diff --git a/google/genai/client.py b/google/genai/client.py
index 55663b888..942391353 100644
--- a/google/genai/client.py
+++ b/google/genai/client.py
@@ -43,6 +43,7 @@
 from . import _common
 
 from ._interactions import AsyncGeminiNextGenAPIClient, DEFAULT_MAX_RETRIES, GeminiNextGenAPIClient
+from ._interactions._types import NOT_GIVEN
 from . import _interactions
 
 from ._interactions.resources import AsyncInteractionsResource as AsyncNextGenInteractionsResource, InteractionsResource as NextGenInteractionsResource
@@ -175,7 +176,9 @@ def _nextgen_client(self) -> AsyncGeminiNextGenAPIClient:
         default_headers=http_opts.headers,
         http_client=http_client,
         # uSDk expects ms, nextgen uses a httpx Timeout -> expects seconds.
-        timeout=http_opts.timeout / 1000 if http_opts.timeout else None,
+        # Pass NOT_GIVEN (not None) when unset so the Stainless client uses
+        # its DEFAULT_TIMEOUT.  httpx treats None as "no timeout".
+        timeout=http_opts.timeout / 1000 if http_opts.timeout else NOT_GIVEN,
         max_retries=max_retries,
         client_adapter=AsyncGeminiNextGenAPIClientAdapter(self._api_client)
     )
@@ -552,7 +555,9 @@ def _nextgen_client(self) -> GeminiNextGenAPIClient:
         default_headers=http_opts.headers,
         http_client=self._api_client._httpx_client,
         # uSDk expects ms, nextgen uses a httpx Timeout -> expects seconds.
-        timeout=http_opts.timeout / 1000 if http_opts.timeout else None,
+        # Pass NOT_GIVEN (not None) when unset so the Stainless client uses
+        # its DEFAULT_TIMEOUT.  httpx treats None as "no timeout".
+        timeout=http_opts.timeout / 1000 if http_opts.timeout else NOT_GIVEN,
         max_retries=max_retries,
         client_adapter=GeminiNextGenAPIClientAdapter(self._api_client),
     )
diff --git a/google/genai/tests/interactions/test_default_timeout.py b/google/genai/tests/interactions/test_default_timeout.py
new file mode 100644
index 000000000..125b5c38e
--- /dev/null
+++ b/google/genai/tests/interactions/test_default_timeout.py
@@ -0,0 +1,131 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for default timeout behavior of the interactions client.
+
+When no timeout is configured in http_options, the wrapper must pass
+NOT_GIVEN to the Stainless client so it falls back to DEFAULT_TIMEOUT
+(60s).  Previously it passed None, which httpx interprets as "no timeout"
+causing requests to hang indefinitely.
+"""
+
+from unittest import mock
+
+import pytest
+
+from ... import client as client_lib
+from ..._interactions._types import NotGiven
+
+pytest_plugins = ("pytest_asyncio",)
+
+
+class TestSyncDefaultTimeout:
+  """Sync client default timeout tests."""
+
+  def test_default_timeout_is_not_given(self):
+    """When no timeout is set, NOT_GIVEN (not None) should be passed."""
+    with mock.patch.object(
+        client_lib, "GeminiNextGenAPIClient", spec_set=True
+    ) as mock_nextgen_client:
+      client = client_lib.Client(
+          api_key="placeholder",
+          http_options={"api_version": "v1alpha"},
+      )
+      _ = client.interactions
+
+      timeout_arg = mock_nextgen_client.call_args.kwargs["timeout"]
+      assert isinstance(timeout_arg, NotGiven), (
+          f"Expected timeout to be NOT_GIVEN but got {timeout_arg!r}. "
+          f"None would disable httpx timeouts entirely."
+      )
+
+  def test_explicit_timeout_passes_through(self):
+    """When timeout is set, it should pass through as seconds (ms / 1000)."""
+    with mock.patch.object(
+        client_lib, "GeminiNextGenAPIClient", spec_set=True
+    ) as mock_nextgen_client:
+      client = client_lib.Client(
+          api_key="placeholder",
+          http_options={"api_version": "v1alpha", "timeout": 30000},
+      )
+      _ = client.interactions
+
+      mock_nextgen_client.assert_called_once_with(
+          base_url=mock.ANY,
+          api_key="placeholder",
+          api_version="v1alpha",
+          default_headers=mock.ANY,
+          http_client=mock.ANY,
+          timeout=30.0,
+          max_retries=mock.ANY,
+          client_adapter=mock.ANY,
+      )
+
+  def test_no_http_options_uses_not_given(self):
+    """When no http_options at all, timeout should still be NOT_GIVEN."""
+    with mock.patch.object(
+        client_lib, "GeminiNextGenAPIClient", spec_set=True
+    ) as mock_nextgen_client:
+      client = client_lib.Client(api_key="placeholder")
+      _ = client.interactions
+
+      timeout_arg = mock_nextgen_client.call_args.kwargs["timeout"]
+      assert isinstance(timeout_arg, NotGiven), (
+          f"Expected timeout to be NOT_GIVEN but got {timeout_arg!r}."
+      )
+
+
+class TestAsyncDefaultTimeout:
+  """Async client default timeout tests."""
+
+  @pytest.mark.asyncio
+  async def test_default_timeout_is_not_given(self):
+    """When no timeout is set, NOT_GIVEN (not None) should be passed."""
+    with mock.patch.object(
+        client_lib, "AsyncGeminiNextGenAPIClient", spec_set=True
+    ) as mock_nextgen_client:
+      client = client_lib.Client(
+          api_key="placeholder",
+          http_options={"api_version": "v1alpha"},
+      )
+      _ = client.aio.interactions
+
+      timeout_arg = mock_nextgen_client.call_args.kwargs["timeout"]
+      assert isinstance(timeout_arg, NotGiven), (
+          f"Expected timeout to be NOT_GIVEN but got {timeout_arg!r}. "
+          f"None would disable httpx timeouts entirely."
+      )
+
+  @pytest.mark.asyncio
+  async def test_explicit_timeout_passes_through(self):
+    """When timeout is set, it should pass through as seconds (ms / 1000)."""
+    with mock.patch.object(
+        client_lib, "AsyncGeminiNextGenAPIClient", spec_set=True
+    ) as mock_nextgen_client:
+      client = client_lib.Client(
+          api_key="placeholder",
+          http_options={"api_version": "v1alpha", "timeout": 30000},
+      )
+      _ = client.aio.interactions
+
+      mock_nextgen_client.assert_called_once_with(
+          base_url=mock.ANY,
+          api_key="placeholder",
+          api_version="v1alpha",
+          default_headers=mock.ANY,
+          http_client=mock.ANY,
+          timeout=30.0,
+          max_retries=mock.ANY,
+          client_adapter=mock.ANY,
+      )