diff --git a/src/google/adk/models/google_llm.py b/src/google/adk/models/google_llm.py
index c9d021f..9c03b28 100644
--- a/src/google/adk/models/google_llm.py
+++ b/src/google/adk/models/google_llm.py
@@ -29,6 +29,7 @@ from google.genai import types
 from typing_extensions import override
 
 from .. import version
+from ..utils.variant_utils import GoogleLLMVariant
 from .base_llm import BaseLlm
 from .base_llm_connection import BaseLlmConnection
 from .gemini_llm_connection import GeminiLlmConnection
@@ -178,8 +179,12 @@ class Gemini(BaseLlm):
     )
 
   @cached_property
-  def _api_backend(self) -> str:
-    return 'vertex' if self.api_client.vertexai else 'ml_dev'
+  def _api_backend(self) -> GoogleLLMVariant:
+    return (
+        GoogleLLMVariant.VERTEX_AI
+        if self.api_client.vertexai
+        else GoogleLLMVariant.GEMINI_API
+    )
 
   @cached_property
   def _tracking_headers(self) -> dict[str, str]:
@@ -196,7 +201,7 @@ class Gemini(BaseLlm):
 
   @cached_property
   def _live_api_client(self) -> Client:
-    if self._api_backend == 'vertex':
+    if self._api_backend == GoogleLLMVariant.VERTEX_AI:
       # use beta version for vertex api
       api_version = 'v1beta1'
       # use default api version for vertex
@@ -206,7 +211,7 @@ class Gemini(BaseLlm):
           )
       )
     else:
-      # use v1alpha for ml_dev
+      # use v1alpha for using API KEY from Google AI Studio
       api_version = 'v1alpha'
       return Client(
           http_options=types.HttpOptions(
@@ -239,7 +244,7 @@ class Gemini(BaseLlm):
 
   def _preprocess_request(self, llm_request: LlmRequest) -> None:
 
-    if llm_request.config and self._api_backend == 'ml_dev':
+    if llm_request.config and self._api_backend == GoogleLLMVariant.GEMINI_API:
       # Using API key from Google AI Studio to call model doesn't support labels.
       llm_request.config.labels = None
 
diff --git a/src/google/adk/tools/_automatic_function_calling_util.py b/src/google/adk/tools/_automatic_function_calling_util.py
index 6bd117e..97d89cb 100644
--- a/src/google/adk/tools/_automatic_function_calling_util.py
+++ b/src/google/adk/tools/_automatic_function_calling_util.py
@@ -31,6 +31,7 @@ from pydantic import create_model
 from pydantic import fields as pydantic_fields
 
 from . import _function_parameter_parse_util
+from ..utils.variant_utils import GoogleLLMVariant
 
 _py_type_2_schema_type = {
     'str': types.Type.STRING,
@@ -194,7 +195,7 @@ def _get_return_type(func: Callable) -> Any:
 def build_function_declaration(
     func: Union[Callable, BaseModel],
     ignore_params: Optional[list[str]] = None,
-    variant: Literal['GOOGLE_AI', 'VERTEX_AI', 'DEFAULT'] = 'GOOGLE_AI',
+    variant: GoogleLLMVariant = GoogleLLMVariant.GEMINI_API,
 ) -> types.FunctionDeclaration:
   signature = inspect.signature(func)
   should_update_signature = False
@@ -291,16 +292,9 @@ def build_function_declaration_util(
 
 def from_function_with_options(
     func: Callable,
-    variant: Literal['GOOGLE_AI', 'VERTEX_AI', 'DEFAULT'] = 'GOOGLE_AI',
+    variant: GoogleLLMVariant = GoogleLLMVariant.GEMINI_API,
 ) -> 'types.FunctionDeclaration':
 
-  supported_variants = ['GOOGLE_AI', 'VERTEX_AI', 'DEFAULT']
-  if variant not in supported_variants:
-    raise ValueError(
-        f'Unsupported variant: {variant}. Supported variants are:'
-        f' {", ".join(supported_variants)}'
-    )
-
   parameters_properties = {}
   for name, param in inspect.signature(func).parameters.items():
     if param.kind in (
@@ -330,7 +324,7 @@ def from_function_with_options(
             declaration.parameters
         )
     )
-  if not variant == 'VERTEX_AI':
+  if variant == GoogleLLMVariant.GEMINI_API:
     return declaration
 
   return_annotation = inspect.signature(func).return_annotation
diff --git a/src/google/adk/tools/_function_parameter_parse_util.py b/src/google/adk/tools/_function_parameter_parse_util.py
index 964a615..1c5ed8d 100644
--- a/src/google/adk/tools/_function_parameter_parse_util.py
+++ b/src/google/adk/tools/_function_parameter_parse_util.py
@@ -28,6 +28,8 @@ from typing import Union
 from google.genai import types
 import pydantic
 
