diff --git a/google/genai/_api_client.py b/google/genai/_api_client.py
index 2dfc0366b..9f33de151 100644
--- a/google/genai/_api_client.py
+++ b/google/genai/_api_client.py
@@ -702,7 +702,21 @@ def __init__(
         )
       self._http_options.api_version = 'v1beta1'
     else:  # Implicit initialization or missing arguments.
-      if not self.api_key:
+      if env_api_key and api_key:
+        # Explicit api_key takes precedence over implicit api_key.
+        logger.info(
+            'The client initializer api_key argument takes '
+            'precedence over the API key from the environment variable.'
+        )
+      if credentials:
+        if env_api_key:
+          logger.info(
+              'The user `credentials` argument will take precedence over the'
+              ' api key from the environment variables.'
+          )
+          self.api_key = None
+
+      if not self.api_key and not credentials:
         raise ValueError(
             'No API key was provided. Please pass a valid API key. Learn how to'
             ' create an API key at'
@@ -1175,20 +1189,21 @@ def _request_once(
       stream: bool = False,
   ) -> HttpResponse:
     data: Optional[Union[str, bytes]] = None
-    # If using proj/location, fetch ADC
-    if self.vertexai and (self.project or self.location):
+
+    uses_vertex_creds = self.vertexai and (self.project or self.location)
+    uses_mldev_creds = not self.vertexai and self._credentials
+    if (uses_vertex_creds or uses_mldev_creds):
       http_request.headers['Authorization'] = f'Bearer {self._access_token()}'
       if self._credentials and self._credentials.quota_project_id:
         http_request.headers['x-goog-user-project'] = (
             self._credentials.quota_project_id
         )
-      data = json.dumps(http_request.data) if http_request.data else None
-    else:
-      if http_request.data:
-        if not isinstance(http_request.data, bytes):
-          data = json.dumps(http_request.data) if http_request.data else None
-        else:
-          data = http_request.data
+
+    if http_request.data:
+      if not isinstance(http_request.data, bytes):
+        data = json.dumps(http_request.data)
+      else:
+        data = http_request.data
 
     if stream:
       httpx_request = self._httpx_client.build_request(
@@ -1241,8 +1256,9 @@ async def _async_request_once(
   ) -> HttpResponse:
     data: Optional[Union[str, bytes]] = None
 
-    # If using proj/location, fetch ADC
-    if self.vertexai and (self.project or self.location):
+    uses_vertex_creds = self.vertexai and (self.project or self.location)
+    uses_mldev_creds = not self.vertexai and self._credentials
+    if (uses_vertex_creds or uses_mldev_creds):
       http_request.headers['Authorization'] = (
           f'Bearer {await self._async_access_token()}'
       )
@@ -1250,13 +1266,12 @@ async def _async_request_once(
         http_request.headers['x-goog-user-project'] = (
             self._credentials.quota_project_id
         )
-      data = json.dumps(http_request.data) if http_request.data else None
-    else:
-      if http_request.data:
-        if not isinstance(http_request.data, bytes):
-          data = json.dumps(http_request.data) if http_request.data else None
-        else:
-          data = http_request.data
+
+    if http_request.data:
+      if not isinstance(http_request.data, bytes):
+        data = json.dumps(http_request.data)
+      else:
+        data = http_request.data
 
     if stream:
       if self._use_aiohttp():
diff --git a/google/genai/_extra_utils.py b/google/genai/_extra_utils.py
index 129c05f7d..19e1bee31 100644
--- a/google/genai/_extra_utils.py
+++ b/google/genai/_extra_utils.py
@@ -16,16 +16,20 @@
 """Extra utils depending on types that are shared between sync and async modules."""
 
 import asyncio
+from collections.abc import Callable, MutableMapping
 import inspect
 import io
 import logging
 import sys
 import typing
-from typing import Any, Callable, Dict, Optional, Union, get_args, get_origin
+from typing import Any, Optional, Union, get_args, get_origin
 import mimetypes
 import os
 import pydantic
 
+import google.auth.transport.requests
+
+
 from . import _common
 from . import _mcp_utils
 from . import _transformers as t
@@ -677,3 +681,18 @@ def prepare_resumable_upload(
         http_options.headers = {}
     http_options.headers['X-Goog-Upload-File-Name'] = os.path.basename(file)
   return http_options, size_bytes, mime_type
+
+
+async def _maybe_update_and_insert_auth_token(
+    headers:MutableMapping[str, str],
+    creds: google.auth.credentials.Credentials) -> None:
+  # Refresh credentials to ensure token is valid
+  if not (creds.token and creds.valid):
+      try:
+          auth_req = google.auth.transport.requests.Request()  # type: ignore[no-untyped-call]
+          await asyncio.to_thread(creds.refresh, auth_req)
+      except Exception as e:
+          raise ConnectionError(f"Failed to refresh credentials") from e
+
+  if not headers.get('Authorization'):
+      headers['Authorization'] = f'Bearer {creds.token}'
diff --git a/google/genai/_interactions/_client.py b/google/genai/_interactions/_client.py
index 64bc99587..d751df1be 100644
--- a/google/genai/_interactions/_client.py
+++ b/google/genai/_interactions/_client.py
@@ -178,7 +178,7 @@ def _validate_headers(self, headers: Headers, custom_headers: Headers) -> None:
     
     @override
     def _prepare_options(self, options: FinalRequestOptions) -> FinalRequestOptions:
-        if not self.client_adapter or not self.client_adapter.is_vertex_ai():
+        if not self.client_adapter:
             return options
 
         headers = options.headers or {}
@@ -400,7 +400,7 @@ def _validate_headers(self, headers: Headers, custom_headers: Headers) -> None:
     
