fix: Add support for injecting a custom google.genai.Client into Gemini models

This change introduces a new `client` parameter to the `Gemini` model's constructor. When provided, this preconfigured `google.genai.Client` instance is used for all API calls, offering fine-grained control over authentication, project, and location settings

Close #2560

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 874628604

Author: GWeale

src/google/adk/models/google_llm.py

@@ -85,6 +85,23 @@ class Gemini(BaseLlm):
 
   Attributes:
     model: The name of the Gemini model.
+    client: An optional preconfigured ``google.genai.Client`` instance.
+      When provided, ADK uses this client for all API calls instead of
+      creating one internally from environment variables or ADC.  This
+      allows fine-grained control over authentication, project, location,
+      and other client-level settings — and enables running agents that
+      target different Vertex AI regions within the same process.
+
+      Example::
+
+        from google import genai
+        from google.adk.models import Gemini
+
+        client = genai.Client(
+            vertexai=True, project="my-project", location="us-central1"
+        )
+        model = Gemini(model="gemini-2.5-flash", client=client)
+
     use_interactions_api: Whether to use the interactions API for model
       invocation.
   """
@@ -131,6 +148,35 @@ class Gemini(BaseLlm):
   ```
   """
 
+  def __init__(self, *, client: Optional[Client] = None, **kwargs: Any):
+    """Initialises a Gemini model wrapper.
+
+    Args:
+      client: An optional preconfigured ``google.genai.Client``.  When
+        provided, ADK uses this client for **all** Gemini API calls
+        (including the Live API) instead of creating one internally.
+
+        .. note::
+          When a custom client is supplied it is used as-is for Live API
+          connections.  ADK will **not** override the client's
+          ``api_version``; you are responsible for setting the correct
+          version (``v1beta1`` for Vertex AI, ``v1alpha`` for the
+          Gemini developer API) on the client yourself.
+
+        .. warning::
+          ``google.genai.Client`` contains threading primitives that
+          cannot be pickled.  If you are deploying to Agent Engine (or
+          any environment that serialises the model), do **not** pass a
+          custom client — let ADK create one from the environment
+          instead.
+
+      **kwargs: Forwarded to the Pydantic ``BaseLlm`` constructor
+        (``model``, ``base_url``, ``retry_options``, etc.).
+    """
+    super().__init__(**kwargs)
+    # Store after super().__init__ so Pydantic validation runs first.
+    object.__setattr__(self, '_client', client)
+
   @classmethod
   @override
   def supported_models(cls) -> list[str]:
@@ -299,9 +345,16 @@ async def _generate_content_via_interactions(
   def api_client(self) -> Client:
     """Provides the api client.
 
+    If a preconfigured ``client`` was passed to the constructor it is
+    returned directly; otherwise a new ``Client`` is created using the
+    default environment/ADC configuration.
+
     Returns:
       The api client.
     """
+    if self._client is not None:
+      return self._client
+
     from google.genai import Client
 
     return Client(
@@ -334,6 +387,9 @@ def _live_api_version(self) -> str:
 
   @cached_property
   def _live_api_client(self) -> Client:
+    if self._client is not None:
+      return self._client
+
     from google.genai import Client
 
     return Client(

tests/unittests/models/test_google_llm.py

@@ -2140,3 +2140,153 @@ async def __aexit__(self, *args):
       # Verify the final speech_config is still None
       assert config_arg.speech_config is None
       assert isinstance(connection, GeminiLlmConnection)
+
+
+# ---------------------------------------------------------------------------
+# Tests for custom client injection (Issue #2560)
+# ---------------------------------------------------------------------------
+
+
+def test_custom_client_is_used_for_api_client():
+  """When a custom client is provided, api_client returns it directly."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  assert gemini.api_client is custom_client
+
+
+def test_custom_client_is_used_for_live_api_client():
+  """When a custom client is provided, _live_api_client returns it directly."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  assert gemini._live_api_client is custom_client
+
+
+def test_default_api_client_when_no_custom_client():
+  """Without a custom client, api_client creates a default Client."""
+  gemini = Gemini(model="gemini-1.5-flash")
+
+  # api_client should construct a real Client (not None)
+  client = gemini.api_client
+  assert client is not None
+  # Verify it is not a mock — it's a real google.genai.Client
+  from google.genai import Client
+
+  assert isinstance(client, Client)
+
+
+def test_default_live_api_client_when_no_custom_client():
+  """Without a custom client, _live_api_client creates a default Client."""
+  gemini = Gemini(model="gemini-1.5-flash")
+
+  client = gemini._live_api_client
+  assert client is not None
+  from google.genai import Client
+
+  assert isinstance(client, Client)
+
+
+def test_custom_client_api_backend_vertexai():
+  """_api_backend reflects the custom client's vertexai setting."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  custom_client.vertexai = True
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  assert gemini._api_backend == GoogleLLMVariant.VERTEX_AI
+
+
+def test_custom_client_api_backend_gemini_api():
+  """_api_backend reflects non-vertexai custom client."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  custom_client.vertexai = False
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  assert gemini._api_backend == GoogleLLMVariant.GEMINI_API
+
+
+@pytest.mark.asyncio
+async def test_custom_client_used_for_generate_content():
+  """Custom client is used when generate_content_async is called."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  custom_client.vertexai = False
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  generate_content_response = types.GenerateContentResponse(
+      candidates=[
+          types.Candidate(
+              content=Content(
+                  role="model",
+                  parts=[Part.from_text(text="Hello from custom client")],
+              ),
+              finish_reason=types.FinishReason.STOP,
+          )
+      ]
+  )
+
+  async def mock_coro():
+    return generate_content_response
+
+  custom_client.aio.models.generate_content.return_value = mock_coro()
+
+  llm_request = LlmRequest(
+      model="gemini-1.5-flash",
+      contents=[Content(role="user", parts=[Part.from_text(text="Hello")])],
+      config=types.GenerateContentConfig(
+          system_instruction="You are a helpful assistant",
+      ),
+  )
+
+  responses = [
+      resp
+      async for resp in gemini.generate_content_async(llm_request, stream=False)
+  ]
+
+  assert len(responses) == 1
+  assert responses[0].content.parts[0].text == "Hello from custom client"
+  custom_client.aio.models.generate_content.assert_called_once()
+
+
+@pytest.mark.asyncio
+async def test_custom_client_used_for_live_connect():
+  """Custom client is used for live API streaming connections."""
+  from google.genai import Client
+
+  custom_client = mock.MagicMock(spec=Client)
+  custom_client.vertexai = False
+  gemini = Gemini(model="gemini-1.5-flash", client=custom_client)
+
+  mock_live_session = mock.AsyncMock()
+
+  class MockLiveConnect:
+
+    async def __aenter__(self):
+      return mock_live_session
+
+    async def __aexit__(self, *args):
+      pass
+
+  custom_client.aio.live.connect.return_value = MockLiveConnect()
+
+  llm_request = LlmRequest(
+      model="gemini-1.5-flash",
+      contents=[Content(role="user", parts=[Part.from_text(text="Hello")])],
+      config=types.GenerateContentConfig(
+          system_instruction="You are a helpful assistant",
+      ),
+  )
+  llm_request.live_connect_config = types.LiveConnectConfig()
+
+  async with gemini.connect(llm_request) as connection:
+    custom_client.aio.live.connect.assert_called_once()
+    assert isinstance(connection, GeminiLlmConnection)