+from ..utils.variant_utils import GoogleLLMVariant
+
 _py_builtin_type_to_schema_type = {
     str: types.Type.STRING,
     int: types.Type.INTEGER,
@@ -63,8 +65,10 @@ def _update_for_default_if_mldev(schema: types.Schema):
     )
 
 
-def _raise_if_schema_unsupported(variant: str, schema: types.Schema):
-  if not variant == 'VERTEX_AI':
+def _raise_if_schema_unsupported(
+    variant: GoogleLLMVariant, schema: types.Schema
+):
+  if variant == GoogleLLMVariant.GEMINI_API:
     _raise_for_any_of_if_mldev(schema)
     _update_for_default_if_mldev(schema)
 
@@ -116,7 +120,7 @@ def _is_default_value_compatible(
 
 
 def _parse_schema_from_parameter(
-    variant: str, param: inspect.Parameter, func_name: str
+    variant: GoogleLLMVariant, param: inspect.Parameter, func_name: str
 ) -> types.Schema:
   """parse schema from parameter.
 
diff --git a/src/google/adk/tools/base_tool.py b/src/google/adk/tools/base_tool.py
index 60eea5a..ad698db 100644
--- a/src/google/adk/tools/base_tool.py
+++ b/src/google/adk/tools/base_tool.py
@@ -15,13 +15,14 @@
 from __future__ import annotations
 
 from abc import ABC
-import os
 from typing import Any
 from typing import Optional
 from typing import TYPE_CHECKING
 
 from google.genai import types
 
+from ..utils.variant_utils import get_google_llm_variant
+from ..utils.variant_utils import GoogleLLMVariant
 from .tool_context import ToolContext
 
 if TYPE_CHECKING:
@@ -118,12 +119,8 @@ class BaseTool(ABC):
       )
 
   @property
-  def _api_variant(self) -> str:
-    use_vertexai = os.environ.get('GOOGLE_GENAI_USE_VERTEXAI', '0').lower() in [
-        'true',
-        '1',
-    ]
-    return 'VERTEX_AI' if use_vertexai else 'GOOGLE_AI'
+  def _api_variant(self) -> GoogleLLMVariant:
+    return get_google_llm_variant()
 
 
 def _find_tool_with_function_declarations(
diff --git a/src/google/adk/utils/variant_utils.py b/src/google/adk/utils/variant_utils.py
new file mode 100644
index 0000000..0eef616
--- /dev/null
+++ b/src/google/adk/utils/variant_utils.py
@@ -0,0 +1,51 @@
+# 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.
+
+"""Utilities for Google LLM variants.
+
+This module is for ADK internal use only.
+Please do not rely on the implementation details.
+"""
+
+from __future__ import annotations
+
+from enum import Enum
+import os
+
+_GOOGLE_LLM_VARIANT_VERTEX_AI = 'VERTEX_AI'
+_GOOGLE_LLM_VARIANT_GEMINI_API = 'GEMINI_API'
+
+
+class GoogleLLMVariant(Enum):
+  """
+  The Google LLM variant to use.
+  see https://google.github.io/adk-docs/get-started/quickstart/#set-up-the-model
+  """
+
+  VERTEX_AI = _GOOGLE_LLM_VARIANT_VERTEX_AI
+  """For using credentials from Google Vertex AI"""
+  GEMINI_API = _GOOGLE_LLM_VARIANT_GEMINI_API
+  """For using API Key from Google AI Studio"""
+
+
+def get_google_llm_variant() -> str:
+  return (
+      GoogleLLMVariant.VERTEX_AI
+      if os.environ.get('GOOGLE_GENAI_USE_VERTEXAI', '0').lower()
+      in [
+          'true',
+          '1',
+      ]
+      else GoogleLLMVariant.GEMINI_API
+  )
diff --git a/tests/unittests/tools/test_build_function_declaration.py b/tests/unittests/tools/test_build_function_declaration.py
index 508608c..eb95a6e 100644
--- a/tests/unittests/tools/test_build_function_declaration.py
+++ b/tests/unittests/tools/test_build_function_declaration.py
@@ -17,22 +17,9 @@ from typing import List
 
 from google.adk.tools import _automatic_function_calling_util
 from google.adk.tools.agent_tool import ToolContext
-from google.adk.tools.langchain_tool import LangchainTool
 # TODO: crewai requires python 3.10 as minimum
 # from crewai_tools import FileReadTool
-from langchain_community.tools import ShellTool
 from pydantic import BaseModel
-import pytest
-
-
-def test_unsupported_variant():
-  def simple_function(input_str: str) -> str:
-    return {'result': input_str}
-
-  with pytest.raises(ValueError):
-    _automatic_function_calling_util.build_function_declaration(
-        func=simple_function, variant='Unsupported'
-    )
 
 
 def test_string_input():