diff --git a/hermes_cli/gateway.py b/hermes_cli/gateway.py
index 3181751005a9..cc34b44281f7 100644
--- a/hermes_cli/gateway.py
+++ b/hermes_cli/gateway.py
@@ -752,9 +752,25 @@ def _spawn_gateway_restart_watcher(old_pid: int, run_argv: list[str]) -> bool:
         # been spawned inside a job object (Electron/Tauri parent), and
         # without breakaway the respawned gateway would die when that job
         # tears down. See _subprocess_compat.windows_detach_flags().
+        #
+        # Mark the respawn as a detached service launch and sever its stdin.
+        # _windows_gateway_should_absorb_console_controls() keys off
+        # HERMES_GATEWAY_DETACHED (and, failing that, an interactive stdin):
+        # without this marker the respawned gateway inherits the spawning
+        # process's console handle, decides it is "interactive", and SKIPS the
+        # SetConsoleCtrlHandler(NULL, TRUE) guard. It then dies the instant
+        # Windows broadcasts CTRL_CLOSE_EVENT / CTRL_LOGOFF_EVENT when the
+        # parent console (e.g. the post-update desktop shell) goes away —
+        # silently, with no shutdown log. Mirror gateway_windows._spawn_detached:
+        # set HERMES_GATEWAY_DETACHED=1 and redirect stdin to DEVNULL so the
+        # respawn installs the console-control guard and survives. (#21301-followup)
+        _env = dict(os.environ)
+        _env["HERMES_GATEWAY_DETACHED"] = "1"
         _popen_kwargs = {
+            "stdin": subprocess.DEVNULL,
             "stdout": subprocess.DEVNULL,
             "stderr": subprocess.DEVNULL,
+            "env": _env,
         }
         if sys.platform == "win32":
             try:
diff --git a/tests/hermes_cli/test_gateway.py b/tests/hermes_cli/test_gateway.py
index 9fb3e99caca5..64c3d18342e5 100644
--- a/tests/hermes_cli/test_gateway.py
+++ b/tests/hermes_cli/test_gateway.py
@@ -3,6 +3,7 @@
 import argparse
 import signal
 import sys
+from pathlib import Path
 from types import ModuleType, SimpleNamespace
 
 import pytest
@@ -374,6 +375,70 @@ def fake_signal(sig, handler):
     assert (gateway.signal.SIGINT, gateway.signal.SIG_IGN) in signal_calls
 
 
+class TestGatewayRestartWatcher:
+    """Regression tests for the post-update detached respawn watcher.
+
+    The watcher must hand the respawned gateway the same detach context that
+    gateway_windows._spawn_detached gives the autostart path — otherwise the
+    respawn inherits the spawning console, decides it is interactive, skips the
+    SetConsoleCtrlHandler guard, and dies on the next CTRL_CLOSE/LOGOFF event
+    (the silent post-update death this fix addresses).
+    """
+
+    def _watcher_source(self):
+        import ast
+        import re
+        import textwrap
+
+        content = (
+            Path(gateway.__file__).read_text(encoding="utf-8")
+        )
+        start = content.index("watcher = textwrap.dedent(")
+        seg = content[start : start + 3500]
+        m = re.search(
+            r'textwrap\.dedent\(\s*"""(.*?)"""\s*\)\.strip\(\)', seg, re.DOTALL
+        )
+        assert m, "could not locate watcher template literal"
+        inner = textwrap.dedent(m.group(1)).strip()
+        # Must be valid Python — it is executed via `python -c`.
+        ast.parse(inner)
+        return inner
+
+    def test_watcher_marks_respawn_detached_and_severs_stdin(self):
+        src = self._watcher_source()
+        # The respawned gateway must be tagged as a detached service launch so
+        # _windows_gateway_should_absorb_console_controls() returns True.
+        assert 'HERMES_GATEWAY_DETACHED' in src
+        assert '"HERMES_GATEWAY_DETACHED"] = "1"' in src
+        # And its stdin must be severed so the isatty() fallback can't class it
+        # interactive even if the env marker were somehow dropped.
+        assert '"stdin": subprocess.DEVNULL' in src
+        # The env overlay must be passed to Popen, not just constructed.
+        assert '"env": _env' in src
+
+    def test_by_cmdline_spawns_watcher(self, monkeypatch):
+        captured = {}
+
+        def fake_popen(argv, **kwargs):
+            captured["argv"] = argv
+            captured["kwargs"] = kwargs
+            return SimpleNamespace(pid=4321)
+
+        monkeypatch.setattr(gateway.subprocess, "Popen", fake_popen)
+        ok = gateway.launch_detached_gateway_restart_by_cmdline(
+            999999, ["pythonw.exe", "-m", "hermes_cli.main", "gateway", "run"]
+        )
+        assert ok is True
+        # argv = [python, "-c", watcher, old_pid, *run_argv]
+        assert captured["argv"][1] == "-c"
+        assert captured["argv"][3] == "999999"
+        assert captured["argv"][-3:] == ["hermes_cli.main", "gateway", "run"]
+
+    def test_rejects_empty_inputs(self):
+        assert gateway.launch_detached_gateway_restart_by_cmdline(0, ["x"]) is False
+        assert gateway.launch_detached_gateway_restart_by_cmdline(123, []) is False
+
+
 class TestSystemdLingerStatus:
     def test_reports_enabled(self, monkeypatch):
         monkeypatch.setattr(gateway, "is_linux", lambda: True)