fix: Set response schema for function that returns None

src/google/adk/tools/_automatic_function_calling_util.py

@@ -20,7 +20,6 @@
 from typing import Any
 from typing import Callable
 from typing import Dict
-from typing import Literal
 from typing import Optional
 from typing import Union
 
@@ -329,7 +328,26 @@ def from_function_with_options(
     return declaration
 
   return_annotation = inspect.signature(func).return_annotation
-  if return_annotation is inspect._empty:
+
+  # Handle functions with no return annotation or that return None
+  if (
+      return_annotation is inspect._empty
+      or return_annotation is None
+      or return_annotation is type(None)
+  ):
+    # Create a response schema for None/null return
+    return_value = inspect.Parameter(
+        'return_value',
+        inspect.Parameter.POSITIONAL_OR_KEYWORD,
+        annotation=None,
+    )
+    declaration.response = (
+        _function_parameter_parse_util._parse_schema_from_parameter(
+            variant,
+            return_value,
+            func.__name__,
+        )
+    )
     return declaration
 
   return_value = inspect.Parameter(

tests/unittests/tools/test_build_function_declaration.py

@@ -17,6 +17,8 @@
 
 from google.adk.tools import _automatic_function_calling_util
 from google.adk.tools.agent_tool import ToolContext
+from google.adk.utils.variant_utils import GoogleLLMVariant
+from google.genai import types
 # TODO: crewai requires python 3.10 as minimum
 # from crewai_tools import FileReadTool
 from pydantic import BaseModel
@@ -262,3 +264,118 @@ def simple_function(input_str: List[CustomInput]) -> str:
 #   assert function_decl.name == 'directory_read_tool'
 #   assert function_decl.parameters.type == 'OBJECT'
 #   assert function_decl.parameters.properties['file_path'].type == 'STRING'
+
+
+def test_function_no_return_annotation_gemini_api():
+  """Test function with no return annotation using GEMINI_API variant."""
+
+  def function_no_return(param: str):
+    """A function with no return annotation."""
+    return None
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=function_no_return, variant=GoogleLLMVariant.GEMINI_API
+  )
+
+  assert function_decl.name == 'function_no_return'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['param'].type == 'STRING'
+  # GEMINI_API should not have response schema
+  assert function_decl.response is None
+
+
+def test_function_no_return_annotation_vertex_ai():
+  """Test function with no return annotation using VERTEX_AI variant."""
+
+  def function_no_return(param: str):
+    """A function with no return annotation."""
+    return None
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=function_no_return, variant=GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert function_decl.name == 'function_no_return'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for None return
+  assert function_decl.response is not None
+  assert function_decl.response.type == types.Type.NULL
+
+
+def test_function_explicit_none_return_vertex_ai():
+  """Test function with explicit None return annotation using VERTEX_AI variant."""
+
+  def function_none_return(param: str) -> None:
+    """A function that explicitly returns None."""
+    pass
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=function_none_return, variant=GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert function_decl.name == 'function_none_return'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for explicit None return
+  assert function_decl.response is not None
+  assert function_decl.response.type == types.Type.NULL
+
+
+def test_function_explicit_none_return_gemini_api():
+  """Test function with explicit None return annotation using GEMINI_API variant."""
+
+  def function_none_return(param: str) -> None:
+    """A function that explicitly returns None."""
+    pass
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=function_none_return, variant=GoogleLLMVariant.GEMINI_API
+  )
+
+  assert function_decl.name == 'function_none_return'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['param'].type == 'STRING'
+  # GEMINI_API should not have response schema
+  assert function_decl.response is None
+
+
+def test_function_regular_return_type_vertex_ai():
+  """Test function with regular return type using VERTEX_AI variant."""
+
+  def function_string_return(param: str) -> str:
+    """A function that returns a string."""
+    return param
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=function_string_return, variant=GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert function_decl.name == 'function_string_return'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for string return
+  assert function_decl.response is not None
+  assert function_decl.response.type == types.Type.STRING
+
+
+def test_transfer_to_agent_like_function():
+  """Test a function similar to transfer_to_agent that caused the original issue."""
+
+  def transfer_to_agent(agent_name: str, tool_context: ToolContext):
+    """Transfer the question to another agent."""
+    tool_context.actions.transfer_to_agent = agent_name
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=transfer_to_agent,
+      ignore_params=['tool_context'],
+      variant=GoogleLLMVariant.VERTEX_AI,
+  )
+
+  assert function_decl.name == 'transfer_to_agent'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['agent_name'].type == 'STRING'
+  assert 'tool_context' not in function_decl.parameters.properties
+  # This should now have a response schema for VERTEX_AI variant
+  assert function_decl.response is not None
+  assert function_decl.response.type == types.Type.NULL

tests/unittests/tools/test_from_function_with_options.py

@@ -0,0 +1,172 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Dict
+
+from google.adk.tools import _automatic_function_calling_util
+from google.adk.utils.variant_utils import GoogleLLMVariant
+from google.genai import types
+
+
+def test_from_function_with_options_no_return_annotation_gemini():
+  """Test from_function_with_options with no return annotation for GEMINI_API."""
+
+  def test_function(param: str):
+    """A test function with no return annotation."""
+    return None
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.GEMINI_API
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # GEMINI_API should not have response schema
+  assert declaration.response is None
+
+
+def test_from_function_with_options_no_return_annotation_vertex():
+  """Test from_function_with_options with no return annotation for VERTEX_AI."""
+
+  def test_function(param: str):
+    """A test function with no return annotation."""
+    return None
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for None return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.NULL
+
+
+def test_from_function_with_options_explicit_none_return_vertex():
+  """Test from_function_with_options with explicit None return for VERTEX_AI."""
+
+  def test_function(param: str) -> None:
+    """A test function that explicitly returns None."""
+    pass
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for explicit None return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.NULL
+
+
+def test_from_function_with_options_explicit_none_return_gemini():
+  """Test from_function_with_options with explicit None return for GEMINI_API."""
+
+  def test_function(param: str) -> None:
+    """A test function that explicitly returns None."""
+    pass
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.GEMINI_API
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # GEMINI_API should not have response schema
+  assert declaration.response is None
+
+
+def test_from_function_with_options_string_return_vertex():
+  """Test from_function_with_options with string return for VERTEX_AI."""
+
+  def test_function(param: str) -> str:
+    """A test function that returns a string."""
+    return param
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for string return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.STRING
+
+
+def test_from_function_with_options_dict_return_vertex():
+  """Test from_function_with_options with dict return for VERTEX_AI."""
+
+  def test_function(param: str) -> Dict[str, str]:
+    """A test function that returns a dict."""
+    return {'result': param}
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for dict return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.OBJECT
+
+
+def test_from_function_with_options_int_return_vertex():
+  """Test from_function_with_options with int return for VERTEX_AI."""
+
+  def test_function(param: str) -> int:
+    """A test function that returns an int."""
+    return 42
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  assert declaration.parameters.properties['param'].type == 'STRING'
+  # VERTEX_AI should have response schema for int return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.INTEGER
+
+
+def test_from_function_with_options_no_params():
+  """Test from_function_with_options with no parameters."""
+
+  def test_function() -> None:
+    """A test function with no parameters that returns None."""
+    pass
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  # No parameters should result in no parameters field or empty parameters
+  assert (
+      declaration.parameters is None
+      or len(declaration.parameters.properties) == 0
+  )
+  # VERTEX_AI should have response schema for None return
+  assert declaration.response is not None
+  assert declaration.response.type == types.Type.NULL