     @override
     async def _prepare_options(self, options: FinalRequestOptions) -> FinalRequestOptions:
-        if not self.client_adapter or not self.client_adapter.is_vertex_ai():
+        if not self.client_adapter:
             return options
 
         headers = options.headers or {}
diff --git a/google/genai/client.py b/google/genai/client.py
index cec5cef4c..2ebf26e47 100644
--- a/google/genai/client.py
+++ b/google/genai/client.py
@@ -174,14 +174,12 @@ def _nextgen_client(self) -> AsyncGeminiNextGenAPIClient:
         # uSDk expects ms, nextgen uses a httpx Timeout -> expects seconds.
         timeout=http_opts.timeout / 1000 if http_opts.timeout else None,
         max_retries=max_retries,
-        client_adapter=AsyncGeminiNextGenAPIClientAdapter(self._api_client)
+        client_adapter=AsyncGeminiNextGenAPIClientAdapter(self._api_client),
     )
 
-    client = self._nextgen_client_instance
-    if self._api_client.vertexai:
-      client._is_vertex = True
-      client._vertex_project = self._api_client.project
-      client._vertex_location = self._api_client.location
+    self._nextgen_client_instance._is_vertex = self._api_client.vertexai or False
+    self._nextgen_client_instance._vertex_project = self._api_client.project
+    self._nextgen_client_instance._vertex_location = self._api_client.location
 
     return self._nextgen_client_instance
 
@@ -525,11 +523,9 @@ def _nextgen_client(self) -> GeminiNextGenAPIClient:
         client_adapter=GeminiNextGenAPIClientAdapter(self._api_client),
     )
 
-    client = self._nextgen_client_instance
-    if self._api_client.vertexai:
-      client._is_vertex = True
-      client._vertex_project = self._api_client.project
-      client._vertex_location = self._api_client.location
+    self._nextgen_client_instance._is_vertex = self._api_client.vertexai or False
+    self._nextgen_client_instance._vertex_project = self._api_client.project
+    self._nextgen_client_instance._vertex_location = self._api_client.location
 
     return self._nextgen_client_instance
 
diff --git a/google/genai/live.py b/google/genai/live.py
index 93953a02f..08a2f3d78 100644
--- a/google/genai/live.py
+++ b/google/genai/live.py
@@ -29,6 +29,7 @@
 import websockets
 
 from . import _api_module
+from . import _extra_utils
 from . import _common
 from . import _live_converters as live_converters
 from . import _mcp_utils
@@ -946,17 +947,103 @@ async def connect(
     base_url = self._api_client._websocket_base_url()
     if isinstance(base_url, bytes):
       base_url = base_url.decode('utf-8')
-    transformed_model = t.t_model(self._api_client, model)  # type: ignore
 
     parameter_model = await _t_live_connect_config(self._api_client, config)
 
-    if self._api_client.api_key and not self._api_client.vertexai:
-      version = self._api_client._http_options.api_version
-      api_key = self._api_client.api_key
-      method = 'BidiGenerateContent'
-      original_headers = self._api_client._http_options.headers
-      headers = original_headers.copy() if original_headers is not None else {}
+    if self._api_client.vertexai:
+      uri, headers, request = await self._prepare_connection_vertex(
+          base_url=base_url, model=model, parameter_model=parameter_model
+      )
+    else:
+      uri, headers, request = await self._prepare_connection_mldev(
+          base_url=base_url, model=model, parameter_model=parameter_model
+      )
+
+    if parameter_model.tools and _mcp_utils.has_mcp_tool_usage(
+        parameter_model.tools
+    ):
+      if headers is None:
+        headers = {}
+      _mcp_utils.set_mcp_usage_header(headers)
+
+    async with ws_connect(
+        uri, additional_headers=headers, **self._api_client._websocket_ssl_ctx
+    ) as ws:
+      await ws.send(request)
+      try:
+        # websockets 14.0+
+        raw_response = await ws.recv(decode=False)
+      except TypeError:
+        raw_response = await ws.recv()  # type: ignore[assignment]
+      except ConnectionClosed as e:
+        if e.rcvd:
+          code = e.rcvd.code
+          reason = e.rcvd.reason
+        else:
+          code = 1006
+          reason = 'Abnormal closure.'
+        errors.APIError.raise_error(code, reason, None)
+      if raw_response:
+        try:
+          response = json.loads(raw_response)
+        except json.decoder.JSONDecodeError as e:
+          raise ValueError(f'Failed to parse response: {raw_response!r}') from e
+      else:
+        response = {}
+
+      if self._api_client.vertexai:
+        response_dict = live_converters._LiveServerMessage_from_vertex(response)
+      else:
+        response_dict = response
+
+      setup_response = types.LiveServerMessage._from_response(
+          response=response_dict, kwargs=parameter_model.model_dump()
+      )
+      if setup_response.setup_complete:
+        session_id = setup_response.setup_complete.session_id
+      else:
+        session_id = None
+      yield AsyncSession(
+          api_client=self._api_client,
+          websocket=ws,
+          session_id=session_id,
+      )
+
+  async def _prepare_connection_mldev(
+      self, *,
+      base_url: str,
+      model: str,
+      parameter_model: types.LiveConnectConfig,
+  ) -> tuple[str, _common.StringDict, str]:
+    """Prepares live connection parameters for the MLDev API.
+
+    Constructs the WebSocket URI, headers, and request body necessary
+    to establish a connection with the MLDev backend.
+
+    Args:
+        base_url: The base URL for the WebSocket connection.
+        model: The name of the model to use.
+        parameter_model: Configuration parameters for the connection.
+
+    Returns:
+        A tuple containing:
+            - uri: The WebSocket connection URI.
+            - headers: A dictionary of headers for the connection.
+            - request: The JSON-serialized request body.
+
+    Raises:
+        ValueError: If an API key is not provided.
+    """
+    transformed_model = t.t_model(self._api_client, model)  # type: ignore
+    version = self._api_client._http_options.api_version
+    method = 'BidiGenerateContent'
+    original_headers = self._api_client._http_options.headers
+    headers = original_headers.copy() if original_headers is not None else {}
+
+    if api_key := self._api_client.api_key:
       if api_key.startswith('auth_tokens/'):
+        method = 'BidiGenerateContentConstrained'
+        headers['Authorization'] = f'Token {api_key}'
         warnings.warn(
             message=(
                 "The SDK's ephemeral token support is experimental, and may"
@@ -964,8 +1051,6 @@ async def connect(
             ),
             category=errors.ExperimentalWarning,
         )
-        method = 'BidiGenerateContentConstrained'
-        headers['Authorization'] = f'Token {api_key}'
         if version != 'v1alpha':
           warnings.warn(
               message=(
@@ -976,47 +1061,68 @@ async def connect(
               ),
               category=errors.ExperimentalWarning,
           )
-      uri = f'{base_url}/ws/google.ai.generativelanguage.{version}.GenerativeService.{method}'
-
-      request_dict = _common.convert_to_dict(
-          live_converters._LiveConnectParameters_to_mldev(
-              api_client=self._api_client,
-              from_object=types.LiveConnectParameters(
-                  model=transformed_model,
-                  config=parameter_model,
-              ).model_dump(exclude_none=True),
-          )
-      )
+    elif creds := self._api_client._credentials:
+      await _extra_utils._maybe_update_and_insert_auth_token(headers, creds)
+    else:
+      # this shouldn't happen.
+      raise ValueError('Genai live connection requires credentials or API key provided.')
+
+    uri = f'{base_url}/ws/google.ai.generativelanguage.{version}.GenerativeService.{method}'
+
+    request_dict = _common.convert_to_dict(
+        live_converters._LiveConnectParameters_to_mldev(
+            api_client=self._api_client,
+            from_object=types.LiveConnectParameters(
+                model=transformed_model,
+                config=parameter_model,
+            ).model_dump(exclude_none=True),
+        )
+    )
+    del request_dict['config']
+    request_dict = _common.encode_unserializable_types(request_dict)
 
