diff --git a/README.md b/README.md
index 6cca3cd0a9..f2388d4255 100644
--- a/README.md
+++ b/README.md
@@ -250,37 +250,37 @@ client = genai.Client(
 )
 ```
 
-### HTTP client option: httpx2
+### HTTP client option: httpx
 
-By default, the SDK uses `httpx` for its HTTP transport. You can opt in to
-[`httpx2`](https://github.com/pydantic/httpx2) by installing `google-genai[httpx2]`
+By default, the SDK uses [`httpx2`](https://github.com/pydantic/httpx2) for its HTTP transport. You can opt in to
+legacy `httpx` by installing `google-genai[httpx]`
 and setting the `GOOGLE_GENAI_HTTP_CLIENT` environment variable:
 
 ```bash
-export GOOGLE_GENAI_HTTP_CLIENT=httpx2
+export GOOGLE_GENAI_HTTP_CLIENT=httpx
 ```
 
-Alternatively, you can directly inject an `httpx2.Client` or `httpx2.AsyncClient`
+Alternatively, you can directly inject an `httpx.Client` or `httpx.AsyncClient`
 via `HttpOptions`:
 
 ```python
-import httpx2
+import httpx
 from google import genai
 from google.genai import types
 
 client = genai.Client(
     http_options=types.HttpOptions(
-        httpx_client=httpx2.Client(),
-        httpx_async_client=httpx2.AsyncClient(),
+        httpx_client=httpx.Client(),
+        httpx_async_client=httpx.AsyncClient(),
     )
 )
 ```
 
 ### Faster async client option: Aiohttp
 
-By default we use httpx for both sync and async client implementations. In order
+By default we use httpx2 for both sync and async client implementations. In order
 to have faster performance, you may install `google-genai[aiohttp]`. In Gen AI
-SDK we configure `trust_env=True` to match with the default behavior of httpx.
+SDK we configure `trust_env=True` to match with the default behavior of httpx2.
 Additional args of `aiohttp.ClientSession.request()` ([see `_RequestOptions` args](https://github.com/aio-libs/aiohttp/blob/v3.12.13/aiohttp/client.py#L170)) can be passed
 through the following way:
 
@@ -295,7 +295,7 @@ client=Client(..., http_options=http_options)
 
 ### Proxy
 
-Both httpx and aiohttp libraries use `urllib.request.getproxies` from
+Both httpx2 and aiohttp libraries use `urllib.request.getproxies` from
 environment variables. Before client initialization, you may set proxy (and
 optional `SSL_CERT_FILE`) by setting the environment variables:
 
@@ -304,8 +304,8 @@ export HTTPS_PROXY='http://username:password@proxy_uri:port'
 export SSL_CERT_FILE='client.pem'
 ```
 
-If you need `socks5` proxy, httpx [supports](https://www.python-httpx.org/advanced/proxies/#socks) `socks5` proxy if you pass it via
-args to `httpx.Client()`. You may install `httpx[socks]` to use it.
+If you need `socks5` proxy, httpx2 [supports](https://www.python-httpx.org/advanced/proxies/#socks) `socks5` proxy if you pass it via
+args to `httpx2.Client()`. You may install `httpx2[socks]` to use it.
 Then, you can pass it through the following way:
 
 ```python
diff --git a/docs/source/index.rst b/docs/source/index.rst
index a285fcf808..2d8be4b0dc 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -218,9 +218,9 @@ To set the API version to `v1alpha` for the Gemini Developer API:
 Faster async client option: Aiohttp
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-By default we use httpx for both sync and async client implementations. In order
+By default we use httpx2 for both sync and async client implementations. In order
 to have faster performance, you may install `google-genai[aiohttp]`. In Gen AI
-SDK we configure `trust_env=True` to match with the default behavior of httpx.
+SDK we configure `trust_env=True` to match with the default behavior of httpx2.
 Additional args of `aiohttp.ClientSession.request()`
 (`see _RequestOptions args <https://github.com/aio-libs/aiohttp/blob/v3.12.13/aiohttp/client.py#L170>`_)
 can be passed through the following way:
@@ -237,7 +237,7 @@ can be passed through the following way:
 Proxy
 ^^^^^^^
 
-Both httpx and aiohttp libraries use `urllib.request.getproxies` from
+Both httpx2 and aiohttp libraries use `urllib.request.getproxies` from
 environment variables. Before client initialization, you may set proxy (and
 optional SSL_CERT_FILE) by setting the environment variables:
 
@@ -248,9 +248,9 @@ optional SSL_CERT_FILE) by setting the environment variables:
     export SSL_CERT_FILE='client.pem'
 
 
-If you need `socks5` proxy, httpx `supports <https://www.python-httpx.org/advanced/proxies/#socks>`_
-`socks5` proxy if you pass it via args to httpx.Client(). You may install
-`httpx[socks]` to use it. Then you can pass it through the following way:
+If you need `socks5` proxy, httpx2 `supports <https://www.python-httpx.org/advanced/proxies/#socks>`_
+`socks5` proxy if you pass it via args to httpx2.Client(). You may install
+`httpx2[socks]` to use it. Then you can pass it through the following way:
 
 .. code:: python
 
diff --git a/google/genai/_api_client.py b/google/genai/_api_client.py
index 36d7a2638d..ca4878f4ae 100644
--- a/google/genai/_api_client.py
+++ b/google/genai/_api_client.py
@@ -809,13 +809,19 @@ def __del__(self) -> None:
     AsyncHttpx2Client = None  # type: ignore[assignment,misc]
 
 
+# The HTTP client backend used when GOOGLE_GENAI_HTTP_CLIENT is unset or 'auto'.
+# If it is not installed, the other backend is used instead.
+_DEFAULT_HTTP_CLIENT_BACKEND = 'httpx2'
+
+
 def _get_http_client_backend() -> str:
   """Determines whether to use 'httpx' or 'httpx2' as the default HTTP client.
 
   Can be controlled via the GOOGLE_GENAI_HTTP_CLIENT environment variable:
     - 'httpx2': Use httpx2 (raises ImportError if not installed).
     - 'httpx': Use httpx (raises ImportError if not installed).
-    - 'auto' or unset: Default to httpx if installed, otherwise fall back to httpx2.
+    - 'auto' or unset: Use `_DEFAULT_HTTP_CLIENT_BACKEND` if installed,
+      otherwise fall back to the other backend.
   """
   env = os.environ.get('GOOGLE_GENAI_HTTP_CLIENT', '').lower().strip()
   if env == 'httpx2':
@@ -837,14 +843,14 @@ def _get_http_client_backend() -> str:
         'Unrecognized GOOGLE_GENAI_HTTP_CLIENT=%r; falling back to default.', env
     )
 
-  if httpx is not None:
-    return 'httpx'
-  # httpx is a required dependency so it _should_ always be present in a
-  # standard install, but if a user has uninstalled httpx in an environment
-  # that only has httpx2 installed, fall back to httpx2 before raising
+  installed = {'httpx': httpx is not None, 'httpx2': httpx2 is not None}
+  fallback = 'httpx' if _DEFAULT_HTTP_CLIENT_BACKEND == 'httpx2' else 'httpx2'
+  # Either backend may be missing (httpx2 is an optional extra, and a user may
+  # have uninstalled httpx), so fall back to the other one before raising
   # ImportError.
-  if httpx2 is not None:
-    return 'httpx2'
+  for backend in (_DEFAULT_HTTP_CLIENT_BACKEND, fallback):
+    if installed[backend]:
+      return backend
   raise ImportError(
       'Neither httpx nor httpx2 is installed. Please install httpx2 or httpx.'
