diff --git a/services/constrained_transforms.py b/services/constrained_transforms.py
index a846451..5ad7abe 100644
--- a/services/constrained_transforms.py
+++ b/services/constrained_transforms.py
@@ -80,10 +80,24 @@ class TransformTimeoutError(Exception):
 
 class TransformExecutor:
     """
-    Executes registered transforms with strict runtime constraints.
+    Legacy in-process transform executor (DEPRECATED — use TransformProcessRunner).
+
+    WARNING: This executor runs transforms in a daemon thread within the host
+    process.  Python threads cannot be forcefully terminated, so a malicious or
+    runaway transform can escape the timeout constraint and continue executing
+    indefinitely.  It also shares the host process address space, making
+    network/filesystem isolation best-effort only.
+
+    New code should always use ``TransformProcessRunner`` (S35) which executes
+    transforms in an isolated subprocess with enforced timeouts via
+    ``subprocess.run(timeout=…)`` and network access denied via socket
+    monkeypatching.
+
+    ``get_transform_executor()`` will refuse to fall back to this class and
+    instead fail closed if ``TransformProcessRunner`` is unavailable.
 
     Enforces:
-    - Timeout per transform
+    - Timeout per transform (best-effort — thread cannot be killed)
     - Output size cap
     - Integrity verification before execution
     - No network/filesystem access (best-effort: module is pre-vetted)
@@ -288,26 +302,33 @@ def get_transform_executor() -> TransformExecutor:
     """
     Get or create the global transform executor.
 
-    If S35 isolation is enabled (default: True in this hardening wave),
-    returns a TransformProcessRunner instance.
+    Always returns a ``TransformProcessRunner`` instance (S35 process
+    isolation).  If the subprocess runner cannot be imported, this function
+    raises ``RuntimeError`` rather than falling back to the in-process
+    ``TransformExecutor``, which cannot enforce timeouts or isolation.
     """
     global _executor
     if _executor is None:
-        # Check for process isolation flag (defaulting to on for S35)
-        # We can use the same enable flag, or a specific isolation one.
-        # Let's assume strict isolation is part of the enabling.
-
         # Local import to avoid circular dependency
         try:
             from .transform_runner import TransformProcessRunner
 
             registry = get_transform_registry()
-            # We treat TransformProcessRunner as compatible with TransformExecutor interface
             _executor = TransformProcessRunner(registry)  # type: ignore
         except ImportError as e:
-            logger.warning(
-                f"S35: Could not import transform_runner ({e}), falling back to thread executor."
+            # Fail closed: do NOT fall back to the thread-based executor.
+            # The in-process TransformExecutor uses threading.Thread which
+            # cannot be forcefully terminated, allowing a malicious transform
+            # to escape timeout constraints and continue executing in the
+            # host process indefinitely.
+            logger.error(
+                f"S35: Could not import transform_runner ({e}). "
+                "Refusing to fall back to in-process executor for security reasons. "
+                "Transforms will be unavailable until the subprocess runner is restored."
             )
-            _executor = TransformExecutor(get_transform_registry())
+            raise RuntimeError(
+                "TransformProcessRunner unavailable — transforms disabled for security. "
+                f"Import error: {e}"
+            ) from e
 
     return _executor
diff --git a/tests/test_s35_transform_isolation.py b/tests/test_s35_transform_isolation.py
index 20414d3..784afe1 100644
--- a/tests/test_s35_transform_isolation.py
+++ b/tests/test_s35_transform_isolation.py
@@ -131,5 +131,55 @@ def transform(data):
                 os.remove(module_path)
 
 
+class TestGetTransformExecutorFailClosed(unittest.TestCase):
+    """Test that get_transform_executor refuses to fall back to in-process executor."""
+
+    def setUp(self):
+        # Reset the global singleton before each test
+        import services.constrained_transforms as ct
+
+        ct._executor = None
+
+    def tearDown(self):
+        import services.constrained_transforms as ct
+
+        ct._executor = None
+
+    def test_raises_when_process_runner_unavailable(self):
+        """get_transform_executor must raise RuntimeError, not fall back."""
+        import importlib
+
+        import services.constrained_transforms as ct
+
+        # Force the module to re-evaluate the local import by removing the
+        # transform_runner module from sys.modules temporarily.
+        saved = sys.modules.pop("services.transform_runner", None)
+        # Also remove any cached reference
+        sys.modules["services.transform_runner"] = None  # type: ignore[assignment]
+        try:
+            with self.assertRaises((RuntimeError, ImportError)) as ctx:
+                ct.get_transform_executor()
+            error_msg = str(ctx.exception).lower()
+            self.assertTrue(
+                "unavailable" in error_msg or "import" in error_msg,
+                f"Expected 'unavailable' or 'import' in error: {ctx.exception}",
+            )
+        finally:
+            # Restore
+            if saved is not None:
+                sys.modules["services.transform_runner"] = saved
+            else:
+                sys.modules.pop("services.transform_runner", None)
+
+    def test_returns_process_runner_when_available(self):
+        """get_transform_executor returns TransformProcessRunner normally."""
+        from services.constrained_transforms import get_transform_executor
+
+        executor = get_transform_executor()
+        from services.transform_runner import TransformProcessRunner
+
+        self.assertIsInstance(executor, TransformProcessRunner)
+
+
 if __name__ == "__main__":
     unittest.main()