-      del request_dict['config']
-      request_dict = _common.encode_unserializable_types(request_dict)
-      setv(request_dict, ['setup', 'model'], transformed_model)
+    setv(request_dict, ['setup', 'model'], transformed_model)
 
-      request = json.dumps(request_dict)
-    elif self._api_client.api_key and self._api_client.vertexai:
-      # Headers already contains api key for express mode.
-      api_key = self._api_client.api_key
-      version = self._api_client._http_options.api_version
-      uri = f'{base_url}/ws/google.cloud.aiplatform.{version}.LlmBidiService/BidiGenerateContent'
-      original_headers = self._api_client._http_options.headers
-      headers = original_headers.copy() if original_headers is not None else {}
-
-      request_dict = _common.convert_to_dict(
-          live_converters._LiveConnectParameters_to_vertex(
-              api_client=self._api_client,
-              from_object=types.LiveConnectParameters(
-                  model=transformed_model,
-                  config=parameter_model,
-              ).model_dump(exclude_none=True),
-          )
-      )
-      del request_dict['config']
-      request_dict = _common.encode_unserializable_types(request_dict)
-      setv(request_dict, ['setup', 'model'], transformed_model)
+    return uri, headers, json.dumps(request_dict)
 
-      request = json.dumps(request_dict)
+
+  async def _prepare_connection_vertex(
+      self, *,
+      base_url: str,
+      model: str,
+      parameter_model: types.LiveConnectConfig,
+  ) -> tuple[str, _common.StringDict, str]:
+    """Prepares live connection parameters for the Vertex AI API.
+
+    Constructs the WebSocket URI, headers, and request body necessary
+    to establish a connection with the Vertex AI backend. Handles
+    authentication using either an API key or default credentials.
+
+    Args:
+        base_url: The base URL for the WebSocket connection.
+        model: The name of the model to use.
+        parameter_model: Configuration parameters for the connection.
+
+    Returns:
+        A tuple containing:
+            - uri: The WebSocket connection URI.
+            - headers: A dictionary of headers for the connection.
+            - request: The JSON-serialized request body.
+
+    Raises:
+        ValueError: If project and location are not provided when
+            default credentials are used.
+    """
+    transformed_model = t.t_model(self._api_client, model)  # type: ignore
+    version = self._api_client._http_options.api_version
+    original_headers = self._api_client._http_options.headers
+    headers = (
+        original_headers.copy() if original_headers is not None else {}
+    )
+    if api_key := self._api_client.api_key:
+      # Headers already contains api key
+      uri = f'{base_url}/ws/google.cloud.aiplatform.{version}.LlmBidiService/BidiGenerateContent'
     else:
-      version = self._api_client._http_options.api_version
       has_sufficient_auth = (
           self._api_client.project and self._api_client.location
       )
@@ -1044,19 +1150,8 @@ async def connect(
           creds = self._api_client._credentials
         # creds.valid is False, and creds.token is None
         # Need to refresh credentials to populate those
-        if not (creds.token and creds.valid):
-          if requests is None:
-            raise ValueError('The requests module is required to refresh google-auth credentials. Please install with `pip install google-auth[requests]`')
-          auth_req = requests.Request()  # type: ignore
-          creds.refresh(auth_req)  # type: ignore[no-untyped-call]
-        bearer_token = creds.token
+        await _extra_utils._maybe_update_and_insert_auth_token(headers, creds)
 