   )
@@ -1377,19 +1383,70 @@ def _ensure_httpx_ssl_ctx(
         else None
     )
 
+    default_backend = (
+        _get_http_client_backend()
+        if (client_cls is None or async_client_cls is None)
+        else None
+    )
+    if client_cls is not None:
+      sync_cls = client_cls
+    elif default_backend == 'httpx2' and SyncHttpx2Client is not None:
+      sync_cls = SyncHttpx2Client
+    else:
+      sync_cls = (
+          SyncHttpxClient
+          if SyncHttpxClient is not None
+          else (httpx.Client if httpx is not None else None)
+      )
+
+    if async_client_cls is not None:
+      async_cls = async_client_cls
+    elif default_backend == 'httpx2' and AsyncHttpx2Client is not None:
+      async_cls = AsyncHttpx2Client
+    else:
+      async_cls = (
+          AsyncHttpxClient
+          if AsyncHttpxClient is not None
+          else (httpx.AsyncClient if httpx is not None else None)
+      )
+
+    use_httpx2 = httpx2 is not None and (
+        (
+            sync_cls is not None
+            and isinstance(getattr(httpx2, 'Client', None), type)
+            and issubclass(sync_cls, httpx2.Client)
+        )
+        or (
+            async_cls is not None
+            and isinstance(getattr(httpx2, 'AsyncClient', None), type)
+            and issubclass(async_cls, httpx2.AsyncClient)
+        )
+    )
+
     if ctx is None:
-      # Initialize the SSL context for the httpx client.
+      # Initialize the SSL context for the httpx/httpx2 client.
       # Unlike requests, the httpx package does not automatically pull in the
       # environment variables SSL_CERT_FILE or SSL_CERT_DIR. They need to be
-      # enabled explicitly.
+      # enabled explicitly. When using httpx2 without SSL_CERT_FILE or
+      # SSL_CERT_DIR, delegate to httpx2.create_ssl_context(), which uses
+      # truststore (OS native trust store).
       if vertexai:
         ctx = _get_client_cert_ssl_ctx()
 
       if ctx is None:
-        ctx = ssl.create_default_context(
-            cafile=os.environ.get('SSL_CERT_FILE', certifi.where()),
-            capath=os.environ.get('SSL_CERT_DIR'),
-        )
+        if use_httpx2:
+          if os.environ.get('SSL_CERT_FILE') or os.environ.get('SSL_CERT_DIR'):
+            ctx = ssl.create_default_context(
+                cafile=os.environ.get('SSL_CERT_FILE'),
+                capath=os.environ.get('SSL_CERT_DIR'),
+            )
+          else:
+            ctx = httpx2.create_ssl_context()
+        else:
+          ctx = ssl.create_default_context(
+              cafile=os.environ.get('SSL_CERT_FILE', certifi.where()),
+              capath=os.environ.get('SSL_CERT_DIR'),
+          )
 
     def _maybe_set(
         raw_args: Optional[_common.StringDict],
@@ -1432,17 +1489,6 @@ def _maybe_set(
             break
       return args
 
-    sync_cls = client_cls or (
-        SyncHttpxClient
-        if SyncHttpxClient is not None
-        else (httpx.Client if httpx is not None else None)
-    )
-    async_cls = async_client_cls or (
-        AsyncHttpxClient
-        if AsyncHttpxClient is not None
-        else (httpx.AsyncClient if httpx is not None else None)
-    )
-
     return (
         _maybe_set(args, ctx, sync_cls),
         _maybe_set(async_args, ctx, async_cls),
diff --git a/google/genai/_gaos/_hooks/adapters.py b/google/genai/_gaos/_hooks/adapters.py
index 5a9c41c70e..a2809297d4 100644
--- a/google/genai/_gaos/_hooks/adapters.py
+++ b/google/genai/_gaos/_hooks/adapters.py
@@ -34,7 +34,10 @@
     BeforeRequestHook,
 )
 import asyncio
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional, Tuple, Union
 
 # ============================================================================
diff --git a/google/genai/_gaos/_hooks/asyncsdkhooks.py b/google/genai/_gaos/_hooks/asyncsdkhooks.py
index ad81b1d067..bfcdff468e 100644
--- a/google/genai/_gaos/_hooks/asyncsdkhooks.py
+++ b/google/genai/_gaos/_hooks/asyncsdkhooks.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from .asynctypes import (
     AsyncBeforeRequestHook,
     AsyncAfterSuccessHook,
diff --git a/google/genai/_gaos/_hooks/asynctypes.py b/google/genai/_gaos/_hooks/asynctypes.py
index 0869abee1f..a2464421b8 100644
--- a/google/genai/_gaos/_hooks/asynctypes.py
+++ b/google/genai/_gaos/_hooks/asynctypes.py
@@ -24,7 +24,10 @@
     BeforeRequestContext,
 )
 from abc import ABC, abstractmethod
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional, Tuple, Union
 
 
diff --git a/google/genai/_gaos/_hooks/google_genai_auth.py b/google/genai/_gaos/_hooks/google_genai_auth.py
index 7ec30b9d87..989ddfb753 100644
--- a/google/genai/_gaos/_hooks/google_genai_auth.py
+++ b/google/genai/_gaos/_hooks/google_genai_auth.py
@@ -21,7 +21,10 @@
 
 from typing import Any, Callable, Mapping, Optional, Union, cast
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 from .. import types, utils
 from .types import BeforeRequestContext, BeforeRequestHook
diff --git a/google/genai/_gaos/_hooks/sdkhooks.py b/google/genai/_gaos/_hooks/sdkhooks.py
index 5dc2b7bc32..dcb716b345 100644
--- a/google/genai/_gaos/_hooks/sdkhooks.py
+++ b/google/genai/_gaos/_hooks/sdkhooks.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from .types import (
     SDKInitHook,
     BeforeRequestContext,
diff --git a/google/genai/_gaos/_hooks/types.py b/google/genai/_gaos/_hooks/types.py
index fdee868291..a1336373b8 100644
--- a/google/genai/_gaos/_hooks/types.py
+++ b/google/genai/_gaos/_hooks/types.py
@@ -19,7 +19,10 @@
 
 from ..sdkconfiguration import SDKConfiguration
 from abc import ABC, abstractmethod
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Callable, Dict, List, Literal, Optional, Tuple, Union
 
 
diff --git a/google/genai/_gaos/_internal.py b/google/genai/_gaos/_internal.py
index abd8077f0f..2d65147fa8 100644
--- a/google/genai/_gaos/_internal.py
+++ b/google/genai/_gaos/_internal.py
@@ -22,7 +22,10 @@
 from .basesdk import AsyncBaseSDK, BaseSDK
 from .types import OptionalNullable, UNSET
 from .utils import get_security_from_env, response_helpers
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/agents.py b/google/genai/_gaos/agents.py
index 1118eeed94..784af73a8b 100644
--- a/google/genai/_gaos/agents.py
+++ b/google/genai/_gaos/agents.py
@@ -24,7 +24,10 @@
 from .types.agents import agent as agents_agent, agenttool as agents_agenttool
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Iterable, List, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/basesdk.py b/google/genai/_gaos/basesdk.py
index 0306654c11..8d77ec76c9 100644
--- a/google/genai/_gaos/basesdk.py
+++ b/google/genai/_gaos/basesdk.py
@@ -26,7 +26,12 @@
 )
 from .sdkconfiguration import SDKConfiguration
 from .utils import RetryConfig, SerializedRequestBody, get_body_content
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
+from typing import Any, Callable, List, Mapping, Optional, Tuple
+from urllib.parse import parse_qs, urlparse
 
 try:
     import httpx2
@@ -38,8 +43,14 @@
     if httpx2 is None
     else (httpx.Timeout, httpx2.Timeout)
 )
-from typing import Any, Callable, List, Mapping, Optional, Tuple
-from urllib.parse import parse_qs, urlparse
+
+
+def _use_client_default(client: Any) -> Any:
+    if httpx2 is not None and isinstance(
+        client, (httpx2.Client, httpx2.AsyncClient)
+    ):
+        return httpx2.USE_CLIENT_DEFAULT
+    return httpx.USE_CLIENT_DEFAULT
 
 
 class BaseSDK:
@@ -234,7 +245,9 @@ def _build_request_with_client(
             data=serialized_request_body.data,
             files=serialized_request_body.files,
             headers=headers,
-            timeout=timeout if timeout is not None else httpx.USE_CLIENT_DEFAULT,
+            timeout=(
+                timeout if timeout is not None else _use_client_default(client)
+            ),
         )
 
     def do_request(
