diff --git a/app/main.py b/app/main.py
index 592a5a5..bebcd2d 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1069,6 +1069,30 @@ async def start(request):
 
 COOKIES_PATH = os.path.join(config.STATE_DIR, 'cookies.txt')
 
+
+def warn_if_cookiefile_shadowed():
+    """Warn before an uploaded cookies file displaces an operator-configured one.
+
+    Uploaded cookies deliberately win: the upload exists so cookies can be
+    refreshed without restarting the container, and letting YTDL_OPTIONS win
+    would leave a visible UI button doing nothing. But set_runtime_override
+    writes straight into YTDL_OPTIONS, so the configured path is gone from the
+    live config the moment an uploaded file is applied — after that, nothing
+    downstream can report the conflict (delete_cookies' has_manual_cookiefile
+    check cannot fire once the value has been replaced). This is the only point
+    where both are still visible, so it is the only place the warning can be
+    issued. Must be called before set_runtime_override. See issue #881, where
+    the silence cost the reporter days of debugging.
+    """
+    configured = config.YTDL_OPTIONS.get('cookiefile')
+    if isinstance(configured, str) and configured and configured != COOKIES_PATH:
+        log.warning(
+            'Uploaded cookies at %s take precedence over the cookiefile configured in '
+            'YTDL_OPTIONS (%s), which will not be used. Delete the uploaded cookies from '
+            'the UI to go back to the configured file.',
+            COOKIES_PATH, configured)
+
+
 @routes.post(config.URL_PREFIX + 'upload-cookies')
 async def upload_cookies(request):
     reader = await request.multipart()
@@ -1098,6 +1122,7 @@ async def upload_cookies(request):
     except OSError as exc:
         log.warning(f'Could not restrict permissions on cookies file: {exc}')
     os.replace(tmp_cookie_path, COOKIES_PATH)
+    warn_if_cookiefile_shadowed()
     config.set_runtime_override('cookiefile', COOKIES_PATH)
     log.info(f'Cookies file uploaded ({size} bytes)')
     return web.Response(text=serializer.encode({'status': 'ok', 'msg': f'Cookies uploaded ({size} bytes)'}))
@@ -1355,6 +1380,7 @@ if __name__ == '__main__':
 
     # Auto-detect cookie file on startup
     if os.path.exists(COOKIES_PATH):
+        warn_if_cookiefile_shadowed()
         config.set_runtime_override('cookiefile', COOKIES_PATH)
         log.info(f'Cookie file detected at {COOKIES_PATH}')
 
diff --git a/app/tests/test_main_helpers.py b/app/tests/test_main_helpers.py
index 282e940..d570b1e 100644
--- a/app/tests/test_main_helpers.py
+++ b/app/tests/test_main_helpers.py
@@ -313,3 +313,40 @@ class GetCustomDirsTests(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class WarnIfCookiefileShadowedTests(unittest.TestCase):
+    """Issue #881: an uploaded cookies file wins over an operator-configured
+    cookiefile, and used to do so with no way for anyone to notice."""
+
+    def setUp(self):
+        self._saved = main.config.YTDL_OPTIONS
+        main.config.YTDL_OPTIONS = dict(self._saved)
+
+    def tearDown(self):
+        main.config.YTDL_OPTIONS = self._saved
+
+    def test_warns_when_a_different_cookiefile_is_configured(self):
+        main.config.YTDL_OPTIONS["cookiefile"] = "/cookies/cookies.txt"
+        with self.assertLogs("main", level="WARNING") as cm:
+            main.warn_if_cookiefile_shadowed()
+        joined = "\n".join(cm.output)
+        self.assertIn("/cookies/cookies.txt", joined)
+        self.assertIn(main.COOKIES_PATH, joined)
+
+    def test_silent_when_no_cookiefile_configured(self):
+        main.config.YTDL_OPTIONS.pop("cookiefile", None)
+        with self.assertNoLogs("main", level="WARNING"):
+            main.warn_if_cookiefile_shadowed()
+
+    def test_silent_when_configured_file_is_the_uploaded_one(self):
+        # The steady state after an upload: re-running must not nag.
+        main.config.YTDL_OPTIONS["cookiefile"] = main.COOKIES_PATH
+        with self.assertNoLogs("main", level="WARNING"):
+            main.warn_if_cookiefile_shadowed()
+
+    def test_silent_on_non_string_or_empty_values(self):
+        for value in (None, "", 0, [], {}):
+            main.config.YTDL_OPTIONS["cookiefile"] = value
+            with self.assertNoLogs("main", level="WARNING"):
+                main.warn_if_cookiefile_shadowed()