diff --git a/src/aws_durable_execution_sdk_python/lambda_service.py b/src/aws_durable_execution_sdk_python/lambda_service.py
index 51a0049..aa78e4e 100644
--- a/src/aws_durable_execution_sdk_python/lambda_service.py
+++ b/src/aws_durable_execution_sdk_python/lambda_service.py
@@ -36,6 +36,15 @@
 logger = logging.getLogger(__name__)
 
 
+def _is_in_var_dir(module_file: str = __file__) -> bool:
+    """Return True if this SDK is installed under /var/lang/.
+
+    Lambda bundled Python runtimes install packages at
+    /var/lang/lib/pythonX.Y/site-packages/.
+    """
+    return module_file.startswith("/var/lang/")
+
+
 # region model
 class OperationAction(Enum):
     START = "START"
@@ -1061,7 +1070,7 @@ def initialize_client(cls) -> LambdaClient:
                 config=Config(
                     connect_timeout=5,
                     read_timeout=50,
-                    user_agent_extra=f"aws-durable-execution-sdk-python/{__version__}",
+                    user_agent_extra=f"aws-durable-execution-sdk-python/{__version__}{'-bundled' if _is_in_var_dir() else ''}",
                 ),
             )
         return cls(client=cls._cached_boto_client)
diff --git a/tests/lambda_service_test.py b/tests/lambda_service_test.py
index 9c9def5..626be29 100644
--- a/tests/lambda_service_test.py
+++ b/tests/lambda_service_test.py
@@ -38,6 +38,7 @@
     TimestampConverter,
     WaitDetails,
     WaitOptions,
+    _is_in_var_dir,
 )
 
 
@@ -2047,6 +2048,76 @@ def test_lambda_client_initialize_client_no_endpoint(
     assert isinstance(client, LambdaClient)
 
 
+@patch(
+    "aws_durable_execution_sdk_python.lambda_service._is_in_var_dir",
+    return_value=True,
+)
+@patch("boto3.client")
+def test_lambda_client_user_agent_runtime_bundled(
+    mock_boto_client, _mock_is_in_var_dir, reset_lambda_client_cache
+):
+    """user_agent_extra includes -bundled when SDK is in /var/lang/."""
+    mock_client = Mock()
+    mock_boto_client.return_value = mock_client
+
+    client = LambdaClient.initialize_client()
+
+    call_args = mock_boto_client.call_args
+    config = call_args[1]["config"]
+    assert (
+        config.user_agent_extra
+        == f"aws-durable-execution-sdk-python/{__version__}-bundled"
+    )
+    assert isinstance(client, LambdaClient)
+
+
+@patch(
+    "aws_durable_execution_sdk_python.lambda_service._is_in_var_dir",
+    return_value=False,
+)
+@patch("boto3.client")
+def test_lambda_client_user_agent_not_runtime_bundled(
+    mock_boto_client, _mock_is_in_var_dir, reset_lambda_client_cache
+):
+    """user_agent_extra omits -bundled when SDK is not in /var/lang."""
+    mock_client = Mock()
+    mock_boto_client.return_value = mock_client
+
+    client = LambdaClient.initialize_client()
+
+    call_args = mock_boto_client.call_args
+    config = call_args[1]["config"]
+    assert config.user_agent_extra == f"aws-durable-execution-sdk-python/{__version__}"
+    assert isinstance(client, LambdaClient)
+
+
+@pytest.mark.parametrize(
+    "path,expected",
+    [
+        # Lambda bundled runtime site-packages
+        (
+            "/var/lang/lib/python3.13/site-packages/aws_durable_execution_sdk_python/lambda_service.py",
+            True,
+        ),
+        (
+            "/var/lang/lib/python3.12/site-packages/aws_durable_execution_sdk_python/lambda_service.py",
+            True,
+        ),
+        # Customer deployment package
+        ("/var/task/aws_durable_execution_sdk_python/lambda_service.py", False),
+        # Lambda Layer
+        ("/opt/python/aws_durable_execution_sdk_python/lambda_service.py", False),
+        # Trailing-slash guard: /var/langsurprise must not match
+        ("/var/langsurprise/lib/python3.13/site-packages/x.py", False),
+        # Local dev
+        ("/Users/me/project/.venv/lib/python3.12/site-packages/x.py", False),
+        ("", False),
+    ],
+)
+def test_is_in_var_dir(path, expected):
+    assert _is_in_var_dir(path) is expected
+
+
 def test_lambda_client_checkpoint_with_non_none_client_token():
     """Test LambdaClient.checkpoint with non-None client_token."""
     mock_client = Mock()