@@ -501,7 +514,9 @@ def _build_request_with_client(
             data=serialized_request_body.data,
             files=serialized_request_body.files,
             headers=headers,
-            timeout=timeout if timeout is not None else httpx.USE_CLIENT_DEFAULT,
+            timeout=(
+                timeout if timeout is not None else _use_client_default(client)
+            ),
         )
 
     async def do_request_async(
diff --git a/google/genai/_gaos/credentials.py b/google/genai/_gaos/credentials.py
index f132a9d5dd..58c69fefa3 100644
--- a/google/genai/_gaos/credentials.py
+++ b/google/genai/_gaos/credentials.py
@@ -23,7 +23,10 @@
 from .types import BaseModel, OptionalNullable, UNSET, credentials, interactions
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, List, Literal, Mapping, Optional, Union, cast, overload
 
 
diff --git a/google/genai/_gaos/environments.py b/google/genai/_gaos/environments.py
index 92846eb14b..ab216003f5 100644
--- a/google/genai/_gaos/environments.py
+++ b/google/genai/_gaos/environments.py
@@ -30,7 +30,10 @@
 from .types.interactions import source as interactions_source
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Iterable, List, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/errors/cancelinteractionbyid.py b/google/genai/_gaos/errors/cancelinteractionbyid.py
index 8fe786cbfd..f106e86049 100644
--- a/google/genai/_gaos/errors/cancelinteractionbyid.py
+++ b/google/genai/_gaos/errors/cancelinteractionbyid.py
@@ -22,7 +22,10 @@
 from ..types import BaseModel
 from ..types.interactions import error as interactions_error
 from dataclasses import dataclass, field
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 
 
diff --git a/google/genai/_gaos/errors/createinteraction.py b/google/genai/_gaos/errors/createinteraction.py
index 776c4ed298..7bdb6ce903 100644
--- a/google/genai/_gaos/errors/createinteraction.py
+++ b/google/genai/_gaos/errors/createinteraction.py
@@ -22,7 +22,10 @@
 from ..types import BaseModel
 from ..types.interactions import error as interactions_error
 from dataclasses import dataclass, field
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 
 
diff --git a/google/genai/_gaos/errors/deleteinteraction.py b/google/genai/_gaos/errors/deleteinteraction.py
index 05f2536ef9..1468644024 100644
--- a/google/genai/_gaos/errors/deleteinteraction.py
+++ b/google/genai/_gaos/errors/deleteinteraction.py
@@ -22,7 +22,10 @@
 from ..types import BaseModel
 from ..types.interactions import error as interactions_error
 from dataclasses import dataclass, field
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 
 
diff --git a/google/genai/_gaos/errors/genaidefaulterror.py b/google/genai/_gaos/errors/genaidefaulterror.py
index 318be10505..6094a2729d 100644
--- a/google/genai/_gaos/errors/genaidefaulterror.py
+++ b/google/genai/_gaos/errors/genaidefaulterror.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 from dataclasses import dataclass
 
diff --git a/google/genai/_gaos/errors/genaierror.py b/google/genai/_gaos/errors/genaierror.py
index 516f13a5dd..4fdf49a5ab 100644
--- a/google/genai/_gaos/errors/genaierror.py
+++ b/google/genai/_gaos/errors/genaierror.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 from dataclasses import dataclass, field
 
diff --git a/google/genai/_gaos/errors/getinteractionbyid.py b/google/genai/_gaos/errors/getinteractionbyid.py
index a29d02716d..deea47d7aa 100644
--- a/google/genai/_gaos/errors/getinteractionbyid.py
+++ b/google/genai/_gaos/errors/getinteractionbyid.py
@@ -22,7 +22,10 @@
 from ..types import BaseModel
 from ..types.interactions import error as interactions_error
 from dataclasses import dataclass, field
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 
 
diff --git a/google/genai/_gaos/errors/responsevalidationerror.py b/google/genai/_gaos/errors/responsevalidationerror.py
index 24fe08c107..d9aa158c0c 100644
--- a/google/genai/_gaos/errors/responsevalidationerror.py
+++ b/google/genai/_gaos/errors/responsevalidationerror.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Optional
 from dataclasses import dataclass
 
diff --git a/google/genai/_gaos/files.py b/google/genai/_gaos/files.py
index 6155254a35..693ea60aa4 100644
--- a/google/genai/_gaos/files.py
+++ b/google/genai/_gaos/files.py
@@ -23,7 +23,10 @@
 from .types import OptionalNullable, UNSET, environments
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/google_genai.py b/google/genai/_gaos/google_genai.py
index 95c9fdf7c8..fc2c221219 100644
--- a/google/genai/_gaos/google_genai.py
+++ b/google/genai/_gaos/google_genai.py
@@ -31,7 +31,10 @@
 import mimetypes
 import os
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 from ._hooks.google_genai_auth import (
     GOOGLE_GENAI_API_REVISION as _GOOGLE_GENAI_API_REVISION,
diff --git a/google/genai/_gaos/httpclient.py b/google/genai/_gaos/httpclient.py
index 0e1a8dcd74..c3ed9fe352 100644
--- a/google/genai/_gaos/httpclient.py
+++ b/google/genai/_gaos/httpclient.py
@@ -20,7 +20,10 @@
 # pyright: reportReturnType = false
 import asyncio
 from typing_extensions import Protocol, runtime_checkable
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Optional, Union
 
 
diff --git a/google/genai/_gaos/interactions.py b/google/genai/_gaos/interactions.py
index d8951ad442..1aa4df8a5e 100644
--- a/google/genai/_gaos/interactions.py
+++ b/google/genai/_gaos/interactions.py
@@ -24,7 +24,10 @@
 from .utils import get_security_from_env, response_helpers
 from .utils.eventstreaming import AsyncStream, Stream
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Dict, List, Literal, Mapping, Optional, Union, cast, overload
 
 
diff --git a/google/genai/_gaos/lib/compat_errors.py b/google/genai/_gaos/lib/compat_errors.py
index 7bd524927e..358521972b 100644
--- a/google/genai/_gaos/lib/compat_errors.py
+++ b/google/genai/_gaos/lib/compat_errors.py
@@ -34,7 +34,10 @@
 except ImportError:
     httpx2 = None
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 _HTTPX_TIMEOUT_ERRORS = (
     (httpx.TimeoutException,)
diff --git a/google/genai/_gaos/sdk.py b/google/genai/_gaos/sdk.py
index 2f493dcf71..61920a08c0 100644
--- a/google/genai/_gaos/sdk.py
+++ b/google/genai/_gaos/sdk.py
@@ -31,7 +31,10 @@
 from .types import OptionalNullable, UNSET
 from .utils.logger import Logger, get_default_logger
 from .utils.retries import RetryConfig
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 import importlib
 import importlib.util
 import sys
diff --git a/google/genai/_gaos/triggers.py b/google/genai/_gaos/triggers.py
index df24c69207..3840d6f618 100644
--- a/google/genai/_gaos/triggers.py
+++ b/google/genai/_gaos/triggers.py
@@ -27,7 +27,10 @@
 from .types.triggers import triggerupdate as triggers_triggerupdate
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/utils/eventstreaming.py b/google/genai/_gaos/utils/eventstreaming.py
index 423689b29a..6fc316c114 100644
--- a/google/genai/_gaos/utils/eventstreaming.py
+++ b/google/genai/_gaos/utils/eventstreaming.py
@@ -31,7 +31,10 @@
     AsyncGenerator,
     Tuple,
 )
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 T = TypeVar("T")
 
diff --git a/google/genai/_gaos/utils/headers.py b/google/genai/_gaos/utils/headers.py
index b95606938a..f4b326aa14 100644
--- a/google/genai/_gaos/utils/headers.py
+++ b/google/genai/_gaos/utils/headers.py
@@ -23,7 +23,10 @@
     List,
     Optional,
 )
-from httpx import Headers
+try:
+    from httpx import Headers
+except ImportError:
+    from httpx2 import Headers
 from pydantic import BaseModel
 from pydantic.fields import FieldInfo
 
diff --git a/google/genai/_gaos/utils/logger.py b/google/genai/_gaos/utils/logger.py
index 1085e0b046..65f61771a0 100644
--- a/google/genai/_gaos/utils/logger.py
+++ b/google/genai/_gaos/utils/logger.py
@@ -17,7 +17,10 @@
 
 """Code generated by Speakeasy (https://speakeasy.com). DO NOT EDIT."""
