fix: Return empty list instead of None from _get_required_fields for schemas without properties

src/google/adk/tools/_function_parameter_parse_util.py

@@ -548,7 +548,7 @@ def _parse_schema_from_parameter(
 
 def _get_required_fields(schema: types.Schema) -> list[str]:
   if not schema.properties:
-    return
+    return []
   return [
       field_name
       for field_name, field_schema in schema.properties.items()

tests/unittests/tools/test_build_function_declaration.py

@@ -17,6 +17,7 @@
 from google.adk.features import FeatureName
 from google.adk.features._feature_registry import temporary_feature_override
 from google.adk.tools import _automatic_function_calling_util
+from google.adk.tools import _function_parameter_parse_util
 from google.adk.tools.tool_context import ToolContext
 from google.adk.utils.variant_utils import GoogleLLMVariant
 from google.genai import types
@@ -198,6 +199,46 @@ def simple_function(input_str: str, tool_context: ToolContext) -> str:
     assert function_decl.parameters.properties['input_str'].type == 'STRING'
     assert 'tool_context' not in function_decl.parameters.properties
 
+  def test_get_required_fields_no_properties_returns_empty_list(self):
+    # Regression for https://github.com/google/adk-python/issues/5920: the
+    # declared return type is list[str], so a schema without properties must
+    # yield [] rather than None for any caller that relies on the declared
+    # list[str] return.
+    assert (
+        _function_parameter_parse_util._get_required_fields(
+            types.Schema(type='OBJECT')
+        )
+        == []
+    )
+    assert (
+        _function_parameter_parse_util._get_required_fields(
+            types.Schema(type='OBJECT', properties={})
+        )
+        == []
+    )
+
+  def test_declaration_required_fields_is_list_not_none(self):
+    def simple_function(input_str: str) -> str:
+      return {'result': input_str}
+
+    function_decl = _automatic_function_calling_util.build_function_declaration(
+        func=simple_function
+    )
+
+    assert function_decl.parameters.required == ['input_str']
+
+  def test_toolcontext_only_builds_without_none_required(self):
+    def only_context(tool_context: ToolContext) -> str:
+      return ''
+
+    function_decl = _automatic_function_calling_util.build_function_declaration(
+        func=only_context, ignore_params=['tool_context']
+    )
+
+    # No parameters remain after ignoring tool_context, so the declaration
+    # carries no parameter schema instead of one with required=None.
+    assert function_decl.parameters is None
+
   def test_basemodel(self):
     class SimpleFunction(BaseModel):
       input_str: str