-        original_headers = self._api_client._http_options.headers
-        headers = (
-            original_headers.copy() if original_headers is not None else {}
-        )
-        if not headers.get('Authorization'):
-          headers['Authorization'] = f'Bearer {bearer_token}'
 
       location = self._api_client.location
       project = self._api_client.project
@@ -1064,17 +1159,23 @@ async def connect(
         transformed_model = (
             f'projects/{project}/locations/{location}/' + transformed_model
         )
-      request_dict = _common.convert_to_dict(
-          live_converters._LiveConnectParameters_to_vertex(
-              api_client=self._api_client,
-              from_object=types.LiveConnectParameters(
-                  model=transformed_model,
-                  config=parameter_model,
-              ).model_dump(exclude_none=True),
-          )
-      )
-      del request_dict['config']
-      request_dict = _common.encode_unserializable_types(request_dict)
+
+    request_dict = _common.convert_to_dict(
+        live_converters._LiveConnectParameters_to_vertex(
+            api_client=self._api_client,
+            from_object=types.LiveConnectParameters(
+                model=transformed_model,
+                config=parameter_model,
+            ).model_dump(exclude_none=True),
+        )
+    )
+    del request_dict['config']
+    request_dict = _common.encode_unserializable_types(request_dict)
+
+    if api_key is None:
+      # Refactor note: I'm surprised the two paths are different, you'd have
+      # to test every model to be sure. The goal of this refactor is to not
+      # change any behavior so leaving it as is.
       if (
           getv(
               request_dict, ['setup', 'generationConfig', 'responseModalities']
@@ -1087,57 +1188,7 @@ async def connect(
             ['AUDIO'],
         )
 
-      request = json.dumps(request_dict)
-
-    if parameter_model.tools and _mcp_utils.has_mcp_tool_usage(
-        parameter_model.tools
-    ):
-      if headers is None:
-        headers = {}
-      _mcp_utils.set_mcp_usage_header(headers)
-
-    async with ws_connect(
-        uri, additional_headers=headers, **self._api_client._websocket_ssl_ctx
-    ) as ws:
-      await ws.send(request)
-      try:
-        # websockets 14.0+
-        raw_response = await ws.recv(decode=False)
-      except TypeError:
-        raw_response = await ws.recv()  # type: ignore[assignment]
-      except ConnectionClosed as e:
-        if e.rcvd:
-          code = e.rcvd.code
-          reason = e.rcvd.reason
-        else:
-          code = 1006
-          reason = 'Abnormal closure.'
-        errors.APIError.raise_error(code, reason, None)
-      if raw_response:
-        try:
-          response = json.loads(raw_response)
-        except json.decoder.JSONDecodeError:
-          raise ValueError(f'Failed to parse response: {raw_response!r}')
-      else:
-        response = {}
-
-      if self._api_client.vertexai:
-        response_dict = live_converters._LiveServerMessage_from_vertex(response)
-      else:
-        response_dict = response
-
-      setup_response = types.LiveServerMessage._from_response(
-          response=response_dict, kwargs=parameter_model.model_dump()
-      )
-      if setup_response.setup_complete:
-        session_id = setup_response.setup_complete.session_id
-      else:
-        session_id = None
-      yield AsyncSession(
-          api_client=self._api_client,
-          websocket=ws,
-          session_id=session_id,
-      )
+    return uri, headers, json.dumps(request_dict)
 
 
 async def _t_live_connect_config(
diff --git a/google/genai/live_music.py b/google/genai/live_music.py
index 8730e08f9..cdc2111c8 100644
--- a/google/genai/live_music.py
+++ b/google/genai/live_music.py
@@ -22,6 +22,7 @@
 import websockets
 
 from . import _api_module
+from . import _extra_utils
 from . import _common
 from . import _live_converters as live_converters
 from . import _transformers as t
@@ -169,31 +170,42 @@ class AsyncLiveMusic(_api_module.BaseModule):
   @contextlib.asynccontextmanager
   async def connect(self, *, model: str) -> AsyncIterator[AsyncMusicSession]:
     """[Experimental] Connect to the live music server."""
+    if self._api_client.vertexai:
+      raise NotImplementedError('Live music generation is not supported in Vertex AI.')
+
     base_url = self._api_client._websocket_base_url()
     if isinstance(base_url, bytes):
       base_url = base_url.decode('utf-8')
     transformed_model = t.t_model(self._api_client, model)
 
+    version = self._api_client._http_options.api_version
+    original_headers = self._api_client._http_options.headers
+    headers = original_headers.copy() if original_headers is not None else {}
+
     if self._api_client.api_key:
-      api_key = self._api_client.api_key
-      version = self._api_client._http_options.api_version
-      uri = f'{base_url}/ws/google.ai.generativelanguage.{version}.GenerativeService.BidiGenerateMusic?key={api_key}'
-      headers = self._api_client._http_options.headers
-
-      # Only mldev supported
-      request_dict = _common.convert_to_dict(
-          live_converters._LiveMusicConnectParameters_to_mldev(
-              from_object=types.LiveMusicConnectParameters(
-                model=transformed_model,
-              ).model_dump(exclude_none=True)
-          )
-      )
+      # API key is already included in headers.
+      pass
+    elif creds := self._api_client._credentials:
+      await _extra_utils._maybe_update_and_insert_auth_token(headers, creds)
+    else:
+      # This shouldn't happen.
+      raise ValueError('Genai live music connection requires credentials or API key provided.')
+
+    uri = f'{base_url}/ws/google.ai.generativelanguage.{version}.GenerativeService.BidiGenerateMusic'
+
+    # Only mldev supported
+    request_dict = _common.convert_to_dict(
+        live_converters._LiveMusicConnectParameters_to_mldev(
+            from_object=types.LiveMusicConnectParameters(
+              model=transformed_model,
+            ).model_dump(exclude_none=True)
+        )
+    )
 
-      setv(request_dict, ['setup', 'model'], transformed_model)
+    setv(request_dict, ['setup', 'model'], transformed_model)
+
+    request = json.dumps(request_dict)
 
-      request = json.dumps(request_dict)
-    else:
-      raise NotImplementedError('Live music generation is not supported in Vertex AI.')
 
