diff --git a/google/genai/_gaos/_hooks/google_genai_auth.py b/google/genai/_gaos/_hooks/google_genai_auth.py
index 7ec30b9d8..fba57d60c 100644
--- a/google/genai/_gaos/_hooks/google_genai_auth.py
+++ b/google/genai/_gaos/_hooks/google_genai_auth.py
@@ -24,7 +24,8 @@
 import httpx
 
 from .. import types, utils
-from .types import BeforeRequestContext, BeforeRequestHook
+from ..sdkconfiguration import SDKConfiguration
+from .types import BeforeRequestContext, BeforeRequestHook, SDKInitHook
 
 
 GOOGLE_GENAI_API_REVISION = "2026-05-20"
@@ -57,9 +58,14 @@ def consume(self) -> None:
         self._pending_access_token = None
 
 
-class GoogleGenAIAuthHook(BeforeRequestHook):
+class GoogleGenAIAuthHook(SDKInitHook, BeforeRequestHook):
     """Applies Google GenAI headers and custom auth before sending requests."""
 
+    def sdk_init(self, config: SDKConfiguration) -> SDKConfiguration:
+        if config.globals.api_version is None:
+            config.globals.api_version = "v1beta"
+        return config
+
     def before_request(
         self,
         hook_ctx: BeforeRequestContext,
diff --git a/google/genai/_gaos/_hooks/sdkhooks.py b/google/genai/_gaos/_hooks/sdkhooks.py
index 5dc2b7bc3..32854e369 100644
--- a/google/genai/_gaos/_hooks/sdkhooks.py
+++ b/google/genai/_gaos/_hooks/sdkhooks.py
@@ -45,6 +45,7 @@ def __init__(self) -> None:
         self.after_error_hooks: List[AfterErrorHook] = []
         self.after_parse_error_hooks: List[AfterParseErrorHook] = []
         init_hooks(self)
+        self.register_sdk_init_hook(GoogleGenAIAuthHook())
         self.register_before_request_hook(GoogleGenAIAuthHook())
         self.register_after_error_hook(cast(AfterErrorHook, CompatErrorHook()))
         self.register_after_parse_error_hook(
diff --git a/google/genai/tests/interactions/test_integration.py b/google/genai/tests/interactions/test_integration.py
index 22edf727c..cee40e778 100644
--- a/google/genai/tests/interactions/test_integration.py
+++ b/google/genai/tests/interactions/test_integration.py
@@ -153,7 +153,6 @@ async def test_async_client_genai_default_timeout():
   assert async_sdk_client.timeout != httpx.Timeout(5.0)
 
 
-
 @pytest.mark.filterwarnings("error")
 def test_unrecognized_model_serialization():
   from ..._gaos.types.interactions.createmodelinteraction import CreateModelInteraction
@@ -340,3 +339,21 @@ def test_output_text_across_steps(steps, expected):
 
   wrapped = gaos_google_genai._add_output_properties_if_interaction(payload)  # pylint: disable=protected-access
   assert wrapped["output_text"] == expected
+
+
+def test_sdk_init_hook_defaults_and_preserves_api_version():
+  """Tests SDKInitHook default and explicit api_version behavior."""
+  from ..._gaos import sdk as gaos_sdk  # pylint: disable=g-import-not-at-top
+
+  default_sync = gaos_sdk.GenAI()
+  assert default_sync.sdk_configuration.globals.api_version == "v1beta"
+
+  explicit_sync = gaos_sdk.GenAI(api_version="v1alpha")
+  assert explicit_sync.sdk_configuration.globals.api_version == "v1alpha"
+
+  default_async = gaos_sdk.AsyncGenAI()
+  assert default_async.sdk_configuration.globals.api_version == "v1beta"
+
+  explicit_async = gaos_sdk.AsyncGenAI(api_version="v1alpha")
+  assert explicit_async.sdk_configuration.globals.api_version == "v1alpha"
+