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 import logging
 import os
 from typing import Any, Protocol
diff --git a/google/genai/_gaos/utils/response_helpers.py b/google/genai/_gaos/utils/response_helpers.py
index bbb844d39a..c3a2f6c654 100644
--- a/google/genai/_gaos/utils/response_helpers.py
+++ b/google/genai/_gaos/utils/response_helpers.py
@@ -46,7 +46,10 @@
     overload,
 )
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from pydantic import BaseModel
 from typing_extensions import TypeAliasType
 
diff --git a/google/genai/_gaos/utils/retries.py b/google/genai/_gaos/utils/retries.py
index 1dc0099962..a2bb0879d7 100644
--- a/google/genai/_gaos/utils/retries.py
+++ b/google/genai/_gaos/utils/retries.py
@@ -24,7 +24,10 @@
 from email.utils import parsedate_to_datetime
 from typing import List, Optional
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 
 class BackoffStrategy:
diff --git a/google/genai/_gaos/utils/serializers.py b/google/genai/_gaos/utils/serializers.py
index 99747b4f68..dde9c1a0f1 100644
--- a/google/genai/_gaos/utils/serializers.py
+++ b/google/genai/_gaos/utils/serializers.py
@@ -25,7 +25,10 @@
 import typing_extensions
 from typing_extensions import get_origin
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from pydantic import ConfigDict, create_model
 from pydantic_core import from_json
 
diff --git a/google/genai/_gaos/utils/unmarshal_json_response.py b/google/genai/_gaos/utils/unmarshal_json_response.py
index 4ba1a154be..744af872cf 100644
--- a/google/genai/_gaos/utils/unmarshal_json_response.py
+++ b/google/genai/_gaos/utils/unmarshal_json_response.py
@@ -19,7 +19,10 @@
 
 from typing import Any, Optional, Type, TypeVar, overload
 
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 
 from .serializers import unmarshal_json, construct_unvalidated
 from .. import errors
diff --git a/google/genai/_gaos/utils/values.py b/google/genai/_gaos/utils/values.py
index cc953332d6..63fe86570e 100644
--- a/google/genai/_gaos/utils/values.py
+++ b/google/genai/_gaos/utils/values.py
@@ -24,7 +24,10 @@
 import os
 from typing import Any, Callable, Dict, List, Optional, Tuple, TypeVar, Union, cast
 
-from httpx import Response
+try:
+    from httpx import Response
+except ImportError:
+    from httpx2 import Response
 from pydantic import BaseModel
 from pydantic.fields import FieldInfo
 
diff --git a/google/genai/_gaos/voices.py b/google/genai/_gaos/voices.py
index 454f466f56..3ed2411f90 100644
--- a/google/genai/_gaos/voices.py
+++ b/google/genai/_gaos/voices.py
@@ -24,7 +24,10 @@
 from .types.voices import voice_input as voices_voice_input
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Iterable, List, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/_gaos/webhooks.py b/google/genai/_gaos/webhooks.py
index 145a392590..5017450e57 100644
--- a/google/genai/_gaos/webhooks.py
+++ b/google/genai/_gaos/webhooks.py
@@ -29,7 +29,10 @@
 )
 from .utils import get_security_from_env, response_helpers
 from .utils.unmarshal_json_response import unmarshal_json_response
-import httpx
+try:
+    import httpx
+except ImportError:
+    import httpx2 as httpx
 from typing import Any, Iterable, List, Mapping, Optional, Union, cast
 
 
diff --git a/google/genai/tests/client/test_async_stream.py b/google/genai/tests/client/test_async_stream.py
index 71b626b9f7..43b57eb033 100644
--- a/google/genai/tests/client/test_async_stream.py
+++ b/google/genai/tests/client/test_async_stream.py
@@ -32,13 +32,16 @@
   AIOHTTP_NOT_INSTALLED = True
   aiohttp = mock.MagicMock()
 
-import httpx
-
 from ... import _api_client as api_client
 from ... import Client
 from ... import errors
 from ... import types
 
+if api_client._get_http_client_backend() == 'httpx2':
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
+
 
 EVENT_STREAM_DATA_WITH_ERROR = [
     b'{"candidates":[{"content":{"parts":[{"text":"test"}],"role":"model"}}]}',
@@ -387,7 +390,7 @@ def mock_response(chunks):
   )
 
 
-@patch('httpx.Client.send')
+@patch.object(httpx.Client, 'send')
 def test_error_event_in_streamed_responses_bad_json(mock_send_method):
   with_bad_json = [
       b'{"candidates":[{"content":{"parts":[{"text":"test"}],"role":"model"}}]}',
@@ -412,7 +415,7 @@ def test_error_event_in_streamed_responses_bad_json(mock_send_method):
     next(stream)
 
 
-@patch('httpx.Client.send')
+@patch.object(httpx.Client, 'send')
 def test_error_event_in_streamed_responses(mock_send_method):
   mock_send_method.return_value = mock_response(EVENT_STREAM_DATA_WITH_ERROR)
 
@@ -432,7 +435,7 @@ def test_error_event_in_streamed_responses(mock_send_method):
     next(stream)
 
 
-@patch('httpx.Client.send')
+@patch.object(httpx.Client, 'send')
 def test_error_event_in_generate_content_stream(mock_send_method):
   mock_send_method.return_value = mock_response(EVENT_STREAM_DATA_WITH_ERROR)
 
@@ -464,8 +467,12 @@ def test_error_event_in_generate_content_stream(mock_send_method):
 
 
 async def _async_httpx_response(_):
+  async def _aiter():
+    for chunk in EVENT_STREAM_DATA_WITH_ERROR:
+      yield chunk
+
   mock_stream = MagicMock(spec=httpx.AsyncByteStream)
-  mock_stream.__aiter__.return_value = EVENT_STREAM_DATA_WITH_ERROR
+  mock_stream.__aiter__.side_effect = _aiter
   mock_stream.aclose = AsyncMock()
   return httpx.Response(
       status_code=200,
@@ -473,7 +480,7 @@ async def _async_httpx_response(_):
   )
 
 
-@patch('httpx.AsyncBaseTransport')
+@patch.object(httpx, 'AsyncBaseTransport')
 @pytest.mark.asyncio
 async def test_error_event_in_streamed_responses_async(mock_transport):
   client = api_client.BaseApiClient(
@@ -502,7 +509,7 @@ async def test_error_event_in_streamed_responses_async(mock_transport):
     await anext(resp)
 
 
-@patch('httpx.AsyncBaseTransport')
+@patch.object(httpx, 'AsyncBaseTransport')
 @pytest.mark.asyncio
 async def test_error_event_in_generate_content_stream_async(mock_transport):
   client = Client(
diff --git a/google/genai/tests/client/test_client_initialization.py b/google/genai/tests/client/test_client_initialization.py
index 5df241a7a1..d36706b272 100644
--- a/google/genai/tests/client/test_client_initialization.py
+++ b/google/genai/tests/client/test_client_initialization.py
@@ -28,7 +28,6 @@
 import certifi
 import google.auth
 from google.auth import credentials
-import httpx
 import pytest
 import requests
 
@@ -38,6 +37,15 @@
 from ... import Client
 from ... import types
 
+if api_client._get_http_client_backend() == "httpx2":
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+  _SyncHttpxClient = api_client.SyncHttpx2Client
+  _AsyncHttpxClient = api_client.AsyncHttpx2Client
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
+  _SyncHttpxClient = api_client.SyncHttpxClient
+  _AsyncHttpxClient = api_client.AsyncHttpxClient
+
 try:
   import aiohttp
 
@@ -1550,7 +1558,7 @@ def refresh_side_effect(request):
         text='{"candidates": [{"content": {"parts": [{"text": "response"}]}}]}',
     )
     mock_request = mock.Mock(return_value=mock_httpx_response)
-    monkeypatch.setattr(api_client.SyncHttpxClient, "send", mock_request)
+    monkeypatch.setattr(_SyncHttpxClient, "send", mock_request)
 