     try:
       async with connect(uri, additional_headers=headers) as ws:
diff --git a/google/genai/tests/client/test_client_initialization.py b/google/genai/tests/client/test_client_initialization.py
index 7b0136044..9daf50d38 100644
--- a/google/genai/tests/client/test_client_initialization.py
+++ b/google/genai/tests/client/test_client_initialization.py
@@ -47,6 +47,28 @@
 )
 
 
+class FakeCredentials(credentials.Credentials):
+  def __init__(self, token="fake_token", expired=False, quota_project_id=None):
+    super().__init__()
+    self.token = token
+    self._expired = expired
+    self._quota_project_id = quota_project_id
+    self.refresh_count = 0
+
+  @property
+  def expired(self):
+    return self._expired
+
+  @property
+  def quota_project_id(self):
+    return self._quota_project_id
+
+  def refresh(self, request):
+    self.refresh_count += 1
+    self.token = "refreshed_token"
+    self._expired = False
+
+
 @pytest.fixture(autouse=True)
 def reset_has_aiohttp():
   yield
@@ -1721,3 +1743,142 @@ async def test_get_aiohttp_session():
   assert initial_session is not None
   session = await client._api_client._get_aiohttp_session()
   assert session is initial_session
+
+
+def test_missing_api_key_and_credentials(monkeypatch):
+  monkeypatch.setenv("GOOGLE_API_KEY", "")
+  with pytest.raises(ValueError, match="No API key was provided"):
+    Client()
+
+
+auth_precedence_test_cases = [
+    # client_args, env_vars, expected_headers
+    (
+        {"credentials": FakeCredentials()},
+        {"GOOGLE_API_KEY": "env_api_key"},
+        {"Authorization": "Bearer fake_token"}
+    ),
+    (
+        {"credentials": FakeCredentials(quota_project_id="quota-proj")},
+        {"GOOGLE_API_KEY": "env_api_key"},
+        {
+            "Authorization": "Bearer fake_token",
+            "x-goog-user-project": "quota-proj"
+        }
+    ),
+    (
+        {"api_key": "test_api_key"},
+        {"GOOGLE_API_KEY": "env_api_key"},
+        {"x-goog-api-key": "test_api_key"}
+    ),
+    (
+        {},
+        {"GOOGLE_API_KEY": "env_api_key"},
+        {"x-goog-api-key": "env_api_key"}
+    ),
+]
+
+
+@pytest.mark.parametrize(
+    ["client_kwargs", "env_vars", "expected_headers"],
+    auth_precedence_test_cases,
+)
+@mock.patch.object(httpx.Client, "send", autospec=True)
+def test_auth_precedence_mldev(mock_send, monkeypatch, client_kwargs, env_vars, expected_headers):
+  for key, value in env_vars.items():
+    monkeypatch.setenv(key, value)
+
+  client = Client(**client_kwargs)
+  mock_send.return_value = httpx.Response(
+      status_code=200,
+      json={"candidates": [{"content": {"parts": [{"text": "response"}]}}]}
+  )
+  client.models.generate_content(model="test", contents="hello?")
+  mock_send.assert_called_once()
+  request = mock_send.call_args[0][1]
+
+  for key, value in expected_headers.items():
+    assert key in request.headers
+    assert request.headers[key] == value
+
+  if "Authorization" in expected_headers:
+    assert "x-goog-api-key" not in request.headers
+  if "x-goog-api-key" in expected_headers:
+    assert "Authorization" not in request.headers
+  if "x-goog-user-project" not in expected_headers:
+    assert "x-goog-user-project" not in request.headers
+  assert request.content == b'{"contents": [{"parts": [{"text": "hello?"}], "role": "user"}]}'
+
+@pytest.mark.parametrize(
+    ["client_kwargs", "env_vars", "expected_headers"],
+    auth_precedence_test_cases,
+)
+@pytest.mark.asyncio
+@mock.patch.object(httpx.AsyncClient, "send", autospec=True)
+async def test_async_auth_precedence_mldev(mock_send, monkeypatch, client_kwargs, env_vars, expected_headers):
+  for key, value in env_vars.items():
+    monkeypatch.setenv(key, value)
+
+  client = Client(**client_kwargs)
+  mock_send.return_value = httpx.Response(
+      status_code=200,
+      json={"candidates": [{"content": {"parts": [{"text": "response"}]}}]}
+  )
+  await client.aio.models.generate_content(model="test", contents="hello?")
+  mock_send.assert_called_once()
+  request = mock_send.call_args[0][1]
+
+  for key, value in expected_headers.items():
+    assert key in request.headers
+    assert request.headers[key] == value
+
+  if "Authorization" in expected_headers:
+    assert "x-goog-api-key" not in request.headers
+  if "x-goog-api-key" in expected_headers:
+    assert "Authorization" not in request.headers
+  if "x-goog-user-project" not in expected_headers:
