diff --git a/pandrator_installer/supervisor.py b/pandrator_installer/supervisor.py
index 53f41d7e..8449022a 100644
--- a/pandrator_installer/supervisor.py
+++ b/pandrator_installer/supervisor.py
@@ -211,7 +211,30 @@ def _write_state(self) -> None:
         }
         temporary = self.runtime_state.with_suffix(".tmp")
         temporary.write_text(json.dumps(payload, indent=2), encoding="utf-8")
-        os.replace(temporary, self.runtime_state)
+        attempts = 10
+        for attempt in range(1, attempts + 1):
+            try:
+                os.replace(temporary, self.runtime_state)
+                return
+            except OSError:
+                if attempt == attempts:
+                    # The state file is informational (GUI status display and
+                    # discovery of already-running services) and is rewritten on
+                    # the next monitor tick. On Windows, a concurrent reader
+                    # holds the destination open without FILE_SHARE_DELETE, so
+                    # os.replace() fails with WinError 5; that must not take
+                    # down the whole supervised process tree over a skipped
+                    # status update.
+                    logging.exception(
+                        "Could not update %s; keeping the previous state file.",
+                        self.runtime_state,
+                    )
+                    try:
+                        temporary.unlink()
+                    except OSError:
+                        pass
+                    return
+                time.sleep(0.1)
 
     def _managed_process_environment(self, spec: ManagedProcessSpec) -> dict[str, str]:
         # The frozen installer needs its private libraries, but the installed
diff --git a/tests/test_web_supervisor.py b/tests/test_web_supervisor.py
index 3824930c..d17db309 100644
--- a/tests/test_web_supervisor.py
+++ b/tests/test_web_supervisor.py
@@ -139,6 +139,41 @@ def observe_callback_state():
             finally:
                 supervisor.stop_all()
 
+    def test_write_state_retries_when_replace_is_briefly_blocked(self):
+        # On Windows a concurrent reader of runtime-processes.json (for example
+        # the installer GUI's status poll) holds the file open without
+        # FILE_SHARE_DELETE, so os.replace() fails with PermissionError.
+        with tempfile.TemporaryDirectory() as directory:
+            supervisor = ProcessSupervisor(data_root=directory, specs=[])
+            real_replace = os.replace
+            failures = iter([PermissionError(13, "Access is denied"), PermissionError(13, "Access is denied")])
+
+            def flaky_replace(source, destination):
+                try:
+                    raise next(failures)
+                except StopIteration:
+                    real_replace(source, destination)
+
+            with mock.patch("pandrator_installer.supervisor.os.replace", side_effect=flaky_replace), mock.patch(
+                "pandrator_installer.supervisor.time.sleep"
+            ):
+                supervisor._write_state()
+
+            state = json.loads((Path(directory) / "runtime-processes.json").read_text(encoding="utf-8"))
+            self.assertEqual(state["supervisor_pid"], os.getpid())
+
+    def test_write_state_survives_a_persistently_blocked_replace(self):
+        with tempfile.TemporaryDirectory() as directory:
+            supervisor = ProcessSupervisor(data_root=directory, specs=[])
+            with mock.patch(
+                "pandrator_installer.supervisor.os.replace",
+                side_effect=PermissionError(13, "Access is denied"),
+            ), mock.patch("pandrator_installer.supervisor.time.sleep"):
+                supervisor._write_state()  # must not raise
+
+            self.assertFalse((Path(directory) / "runtime-processes.json").exists())
+            self.assertFalse((Path(directory) / "runtime-processes.tmp").exists())
+
     def test_control_request_stops_only_the_requested_service_without_restart(self):
         with tempfile.TemporaryDirectory() as directory:
             specs = [