   # Reset credentials to test initialization to ensure the sync lock is tested.
   client._api_client._credentials = None
@@ -2161,7 +2169,7 @@ def test_sync_httpx_uses_mtls_endpoint_with_client_cert(mock_client_cert):
   assert not client._api_client._use_google_auth_sync()
   mock_send = mock.Mock(return_value=httpx.Response(200, text="{}"))
 
-  with mock.patch.object(api_client.SyncHttpxClient, "send", mock_send):
+  with mock.patch.object(_SyncHttpxClient, "send", mock_send):
     client._api_client.request("post", "models/gemini:generateContent", {})
 
   assert mock_send.call_args[0][0].url.host == (
@@ -2177,7 +2185,7 @@ async def test_async_httpx_uses_mtls_endpoint_with_client_cert(
   client = _mtls_test_client()
   mock_request = mock.AsyncMock(return_value=httpx.Response(200, text="{}"))
 
-  with mock.patch.object(api_client.AsyncHttpxClient, "request", mock_request):
+  with mock.patch.object(_AsyncHttpxClient, "request", mock_request):
     await client._api_client.async_request(
         "post", "models/gemini:generateContent", {}
     )
@@ -2195,7 +2203,7 @@ async def test_async_httpx_download_uses_mtls_endpoint_with_client_cert(
   client = _mtls_test_client()
   mock_request = mock.AsyncMock(return_value=httpx.Response(200, content=b""))
 
-  with mock.patch.object(api_client.AsyncHttpxClient, "request", mock_request):
+  with mock.patch.object(_AsyncHttpxClient, "request", mock_request):
     await client._api_client.async_download_file("files/abc:download")
 
   assert mock_request.call_args.kwargs["url"].startswith(
diff --git a/google/genai/tests/client/test_http_options.py b/google/genai/tests/client/test_http_options.py
index c5c08de54b..2dec9790b3 100644
--- a/google/genai/tests/client/test_http_options.py
+++ b/google/genai/tests/client/test_http_options.py
@@ -18,12 +18,16 @@
 
 from unittest import mock
 
-import httpx
 import pytest
 
 from ... import _api_client
 from ... import types
 
+if _api_client._get_http_client_backend() == 'httpx2':
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
+
 
 def test_patch_http_options_with_copies_all_fields():
   patch_options = types.HttpOptions(
diff --git a/google/genai/tests/client/test_httpx2_client.py b/google/genai/tests/client/test_httpx2_client.py
index dbb1480022..83cf571736 100644
--- a/google/genai/tests/client/test_httpx2_client.py
+++ b/google/genai/tests/client/test_httpx2_client.py
@@ -34,8 +34,10 @@
 
 try:
   import httpx2
+  import truststore
 except ImportError:
   httpx2 = None
+  truststore = None
 
 # Mark all tests in this module to be skipped if httpx2 is not available
 pytestmark = pytest.mark.skipif(
@@ -166,20 +168,44 @@ def test_httpx2_response_flows_through_sync_stream():
   assert chunks == ['{"first": 1}', '{"second": 2}']
 
 
-def test_default_client_uses_httpx(monkeypatch):
-  monkeypatch.delenv('GOOGLE_GENAI_HTTP_CLIENT', raising=False)
+def _client_classes(backend):
   target_api_client = getattr(api_client, 'public_api_client', api_client)
-  client = Client(api_key='fake_api_key')
-  assert isinstance(client._api_client._httpx_client, httpx.Client)
-  assert isinstance(
-      client._api_client._httpx_client, target_api_client.SyncHttpxClient
+  if backend == 'httpx2':
+    return (
+        httpx2.Client,
+        httpx2.AsyncClient,
+        target_api_client.SyncHttpx2Client,
+        target_api_client.AsyncHttpx2Client,
+    )
+  return (
+      httpx.Client,
+      httpx.AsyncClient,
+      target_api_client.SyncHttpxClient,
+      target_api_client.AsyncHttpxClient,
   )
-  assert isinstance(client._api_client._async_httpx_client, httpx.AsyncClient)
-  assert isinstance(
-      client._api_client._async_httpx_client, target_api_client.AsyncHttpxClient
+
+
+def _set_default_backend(monkeypatch, backend):
+  target_api_client = getattr(api_client, 'public_api_client', api_client)
+  monkeypatch.setattr(
+      target_api_client, '_DEFAULT_HTTP_CLIENT_BACKEND', backend
   )
 
 
+@pytest.mark.parametrize('default_backend', ['httpx', 'httpx2'])
+def test_default_client_uses_default_backend(monkeypatch, default_backend):
+  monkeypatch.delenv('GOOGLE_GENAI_HTTP_CLIENT', raising=False)
+  _set_default_backend(monkeypatch, default_backend)
+  sync_cls, async_cls, sdk_sync_cls, sdk_async_cls = _client_classes(
+      default_backend
+  )
+  client = Client(api_key='fake_api_key')
+  assert isinstance(client._api_client._httpx_client, sync_cls)
+  assert isinstance(client._api_client._httpx_client, sdk_sync_cls)
+  assert isinstance(client._api_client._async_httpx_client, async_cls)
+  assert isinstance(client._api_client._async_httpx_client, sdk_async_cls)
+
+
 def test_env_var_override_to_httpx(monkeypatch):
   monkeypatch.setenv('GOOGLE_GENAI_HTTP_CLIENT', 'httpx')
   target_api_client = getattr(api_client, 'public_api_client', api_client)
@@ -194,6 +220,7 @@ def test_env_var_override_to_httpx(monkeypatch):
   )
 
 
+
 def test_env_var_override_to_httpx2(monkeypatch):
   monkeypatch.setenv('GOOGLE_GENAI_HTTP_CLIENT', 'httpx2')
   target_api_client = getattr(api_client, 'public_api_client', api_client)
@@ -208,19 +235,27 @@ def test_env_var_override_to_httpx2(monkeypatch):
   )
 
 
-def test_env_var_auto(monkeypatch):
+
+
+@pytest.mark.parametrize('default_backend', ['httpx', 'httpx2'])
+def test_env_var_auto(monkeypatch, default_backend):
   monkeypatch.setenv('GOOGLE_GENAI_HTTP_CLIENT', 'auto')
-  target_api_client = getattr(api_client, 'public_api_client', api_client)
+  _set_default_backend(monkeypatch, default_backend)
+  sync_cls, *_ = _client_classes(default_backend)
   client = Client(api_key='fake_api_key')