+    assert "x-goog-user-project" not in request.headers
+  assert request.content == b'{"contents": [{"parts": [{"text": "hello?"}], "role": "user"}]}'
+
+
+@pytest.mark.asyncio
+async def test_both_credentials_mldev():
+  with pytest.raises(ValueError, match="mutually exclusive"):
+    creds = FakeCredentials(expired=True)
+    client = Client(credentials=creds, api_key="test-api-key")
+
+
+@mock.patch.object(httpx.Client, "send", autospec=True)
+def test_refresh_credentials_mldev(mock_send):
+  creds = FakeCredentials(expired=True)
+  client = Client(credentials=creds)
+  mock_send.return_value = httpx.Response(
+      status_code=200,
+      json={"candidates": [{"content": {"parts": [{"text": "response"}]}}]},
+  )
+  client.models.generate_content(model="test", contents="hello?")
+  mock_send.assert_called_once()
+  request = mock_send.call_args[0][1]
+  assert "Authorization" in request.headers
+  assert request.headers["Authorization"] == "Bearer refreshed_token"
+  assert "x-goog-api-key" not in request.headers
+  assert creds.refresh_count == 1
+
+
+@requires_aiohttp
+@pytest.mark.asyncio
+@mock.patch.object(httpx.AsyncClient, "send", autospec=True)
+async def test_async_refresh_credentials_mldev(mock_send):
+  creds = FakeCredentials(expired=True)
+  client = Client(credentials=creds)
+  mock_send.return_value = httpx.Response(
+      status_code=200,
+      json={"candidates": [{"content": {"parts": [{"text": "response"}]}}]},
+  )
+  await client.aio.models.generate_content(model="test", contents="hello?")
+  mock_send.assert_called_once()
+  request = mock_send.call_args[0][1]
+  assert "Authorization" in request.headers
+  assert request.headers["Authorization"] == "Bearer refreshed_token"
+  assert "x-goog-api-key" not in request.headers
+  assert creds.refresh_count == 1
diff --git a/google/genai/tests/interactions/test_auth.py b/google/genai/tests/interactions/test_auth.py
index 5c92a225e..ff5f5ce4d 100644
--- a/google/genai/tests/interactions/test_auth.py
+++ b/google/genai/tests/interactions/test_auth.py
@@ -474,3 +474,64 @@ async def test_async_interactions_vertex_extra_headers_override():
         assert headers['x-goog-api-key'] == 'manual-key'
         assert 'authorization' not in headers
         mock_access_token.assert_not_called()
+
+def test_interactions_mldev_auth_header():
+  from ..._api_client import BaseApiClient
+  from httpx import Client as HTTPClient
+
+  creds = mock.Mock()
+  creds.quota_project_id = "test-quota-project"
+  client = Client(vertexai=False, credentials=creds)
+
+  with (
+      mock.patch.object(
+          BaseApiClient, "_access_token", return_value='fake-mldev-token'
+      ) as  mock_access_token,
+      mock.patch.object(
+          HTTPClient, "send",
+          return_value=mock.Mock(),
+      ) as mock_send,
+  ):
+
+    client.interactions.create(
+        model='gemini-3-flash-preview',
+        input='Hello',
+    )
+
+    mock_send.assert_called_once()
+    mock_access_token.assert_called_once()
+    args, kwargs = mock_send.call_args
+    headers = args[0].headers
+    assert headers['authorization'] == 'Bearer fake-mldev-token'
+    assert headers['x-goog-user-project'] == 'test-quota-project'
+
+@pytest.mark.asyncio
+async def test_async_interactions_mldev_auth_header():
+  from ..._api_client import BaseApiClient
+  from ..._api_client import AsyncHttpxClient
+
+  creds = mock.Mock()
+  creds.quota_project_id = "test-quota-project"
+  client = Client(vertexai=False, credentials=creds)
+
+  with (
+      mock.patch.object(
+          BaseApiClient, "_async_access_token", return_value='fake-mldev-token'
+      ) as  mock_access_token,
+      mock.patch.object(
+          AsyncHttpxClient, "send",
+          return_value=mock.Mock(),
+      ) as mock_send,
+  ):
+
+    await client.aio.interactions.create(
+        model='gemini-3-flash-preview',
+        input='Hello',
+    )
+
+    mock_send.assert_called_once()
+    mock_access_token.assert_called_once()
+    args, kwargs = mock_send.call_args
+    headers = args[0].headers
+    assert headers['authorization'] == 'Bearer fake-mldev-token'
+    assert headers['x-goog-user-project'] == 'test-quota-project'
diff --git a/google/genai/tests/live/test_live.py b/google/genai/tests/live/test_live.py
index c0b60e595..1af139272 100644
--- a/google/genai/tests/live/test_live.py
+++ b/google/genai/tests/live/test_live.py
@@ -29,6 +29,8 @@
 import warnings
 
