diff --git a/python/cocoindex/subprocess_exec.py b/python/cocoindex/subprocess_exec.py
index db8f22f8c..cfb36ab9b 100644
--- a/python/cocoindex/subprocess_exec.py
+++ b/python/cocoindex/subprocess_exec.py
@@ -16,8 +16,11 @@
 import pickle
 import threading
 import asyncio
+import os
+import time
 from .user_app_loader import load_user_app
 
+WATCHDOG_INTERVAL_SECONDS = 10.0
 
 # ---------------------------------------------
 # Main process: single, lazily-created pool
@@ -33,7 +36,9 @@ def _get_pool() -> ProcessPoolExecutor:
         if _pool is None:
             # Single worker process as requested
             _pool = ProcessPoolExecutor(
-                max_workers=1, initializer=_subprocess_init, initargs=(_user_apps,)
+                max_workers=1,
+                initializer=_subprocess_init,
+                initargs=(_user_apps, os.getpid()),
             )
         return _pool
 
@@ -48,7 +53,33 @@ def add_user_app(app_target: str) -> None:
 # ---------------------------------------------
 
 
-def _subprocess_init(user_apps: list[str]) -> None:
+def _start_parent_watchdog(
+    parent_pid: int, interval_seconds: float = WATCHDOG_INTERVAL_SECONDS
+) -> None:
+    """Terminate this process if the parent process exits or PPID changes.
+
+    This runs in a background daemon thread so it never blocks pool work.
+    """
+
+    def _watch() -> None:
+        while True:
+            # If PPID changed (parent died and we were reparented), exit.
+            if os.getppid() != parent_pid:
+                os._exit(1)
+
+            # Best-effort liveness probe in case PPID was reused.
+            try:
+                os.kill(parent_pid, 0)
+            except OSError:
+                os._exit(1)
+
+            time.sleep(interval_seconds)
+
+    threading.Thread(target=_watch, name="parent-watchdog", daemon=True).start()
+
+
+def _subprocess_init(user_apps: list[str], parent_pid: int) -> None:
+    _start_parent_watchdog(parent_pid)
     for app_target in user_apps:
         load_user_app(app_target)