-  assert isinstance(client._api_client._httpx_client, httpx.Client)
+  assert isinstance(client._api_client._httpx_client, sync_cls)
 
 
-def test_env_var_unrecognized_falls_back_to_default(monkeypatch, caplog):
+@pytest.mark.parametrize('default_backend', ['httpx', 'httpx2'])
+def test_env_var_unrecognized_falls_back_to_default(
+    monkeypatch, caplog, default_backend
+):
   monkeypatch.setenv('GOOGLE_GENAI_HTTP_CLIENT', 'unknown_client')
-  target_api_client = getattr(api_client, 'public_api_client', api_client)
+  _set_default_backend(monkeypatch, default_backend)
+  sync_cls, *_ = _client_classes(default_backend)
   with caplog.at_level('WARNING'):
     client = Client(api_key='fake_api_key')
-  assert isinstance(client._api_client._httpx_client, httpx.Client)
+  assert isinstance(client._api_client._httpx_client, sync_cls)
   assert 'Unrecognized GOOGLE_GENAI_HTTP_CLIENT' in caplog.text
 
 
@@ -232,6 +267,7 @@ def test_env_var_httpx2_missing_raises(monkeypatch):
     Client(api_key='fake_api_key')
 
 
+
 def test_env_var_httpx_missing_raises(monkeypatch):
   monkeypatch.setenv('GOOGLE_GENAI_HTTP_CLIENT', 'httpx')
   target_api_client = getattr(api_client, 'public_api_client', api_client)
@@ -240,19 +276,36 @@ def test_env_var_httpx_missing_raises(monkeypatch):
     Client(api_key='fake_api_key')
 
 
-def test_default_falls_back_to_httpx2_when_httpx_missing(monkeypatch):
+
+
+@pytest.mark.parametrize(
+    'default_backend,fallback_backend',
+    [('httpx', 'httpx2'), ('httpx2', 'httpx')],
+)
+def test_default_falls_back_when_default_backend_missing(
+    monkeypatch, default_backend, fallback_backend
+):
   monkeypatch.delenv('GOOGLE_GENAI_HTTP_CLIENT', raising=False)
   target_api_client = getattr(api_client, 'public_api_client', api_client)
-  monkeypatch.setattr(target_api_client, 'httpx', None)
-  client = Client(api_key='fake_api_key')
-  assert isinstance(client._api_client._httpx_client, httpx2.Client)
-  assert isinstance(
-      client._api_client._httpx_client, target_api_client.SyncHttpx2Client
-  )
-  assert isinstance(client._api_client._async_httpx_client, httpx2.AsyncClient)
-  assert isinstance(
-      client._api_client._async_httpx_client, target_api_client.AsyncHttpx2Client
+  _set_default_backend(monkeypatch, default_backend)
+  monkeypatch.setattr(target_api_client, default_backend, None)
+  sync_cls, async_cls, sdk_sync_cls, sdk_async_cls = _client_classes(
+      fallback_backend
   )
+  client = Client(api_key='fake_api_key')
+  assert isinstance(client._api_client._httpx_client, sync_cls)
+  assert isinstance(client._api_client._httpx_client, sdk_sync_cls)
+  assert isinstance(client._api_client._async_httpx_client, async_cls)
+  assert isinstance(client._api_client._async_httpx_client, sdk_async_cls)
+
+
+def test_default_raises_when_no_backend_installed(monkeypatch):
+  monkeypatch.delenv('GOOGLE_GENAI_HTTP_CLIENT', raising=False)
+  target_api_client = getattr(api_client, 'public_api_client', api_client)
+  monkeypatch.setattr(target_api_client, 'httpx', None)
+  monkeypatch.setattr(target_api_client, 'httpx2', None)
+  with pytest.raises(ImportError, match='Neither httpx nor httpx2'):
+    target_api_client._get_http_client_backend()
 
 
 # High-level feature tests: end-to-end generate_content and streaming with httpx2
@@ -866,6 +919,21 @@ def handler(request: httpx2.Request) -> httpx2.Response:
   assert attempts == 2
 
 
+def test_ensure_httpx2_ssl_ctx_default_uses_truststore(monkeypatch):
+  monkeypatch.delenv('SSL_CERT_FILE', raising=False)
+  monkeypatch.delenv('SSL_CERT_DIR', raising=False)
+  client_args, async_client_args = (
+      api_client.BaseApiClient._ensure_httpx_ssl_ctx(
+          HttpOptions(),
+          vertexai=False,
+          client_cls=api_client.SyncHttpx2Client,
+          async_client_cls=api_client.AsyncHttpx2Client,
+      )
+  )
+  assert isinstance(client_args['verify'], truststore.SSLContext)
+  assert isinstance(async_client_args['verify'], truststore.SSLContext)
+
+
 def test_ensure_httpx2_ssl_ctx_custom_verify():
   custom_ctx = ssl.create_default_context()
   client_args, async_client_args = (
diff --git a/google/genai/tests/client/test_retries.py b/google/genai/tests/client/test_retries.py
index 6332f2091f..a70244da67 100644
--- a/google/genai/tests/client/test_retries.py
+++ b/google/genai/tests/client/test_retries.py
@@ -36,13 +36,17 @@
   AsyncAuthorizedSession = mock.MagicMock()
 
 from google.oauth2 import credentials
-import httpx
 import tenacity
 
 from ... import _api_client as api_client
 from ... import errors
 from ... import types
 
+if api_client._get_http_client_backend() == 'httpx2':
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
+
 
 requires_aiohttp = pytest.mark.skipif(
     AIOHTTP_NOT_INSTALLED, reason="aiohttp is not installed, skipping test."
diff --git a/google/genai/tests/imports/test_no_optional_imports.py b/google/genai/tests/imports/test_no_optional_imports.py
index a65a3d4a90..2fe658d6c9 100644
--- a/google/genai/tests/imports/test_no_optional_imports.py
+++ b/google/genai/tests/imports/test_no_optional_imports.py
@@ -13,7 +13,9 @@
 # limitations under the License.
 #
 
+import asyncio
 import os
+import sys
 import pytest
 
 IS_NOT_GITHUB_ACTIONS = os.getenv('GITHUB_ACTIONS') != 'true'
@@ -22,7 +24,99 @@
 @pytest.mark.skipif(IS_NOT_GITHUB_ACTIONS,
                     reason='This test is only run on GitHub Actions.')
 def test_library_can_be_imported_without_optional_dependencies():
-  """Tests that the library can be imported without optional dependencies.
-  """
+  """Tests that the library works without optional dependencies (including httpx)."""