 import certifi
+import google.auth
+from google.auth.transport import requests
 from google.oauth2.credentials import Credentials
 import pytest
 from websockets import client
@@ -40,6 +42,9 @@
 from ... import client as gl_client
 from ... import live
 from ... import types
+from ... import _extra_utils
+from google.auth import credentials
+
 try:
     import aiohttp
     AIOHTTP_NOT_INSTALLED = False
@@ -85,6 +90,23 @@
 }]
 
 
+class FakeCredentials(Credentials):
+  def __init__(self, token='fake_token', valid=True):
+    super().__init__(token='placeholder')
+    self.token = token
+    self._valid = valid
+    self.refresh_called = False
+
+  def refresh(self, request):
+    self.token = 'refreshed_token'
+    self._valid = True
+    self.refresh_called = True
+
+  @property
+  def valid(self):
+    return self._valid
+
+
 def get_current_weather(location: str, unit: str):
   """Get the current weather in a city."""
   return 15 if unit == 'C' else 59
@@ -2141,3 +2163,120 @@ async def mock_connect(uri, additional_headers=None, **kwargs):
   assert capture['headers']['x-goog-api-key'] == 'TEST_API_KEY'
   assert 'BidiGenerateContent' in capture['uri']
 
+
+@pytest.mark.asyncio
+async def test_prepare_connection_vertex_with_api_key(mock_websocket):
+    # Test the branch where api_key is present in vertexai
+    client = Client(vertexai=True, api_key="test_api_key")
+    capture = {}
+
+    @contextlib.asynccontextmanager
+    async def mock_ws_connect(uri, additional_headers=None, **kwargs):
+        capture['uri'] = uri
+        capture['headers'] = additional_headers
+        yield mock_websocket
+
+    with patch.object(live, 'ws_connect', new=mock_ws_connect):
+        live_module = client.aio.live
+        async with live_module.connect(model='test-model'):
+            pass
+
+    headers = capture['headers']
+    uri = capture['uri']
+    assert 'x-goog-api-key' in headers
+    assert headers['x-goog-api-key'] == "test_api_key"
+    # Authorization header should not be added by this method if api_key is used
+    assert 'Authorization' not in headers
+    assert "BidiGenerateContent" in uri
+
+
+@pytest.mark.asyncio
+async def test_prepare_connection_vertex_refresh_creds(mock_websocket):
+    # Test the branch where credentials need refreshing
+    fake_creds = FakeCredentials(token=None, valid=False)
+    capture = {}
+
+    @contextlib.asynccontextmanager
+    async def mock_ws_connect(uri, additional_headers=None, **kwargs):
+        capture['uri'] = uri
+        capture['headers'] = additional_headers
+        yield mock_websocket
+
+    with (
+        patch.object(google.auth, 'default', return_value=(fake_creds, "test-project")),
+        patch.object(requests, 'Request', return_value=Mock()),
+        patch.object(live, 'ws_connect', new=mock_ws_connect)
+    ):
+        client = Client(vertexai=True, project="test-project",
+                        location="us-central1")
+        live_module = client.aio.live
+        async with live_module.connect(model='test-model'):
+            pass
+
+    headers = capture['headers']
+    uri = capture['uri']
+    assert fake_creds.refresh_called
+    assert 'Authorization' in headers
+    assert headers['Authorization'] == f'Bearer refreshed_token'
+    assert "BidiGenerateContent" in uri
+
+
+@pytest.mark.asyncio
+async def test_async_live_connect_with_api_key(mock_websocket):
+  client = api_client.BaseApiClient(api_key='test_api_key')
+  async_live = live.AsyncLive(client)
+  capture = {}
+
+  @contextlib.asynccontextmanager
+  async def mock_connect(uri, additional_headers=None, **kwargs):
+    capture['headers'] = additional_headers
+    yield mock_websocket
+
+  with mock.patch.object(live, 'ws_connect', new=mock_connect):
+    async with async_live.connect(model='models/test-model'):
+      pass
+
+    assert 'headers' in capture
+    headers = capture['headers']
+
+    assert headers['x-goog-api-key'] == 'test_api_key'
+
+    assert 'Authorization' not in headers
+
+@pytest.mark.parametrize(
+    "creds, existing_headers, expected_auth, expect_refresh",
+    [
+        (FakeCredentials(), {}, 'Bearer fake_token', False),
+        (FakeCredentials(valid=False), {}, 'Bearer refreshed_token', True),
+        (FakeCredentials(token=None, valid=False), {}, 'Bearer refreshed_token', True),
+        (FakeCredentials(token='existing_token', valid=True), {}, 'Bearer existing_token', False),
+        (FakeCredentials(token='new_token', valid=True), {'Authorization': 'Bearer old_token'}, 'Bearer old_token', False),
+    ],
+)
+@pytest.mark.asyncio
+async def test_async_live_connect_with_credentials(
+    mock_websocket, creds, existing_headers, expected_auth, expect_refresh
+):
+  client = api_client.BaseApiClient(credentials=creds)
+  if existing_headers:
+    client._http_options.headers = existing_headers
+  async_live = live.AsyncLive(client)
+  capture = {}
+
+  @contextlib.asynccontextmanager
+  async def mock_connect(uri, additional_headers=None, **kwargs):
+    capture['headers'] = additional_headers
+    yield mock_websocket
+
+  with (
+      mock.patch.object(live, 'ws_connect', new=mock_connect),
+      mock.patch.object(requests, 'Request', autospec=True)
+  ):
+    async with async_live.connect(model='models/test-model'):
+      pass
+
+  assert 'headers' in capture
+  headers = capture['headers']
+  assert headers.get('Authorization') == expected_auth
+  assert 'x-goog-api-key' not in headers
+  assert creds.refresh_called == expect_refresh
diff --git a/google/genai/tests/live/test_live_music.py b/google/genai/tests/live/test_live_music.py
index f51f8248d..b7a81509d 100644
--- a/google/genai/tests/live/test_live_music.py
+++ b/google/genai/tests/live/test_live_music.py
@@ -24,6 +24,8 @@
 from unittest.mock import patch
 import warnings
 
+import google.auth
+import google.auth.transport.requests
 from google.oauth2.credentials import Credentials
 import pytest
 from websockets import client
@@ -36,6 +38,7 @@
 from ... import live_music
 from ... import types
 from .. import pytest_helper
+
 try:
     import aiohttp
     AIOHTTP_NOT_INSTALLED = False
@@ -49,10 +52,27 @@
 )
 
 
-def mock_api_client(vertexai=False, credentials=None):
+class FakeCredentials(Credentials):
+  def __init__(self, token='fake_token', valid=True):
+    super().__init__(token='placeholder')
+    self.token = token
+    self._valid = valid
+    self.refresh_called = False
+
+  def refresh(self, request):
+    self.token = 'refreshed_token'
+    self._valid = True
+    self.refresh_called = True
+
+  @property
+  def valid(self):
+    return self._valid
+
+
+def mock_api_client(vertexai=False, credentials=None, api_key='TEST_API_KEY'):
   api_client = mock.MagicMock(spec=gl_client.BaseApiClient)
   if not vertexai:
-    api_client.api_key = 'TEST_API_KEY'
+    api_client.api_key = api_key
     api_client.location = None
     api_client.project = None
   else:
@@ -67,9 +87,24 @@ def mock_api_client(vertexai=False, credentials=None):
   )  # Ensure headers exist
   api_client.vertexai = vertexai
   api_client._api_client = api_client
+  api_client._websocket_base_url = lambda: 'wss://test.com'
   return api_client
 
