diff --git a/roadscore/prototype/native_receiver.sh b/roadscore/prototype/native_receiver.sh
index 6da87a3cfb..0ea31cded2 100755
--- a/roadscore/prototype/native_receiver.sh
+++ b/roadscore/prototype/native_receiver.sh
@@ -26,7 +26,8 @@ if [ "${ROADSCORE_COMPOSER:-ace}" = ace ]; then preparation_wait=1500; fi
 if ! pgrep -f "^/data/sa3-feasibility/venv/bin/python -u ${worker_script}$" >/dev/null; then
   worker_reused=0
   rm -f generated/worker_ready
-  setsid env -u OPENPILOT_PREFIX /usr/local/venv/bin/python -u prototype/power_worker.py > results/native_worker.log 2>&1 < /dev/null &
+  # Only this receiver owns fd 9; the warm resident must not retain its session lock.
+  setsid env -u OPENPILOT_PREFIX /usr/local/venv/bin/python -u prototype/power_worker.py > results/native_worker.log 2>&1 < /dev/null 9>&- &
   power_pid=$!
   if [ "${ROADSCORE_RESIDENT:-0}" = 1 ]; then
     /usr/local/venv/bin/python -c 'import json,sys,time; from pathlib import Path; p=int(sys.argv[1]); Path("generated/resident_owner.json").write_text(json.dumps({"power_worker_pid":p,"process_start_ticks":Path(f"/proc/{p}/stat").read_text().split()[21],"created_wall":time.time(),"stop":"SIGTERM power_worker_pid; it restores CPU and stops its child"}))' "$power_pid"
@@ -55,7 +56,7 @@ mkdir -p results/current
 /usr/local/venv/bin/python prototype/runtime_manifest.py
 audio_args=()
 if [ "${ROADSCORE_AUDIBLE:-0}" = 1 ]; then audio_args+=(--audible); fi
-/usr/local/venv/bin/python -u prototype/app.py "${audio_args[@]}" > results/current/app.log 2>&1 < /dev/null &
+/usr/local/venv/bin/python -u prototype/app.py "${audio_args[@]}" > results/current/app.log 2>&1 < /dev/null 9>&- &
 audio_pid=$!
 for attempt in $(seq 1 30); do
   [ -f results/current/ready ] && break
@@ -65,7 +66,7 @@ done
 [ -f results/current/ready ] || { cat results/current/app.log; exit 1; }
 # Supervise score failures during delivery rather than leaving silent replay running.
 exec 8<&0
-/usr/local/venv/bin/python -u prototype/replay_bridge.py receive --route "$1" <&8 8<&- &
+/usr/local/venv/bin/python -u prototype/replay_bridge.py receive --route "$1" <&8 8<&- 9>&- &
 bridge_pid=$!
 exec 8<&-
 while kill -0 "$bridge_pid" 2>/dev/null; do
diff --git a/roadscore/prototype/test_native_receiver_lock.py b/roadscore/prototype/test_native_receiver_lock.py
new file mode 100644
index 0000000000..f5a24d4dca
--- /dev/null
+++ b/roadscore/prototype/test_native_receiver_lock.py
@@ -0,0 +1,48 @@
+"""Local regression for the session descriptor inherited by detached workers."""
+import fcntl
+import os
+from pathlib import Path
+import shlex
+import signal
+import subprocess
+import sys
+import tempfile
+import unittest
+
+
+class NativeReceiverLockTests(unittest.TestCase):
+  def test_every_background_child_closes_session_descriptor(self):
+    source = Path(__file__).with_name('native_receiver.sh').read_text()
+    launches = [line for line in source.splitlines() if line.rstrip().endswith(' &')]
+    self.assertEqual(len(launches), 3)
+    for line in launches:
+      self.assertIn('9>&-', line, line)
+    self.assertIn('resident_keep=1', source)
+    self.assertNotIn('rm -f generated/native_session.lock', source)
+
+  def test_live_child_cannot_retain_exited_receivers_lock(self):
+    source = Path(__file__).with_name('native_receiver.sh').read_text()
+    launch = next(line for line in source.splitlines() if 'setsid env ' in line)
+    # Exercise the exact fd-close redirection on the resident launch locally.
+    redirect = next(token for token in launch.split() if token == '9>&-')
+    with tempfile.TemporaryDirectory() as directory:
+      lock = Path(directory) / 'session.lock'
+      child_pid = Path(directory) / 'child.pid'
+      acquire = 'import fcntl; fcntl.flock(9, fcntl.LOCK_EX | fcntl.LOCK_NB)'
+      script = f'''exec 9>{shlex.quote(str(lock))}
+{shlex.quote(sys.executable)} -c {shlex.quote(acquire)}
+sleep 30 {redirect} </dev/null >/dev/null 2>&1 &
+echo $! >{shlex.quote(str(child_pid))}
+'''
+      subprocess.run(['bash', '-c', script], check=True, timeout=5)
+      pid = int(child_pid.read_text())
+      try:
+        os.kill(pid, 0)  # resident surrogate stays warm/alive
+        with lock.open('w') as stream:
+          fcntl.flock(stream, fcntl.LOCK_EX | fcntl.LOCK_NB)
+      finally:
+        os.kill(pid, signal.SIGTERM)
+
+
+if __name__ == '__main__':
+  unittest.main()