+  with pytest.raises(ImportError):
+    import httpx  # pylint: disable=g-import-not-at-top,unused-import
+  assert 'httpx' not in sys.modules
+
   from google import genai
+  from google.genai import errors
+  from google.genai import interactions
   from google.genai import types
+  import httpx2
+
+  assert interactions is not None
+
+  def handler(request: httpx2.Request) -> httpx2.Response:
+    path = request.url.path
+    if path.endswith(':streamGenerateContent'):
+      return httpx2.Response(
+          status_code=200,
+          content=(
+              b'data: {"candidates": [{"content": {"parts": [{"text": "Hello "}], "role": "model"}}]}\n\n'
+              b'data: {"candidates": [{"content": {"parts": [{"text": "world"}], "role": "model"}}]}\n\n'
+          ),
+          headers={'content-type': 'text/event-stream'},
+      )
+    if path.endswith('/models/bad-model:generateContent'):
+      return httpx2.Response(
+          status_code=400,
+          json={'error': {'code': 400, 'message': 'Bad request', 'status': 'INVALID_ARGUMENT'}},
+      )
+    if '/interactions/' in path:
+      return httpx2.Response(
+          status_code=200,
+          headers={'content-type': 'application/json'},
+          content=b'{"id": "test-id", "status": "completed"}',
+      )
+    return httpx2.Response(
+        status_code=200,
+        json={
+            'candidates': [{
+                'content': {'parts': [{'text': 'ok'}], 'role': 'model'},
+                'finishReason': 'STOP',
+            }]
+        },
+    )
+
+  transport = httpx2.MockTransport(handler)
+  client = genai.Client(
+      api_key='test-api-key',
+      http_options=types.HttpOptions(
+          client_args={'transport': transport},
+          async_client_args={'transport': transport},
+      ),
+  )
+  assert isinstance(client._api_client._httpx_client, httpx2.Client)
+  assert isinstance(client._api_client._async_httpx_client, httpx2.AsyncClient)
+
+  # Sync generate_content, streaming, error handling, and interactions.
+  resp = client.models.generate_content(model='gemini-2.5-flash', contents='hi')
+  assert resp.text == 'ok'
+
+  chunks = list(
+      client.models.generate_content_stream(
+          model='gemini-2.5-flash', contents='hi'
+      )
+  )
+  assert ''.join(c.text for c in chunks) == 'Hello world'
+
+  with pytest.raises(errors.ClientError):
+    client.models.generate_content(model='bad-model', contents='hi')
+
+  interaction = client.interactions.get(id='test-id')
+  assert interaction.status == 'completed'
+
+  # Async generate_content, streaming, error handling, and interactions.
+  async def _run_async() -> None:
+    async_resp = await client.aio.models.generate_content(
+        model='gemini-2.5-flash', contents='hi'
+    )
+    assert async_resp.text == 'ok'
+
+    async_chunks = [
+        c
+        async for c in await client.aio.models.generate_content_stream(
+            model='gemini-2.5-flash', contents='hi'
+        )
+    ]
+    assert ''.join(c.text for c in async_chunks) == 'Hello world'
+
+    with pytest.raises(errors.ClientError):
+      await client.aio.models.generate_content(model='bad-model', contents='hi')
+
+    async_interaction = await client.aio.interactions.get(id='test-id')
+    assert async_interaction.status == 'completed'
+
+  asyncio.run(_run_async())
+  assert 'httpx' not in sys.modules
diff --git a/google/genai/tests/interactions/test_auth.py b/google/genai/tests/interactions/test_auth.py
index 3873dbde5e..be37fe9267 100644
--- a/google/genai/tests/interactions/test_auth.py
+++ b/google/genai/tests/interactions/test_auth.py
@@ -17,14 +17,20 @@
 """Tests for Interactions API."""
 
 from ... import Client
+from ... import _api_client
 from ... import _base_url
 from unittest import mock
 import pytest
-from httpx import Request, Response
-from ..._api_client import AsyncHttpxClient, BaseApiClient
-from httpx import Client as HTTPClient
+from ..._api_client import BaseApiClient
 import os
 
+if _api_client._get_http_client_backend() == "httpx2":
+    from httpx2 import Client as HTTPClient, Request, Response  # pylint: disable=g-import-not-at-top
+    AsyncHttpxClient = _api_client.AsyncHttpx2Client
+else:
+    from httpx import Client as HTTPClient, Request, Response  # pylint: disable=g-import-not-at-top
+    AsyncHttpxClient = _api_client.AsyncHttpxClient
+
 ENV_VARS = [
     "GOOGLE_CLOUD_PROJECT",
     "GEMINI_API_KEY",
@@ -101,9 +107,6 @@ def test_interactions_gemini_extra_headers(monkeypatch):
 
 
 def test_interactions_vertex_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=True, project='test-project', location='us-central1', credentials=creds)
@@ -135,9 +138,6 @@ def test_interactions_vertex_auth_header():
         for key, value in headers.items())
 
 def test_interactions_vertex_key_no_auth_header():
-  from ..._api_client import BaseApiClient
-  from httpx import Client as HTTPClient
-
   creds = mock.Mock()
   client = Client(vertexai=True, api_key='test-api-key')
 
@@ -171,7 +171,7 @@ def test_interactions_vertex_url():
 
     with (
         mock.patch.object(BaseApiClient, "_access_token", return_value="fake-token") as mock_access_token,
-        mock.patch("httpx.Client.send") as mock_send,
+        mock.patch.object(HTTPClient, "send") as mock_send,
     ):
         mock_send.return_value = Response(200, request=Request('POST', ''), headers={'content-type': 'application/json'}, content='{"status": "completed"}')
         client.interactions.create(
@@ -183,9 +183,6 @@ def test_interactions_vertex_url():
         assert str(request.url) == 'https://us-central1-aiplatform.googleapis.com/v1beta1/projects/test-project/locations/us-central1/interactions'
 
 def test_interactions_vertex_auth_refresh_on_retry():
-    from ..._api_client import BaseApiClient
-    from httpx import Client as HTTPClient
-
     creds = mock.Mock()
     creds.quota_project_id = "test-quota-project"
     client = Client(vertexai=True, project='test-project', location='us-central1', credentials=creds)
@@ -223,9 +220,6 @@ def my_send(req, *args, **kwargs):
 
 
 def test_interactions_vertex_extra_headers_override():
-    from ..._api_client import BaseApiClient
-    from httpx import Client as HTTPClient
-
     creds = mock.Mock()
     creds.quota_project_id = "test-quota-project"
     client = Client(vertexai=True, project='test-project', location='us-central1', credentials=creds)
@@ -326,9 +320,6 @@ async def test_async_interactions_gemini_extra_headers(monkeypatch):
 
 @pytest.mark.asyncio
 async def test_async_interactions_vertex_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=True, project='test-project', location='us-central1', credentials=creds)
@@ -361,7 +352,6 @@ async def test_async_interactions_vertex_auth_header():
 
 @pytest.mark.asyncio
 async def test_async_interactions_vertex_key_no_auth_header():
-  from ..._api_client import BaseApiClient
   client = Client(vertexai=True, api_key='test-api-key')
 
   with (
@@ -389,7 +379,6 @@ async def test_async_interactions_vertex_key_no_auth_header():
 
 @pytest.mark.asyncio
 async def test_async_interactions_vertex_url():
-    from ..._api_client import AsyncHttpxClient
     creds = mock.Mock()
     creds.quota_project_id = "test-quota-project"
     client = Client(vertexai=True, project='test-project', location='us-central1', credentials=creds)