 
+@pytest.fixture(autouse=True)
+def clear_env(monkeypatch):
+  monkeypatch.delenv("GOOGLE_API_KEY", raising=False)
+  monkeypatch.delenv("GEMINI_API_KEY", raising=False)
+  monkeypatch.delenv("GOOGLE_CLOUD_PROJECT", raising=False)
+  monkeypatch.delenv("GOOGLE_CLOUD_LOCATION", raising=False)
+  monkeypatch.delenv("GOOGLE_GENAI_USE_VERTEXAI", raising=False)
+  monkeypatch.delenv("GOOGLE_GENAI_CLIENT_MODE", raising=False)
+  monkeypatch.delenv("GOOGLE_GENAI_REPLAY_ID", raising=False)
+  monkeypatch.delenv("GOOGLE_GENAI_REPLAYS_DIRECTORY", raising=False)
+  monkeypatch.delenv("GOOGLE_GEMINI_BASE_URL", raising=False)
+  monkeypatch.delenv("GOOGLE_VERTEX_BASE_URL", raising=False)
+
+
 @pytest.fixture
 def mock_websocket():
   websocket = AsyncMock(spec=client.ClientConnection)
@@ -115,14 +150,15 @@ async def get_connect_message(api_client, model):
   mock_google_auth_default.return_value = (mock_creds, None)
 
   @contextlib.asynccontextmanager
-  async def mock_connect(uri, additional_headers=None):
+  async def mock_connect(uri, additional_headers=None, **kwargs):
     yield mock_ws
 
-  @patch('google.auth.default', new=mock_google_auth_default)
-  @patch.object(live_music, 'connect', new=mock_connect)
-  async def _test_connect():
-    live_module = live.AsyncLive(api_client)
-    async with live_module.music.connect(
+  with (
+      patch.object(google.auth, 'default', new=mock_google_auth_default),
+      patch.object(live_music, 'connect', new=mock_connect)
+  ):
+    live_module = live_music.AsyncLiveMusic(api_client)
+    async with live_module.connect(
         model=model,
     ):
       pass
@@ -130,7 +166,6 @@ async def _test_connect():
     mock_ws.send.assert_called_once()
     return json.loads(mock_ws.send.call_args[0][0])
 
-  return await _test_connect()
 
 
 def test_mldev_from_env(monkeypatch):
@@ -142,6 +177,7 @@ def test_mldev_from_env(monkeypatch):
   assert not client.aio.live.music._api_client.vertexai
   assert client.aio.live.music._api_client.api_key == api_key
   assert isinstance(client.aio.live._api_client, api_client.BaseApiClient)
+  assert client.aio.live._api_client._http_options.headers['x-goog-api-key'] == api_key
 
 
 @requires_aiohttp
@@ -360,3 +396,70 @@ async def test_setup_to_api(vertexai):
   else:
     expected_result['setup']['model'] = 'models/test_model'
   assert result == expected_result
+
+@pytest.mark.asyncio
+async def test_connect_with_api_key(mock_websocket):
+    client = Client(api_key='TEST_API_KEY', http_options={'api_version': 'v1test'})
+    client._api_client._websocket_base_url = lambda: 'wss://test.com'
+    live_module = client.aio.live.music
+    capture = {}
+
+    @contextlib.asynccontextmanager
+    async def mock_connect(uri, additional_headers=None, **kwargs):
+      capture['uri'] = uri
+      capture['headers'] = additional_headers
+      yield mock_websocket
+
+    with patch.object(live_music, 'connect', new=mock_connect):
+      async with live_module.connect(model='test-model'):
+          pass
+
+    assert capture['uri'] == 'wss://test.com/ws/google.ai.generativelanguage.v1test.GenerativeService.BidiGenerateMusic'
+    assert capture['headers']['x-goog-api-key'] == 'TEST_API_KEY'
+    assert 'Authorization' not in capture['headers']
+
+@pytest.mark.parametrize(
+    "creds, existing_headers, expected_auth, expect_refresh",
+    [
+        (FakeCredentials(), {}, 'Bearer fake_token', False),
+        (FakeCredentials(valid=False), {}, 'Bearer refreshed_token', True),
+        (FakeCredentials(token=None, valid=False), {}, 'Bearer refreshed_token', True),
+        (FakeCredentials(token='existing_token', valid=True), {}, 'Bearer existing_token', False),
+        (FakeCredentials(token='new_token', valid=True), {'Authorization': 'Bearer old_token'}, 'Bearer old_token', False),
+    ],
+)
+@pytest.mark.asyncio
+async def test_connect_with_credentials(
+    mock_websocket, creds, existing_headers, expected_auth, expect_refresh
+):
+    client = api_client.BaseApiClient(credentials=creds, http_options={'api_version': 'v1test'})
+    if existing_headers:
+      client._http_options.headers = existing_headers
+    client._websocket_base_url = lambda: 'wss://test.com'
+    live_module = live_music.AsyncLiveMusic(client)
+    capture = {}
+
+    @contextlib.asynccontextmanager
+    async def mock_connect(uri, additional_headers=None, **kwargs):
+      capture['uri'] = uri
+      capture['headers'] = additional_headers
+      yield mock_websocket
+
+    with patch.object(live_music, 'connect', new=mock_connect):
+      with patch.object(google.auth.transport.requests, 'Request', autospec=True):
+        async with live_module.connect(model='test-model'):
+          pass
+
+    assert capture['uri'] == 'wss://test.com/ws/google.ai.generativelanguage.v1test.GenerativeService.BidiGenerateMusic'
+    headers = capture['headers']
+    assert headers.get('Authorization') == expected_auth
+    assert 'x-goog-api-key' not in headers
+    assert creds.refresh_called == expect_refresh
+
+@pytest.mark.asyncio
+async def test_connect_vertex_unsupported(mock_websocket):
+    client = Client(vertexai=True, project='test', location='us-central1')
+    live_module = client.aio.live.music
+    with pytest.raises(NotImplementedError):
+        async with live_module.connect(model='test-model'):
+            pass