctly populated in that fallback branch.
+
+  Regression test for https://github.com/google/adk-python/issues/4798
+  """
+
+  def complex_tool(
+      query: str,
+      mode: str = "default",
+      tags: list[str] | None = None,
+  ) -> str:
+    """A tool where one param has a complex union type."""
+    return query
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      complex_tool, GoogleLLMVariant.GEMINI_API
+  )
+
+  assert declaration.name == 'complex_tool'
+  assert declaration.parameters.type == 'OBJECT'
+  # 'query' has no default → must be required
+  assert declaration.parameters.required is not None
+  assert 'query' in declaration.parameters.required
+  # 'mode' and 'tags' have defaults → must NOT be required
+  assert 'mode' not in declaration.parameters.required
+  assert 'tags' not in declaration.parameters.required