@@ -409,9 +398,6 @@ async def test_async_interactions_vertex_url():
 
 @pytest.mark.asyncio
 async def test_async_interactions_vertex_auth_refresh_on_retry():
-    from ..._api_client import BaseApiClient
-    from ..._api_client import AsyncHttpxClient
-
     creds = mock.Mock()
     creds.quota_project_id = "test-quota-project"
     client = Client(vertexai=True, project='test-project', location='us-central1', credentials=creds)
@@ -448,9 +434,6 @@ async def my_send(req, *args, **kwargs):
 
 @pytest.mark.asyncio
 async def test_async_interactions_vertex_extra_headers_override():
-    from ..._api_client import BaseApiClient
-    from ..._api_client import AsyncHttpxClient
-
     creds = mock.Mock()
     creds.quota_project_id = "test-quota-project"
     client = Client(vertexai=True, project='test-project', location='us-central1', credentials=creds)
diff --git a/google/genai/tests/interactions/test_integration.py b/google/genai/tests/interactions/test_integration.py
index 22edf727c7..d142509dd9 100644
--- a/google/genai/tests/interactions/test_integration.py
+++ b/google/genai/tests/interactions/test_integration.py
@@ -14,14 +14,19 @@
 
 from unittest import mock
 
-import httpx
 import pytest
 
+from ... import _api_client
 from ... import client as client_lib
 from ..._gaos import google_genai as gaos_google_genai
 from ..._gaos.types.interactions import interaction as gaos_interaction
 from ..._gaos.utils import serializers as gaos_serializers
 
+if _api_client._get_http_client_backend() == "httpx2":
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
+
 
 pytest_plugins = ("pytest_asyncio",)
 
@@ -235,7 +240,7 @@ def test_interaction_lenient_parsing():
 def test_interactions_default_timeout():
   from ... import _interactions
 
-  assert _interactions.DEFAULT_TIMEOUT == httpx.Timeout(
+  assert _interactions.DEFAULT_TIMEOUT == _interactions.Timeout(
       timeout=None, connect=5.0
   )
 
diff --git a/google/genai/tests/interactions/test_paths.py b/google/genai/tests/interactions/test_paths.py
index 118a8e62d7..335a058ad0 100644
--- a/google/genai/tests/interactions/test_paths.py
+++ b/google/genai/tests/interactions/test_paths.py
@@ -19,12 +19,17 @@
 
 from unittest import mock
 import pytest
-from httpx import Request, Response
-from ..._api_client import AsyncHttpxClient
-from httpx import Client as HTTPClient
+from ... import _api_client
 from .. import pytest_helper
 import google.auth
 
+if _api_client._get_http_client_backend() == "httpx2":
+    from httpx2 import Client as HTTPClient, Request, Response  # pylint: disable=g-import-not-at-top
+    AsyncHttpxClient = _api_client.AsyncHttpx2Client
+else:
+    from httpx import Client as HTTPClient, Request, Response  # pylint: disable=g-import-not-at-top
+    AsyncHttpxClient = _api_client.AsyncHttpxClient
+
 @mock.patch.object(google.auth, "default", autospec=True)
 def test_interactions_paths(mock_auth_default, client):
     interaction_id = "test-interaction-id"
@@ -50,6 +55,12 @@ def test_interactions_paths(mock_auth_default, client):
         mock_send.assert_called_once()
         request = mock_send.call_args[0][0]
         assert str(request.url) == f'{expected_base_url}/interactions/{interaction_id}?stream=false'
+        assert request.extensions.get("timeout") == {
+            "connect": None,
+            "read": None,
+            "write": None,
+            "pool": None,
+        }
 
         mock_send.reset_mock()
         mock_send.return_value = Response(200, request=Request('POST', ''), headers={'content-type': 'application/json'}, content='{"status": "completed"}')
@@ -91,6 +102,12 @@ async def test_async_interactions_paths(mock_auth_default, client):
         mock_send.assert_called_once()
         request = mock_send.call_args[0][0]
         assert str(request.url) == f'{expected_base_url}/interactions/{interaction_id}?stream=false'
+        assert request.extensions.get("timeout") == {
+            "connect": None,
+            "read": None,
+            "write": None,
+            "pool": None,
+        }
 
         mock_send.reset_mock()
         mock_send.return_value = Response(200, request=Request('POST', ''), headers={'content-type': 'application/json'}, content='{"status": "completed"}')
diff --git a/google/genai/tests/live/test_live.py b/google/genai/tests/live/test_live.py
index dda2286cd6..1297a0a0c9 100644
--- a/google/genai/tests/live/test_live.py
+++ b/google/genai/tests/live/test_live.py
@@ -31,7 +31,6 @@
 
 import certifi
 from google.oauth2.credentials import Credentials
-import httpx
 import pytest
 from websockets import client
 
@@ -42,6 +41,11 @@
 from ... import client as gl_client
 from ... import live
 from ... import types
+
+if api_client._get_http_client_backend() == 'httpx2':
+  import httpx2 as httpx  # pylint: disable=g-import-not-at-top
+else:
+  import httpx  # pylint: disable=g-import-not-at-top
 try:
     import aiohttp
     AIOHTTP_NOT_INSTALLED = False
diff --git a/google/genai/tests/local_tokenizer/test_local_tokenizer.py b/google/genai/tests/local_tokenizer/test_local_tokenizer.py
index 1d9a12db9f..2bb09c1915 100644
--- a/google/genai/tests/local_tokenizer/test_local_tokenizer.py
+++ b/google/genai/tests/local_tokenizer/test_local_tokenizer.py
@@ -16,7 +16,12 @@
 import unittest
 from unittest.mock import MagicMock, patch
 
-from sentencepiece import sentencepiece_model_pb2
+import pytest
+
+try:
+  from sentencepiece import sentencepiece_model_pb2
+except ImportError:
+  pytest.skip('sentencepiece is not installed', allow_module_level=True)
 
 from ... import local_tokenizer
 from ... import types
diff --git a/google/genai/tests/local_tokenizer/test_local_tokenizer_loader.py b/google/genai/tests/local_tokenizer/test_local_tokenizer_loader.py
index 492963e6c7..e404d8d02b 100644
--- a/google/genai/tests/local_tokenizer/test_local_tokenizer_loader.py
+++ b/google/genai/tests/local_tokenizer/test_local_tokenizer_loader.py
@@ -16,8 +16,13 @@
 import unittest
 from unittest.mock import MagicMock, mock_open, patch
 
-import sentencepiece as spm
-from sentencepiece import sentencepiece_model_pb2
+import pytest
+
+try:
+  import sentencepiece as spm
+  from sentencepiece import sentencepiece_model_pb2
+except ImportError:
+  pytest.skip('sentencepiece is not installed', allow_module_level=True)
 
 from ... import _local_tokenizer_loader as loader
 
diff --git a/pyproject.toml b/pyproject.toml
index d0396e651d..d7fdcb57ba 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,7 +28,7 @@ classifiers = [
 dependencies = [
     "anyio>=4.8.0, <5.0.0",
     "google-auth[requests]>=2.56.0, <3.0.0",
-    "httpx>=0.28.1, <1.0.0",
+    "httpx2>=2.0.0, <3.0.0",
     "pydantic>=2.12.5, <3.0.0",
     "requests>=2.28.1, <3.0.0",
     "tenacity>=8.2.3, <9.2.0",
@@ -40,7 +40,8 @@ dependencies = [
 
 [project.optional-dependencies]
 aiohttp = ["aiohttp>=3.10.11, <4.0.0"]
-httpx2 = ["httpx2>=2.0.0, <3.0.0"]
+httpx = ["httpx>=0.28.1, <1.0.0"]
+httpx2 = []
 local-tokenizer = [
     "sentencepiece>=0.2.0",
     "protobuf",
diff --git a/requirements.txt b/requirements.txt
index c8f6e0aa4d..2453ad31d4 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -34,5 +34,5 @@ typing_extensions>=4.14.1
 urllib3==2.5.0
 websockets==16.0
 mcp>=1.14.0,<2.0.0; python_version > '3.9'
-sentencepiece>=0.2.0
+sentencepiece>=0.2.0; python_version < '3.15'
 protobuf