diff --git a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/conftest.py b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/conftest.py
index 54d457a31..86e3d14df 100644
--- a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/conftest.py
+++ b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/conftest.py
@@ -15,7 +15,7 @@
 import socket
 import struct
 import threading
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 import pytest
 
@@ -388,6 +388,13 @@ def unregister_service(self, service_id: int, instance_id: int):
         ]
 
 
+@pytest.fixture(autouse=True)
+def _mock_get_ext():
+    """Ensure the native-extension guard passes in all tests by default."""
+    with patch("jumpstarter_driver_someip.driver.get_ext", return_value=object()):
+        yield
+
+
 @pytest.fixture
 def stateful_osip():
     """Provide a fresh StatefulOsipClient instance."""
diff --git a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver.py b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver.py
index 53f48f623..b78b79ad7 100755
--- a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver.py
+++ b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver.py
@@ -7,6 +7,10 @@
 
 from opensomeip import ClientConfig, TransportMode
 from opensomeip import SomeIpClient as OsipClient
+try:
+    from opensomeip._bridge import get_ext
+except ImportError:
+    get_ext = lambda: None  # noqa: E731
 from opensomeip.message import Message
 from opensomeip.sd import SdConfig, ServiceInstance
 from opensomeip.transport import Endpoint
@@ -91,6 +95,18 @@ def __post_init__(self):
             raise ValueError(
                 f"Invalid transport_mode: {self.transport_mode!r}. Must be 'TCP' or 'UDP'."
             )
+
+        if get_ext() is None:
+            raise RuntimeError(
+                "opensomeip C++ extension (_opensomeip) is not available. "
+                "The SOME/IP driver requires the native extension for network I/O. "
+                "On macOS, rebuild with the system compiler: "
+                "CC=/usr/bin/clang CXX=/usr/bin/clang++ "
+                "pip install --no-cache-dir --force-reinstall --no-binary=opensomeip opensomeip. "
+                "On other platforms, reinstall opensomeip and ensure the C++ "
+                "build toolchain is available."
+            )
+
         mode = TransportMode.TCP if transport_upper == "TCP" else TransportMode.UDP
 
         local_ep = Endpoint(self.host, self.port)
diff --git a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver_test.py b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver_test.py
index 755fcf1a5..8e36a3416 100644
--- a/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver_test.py
+++ b/python/packages/jumpstarter-driver-someip/jumpstarter_driver_someip/driver_test.py
@@ -345,6 +345,12 @@ def test_someip_invalid_port_type():
         SomeIp(host="127.0.0.1", port="not_a_port")  # ty: ignore[invalid-argument-type]
 
 
+@patch("jumpstarter_driver_someip.driver.get_ext", return_value=None)
+def test_someip_raises_when_native_extension_unavailable(_mock_get_ext):
+    with pytest.raises(RuntimeError, match=r"_opensomeip.*not available"):
+        SomeIp(host="127.0.0.1", port=30490)
+
+
 @patch("jumpstarter_driver_someip.driver.OsipClient")
 def test_someip_invalid_transport_mode(mock_osip_cls):
     mock_osip_cls.return_value = _make_mock_osip_client()