diff --git a/backend/app/channels/slack.py b/backend/app/channels/slack.py
index c9ad6a6e..65cb36cf 100644
--- a/backend/app/channels/slack.py
+++ b/backend/app/channels/slack.py
@@ -16,13 +16,31 @@ logger = logging.getLogger(__name__)
 _slack_md_converter = SlackMarkdownConverter()
 
 
+def _normalize_allowed_users(allowed_users: Any) -> set[str]:
+    if allowed_users is None:
+        return set()
+    if isinstance(allowed_users, str):
+        values = [allowed_users]
+    elif isinstance(allowed_users, list | tuple | set):
+        values = allowed_users
+    else:
+        logger.warning(
+            "Slack allowed_users should be a list of Slack user IDs or a single Slack user ID string; treating %s as one string value",
+            type(allowed_users).__name__,
+        )
+        values = [allowed_users]
+    return {str(user_id) for user_id in values if str(user_id)}
+
+
 class SlackChannel(Channel):
     """Slack IM channel using Socket Mode (WebSocket, no public IP).
 
     Configuration keys (in ``config.yaml`` under ``channels.slack``):
         - ``bot_token``: Slack Bot User OAuth Token (xoxb-...).
         - ``app_token``: Slack App-Level Token (xapp-...) for Socket Mode.
-        - ``allowed_users``: (optional) List of allowed Slack user IDs. Empty = allow all.
+        - ``allowed_users``: (optional) List of allowed Slack user IDs, or a
+          single Slack user ID string as shorthand. Empty = allow all. Other
+          scalar values are treated as a single string with a warning.
     """
 
     def __init__(self, bus: MessageBus, config: dict[str, Any]) -> None:
@@ -30,7 +48,7 @@ class SlackChannel(Channel):
         self._socket_client = None
         self._web_client = None
         self._loop: asyncio.AbstractEventLoop | None = None
-        self._allowed_users: set[str] = {str(user_id) for user_id in config.get("allowed_users", [])}
+        self._allowed_users = _normalize_allowed_users(config.get("allowed_users", []))
 
     async def start(self) -> None:
         if self._running:
diff --git a/backend/tests/test_channels.py b/backend/tests/test_channels.py
index 7fc41265..e6fb0213 100644
--- a/backend/tests/test_channels.py
+++ b/backend/tests/test_channels.py
@@ -2046,6 +2046,11 @@ class TestSlackSendRetry:
 
 
 class TestSlackAllowedUsers:
+    @staticmethod
+    def _submit_coro(coro, loop):
+        coro.close()
+        return MagicMock()
+
     def test_numeric_allowed_users_match_string_event_user_id(self):
         from app.channels.slack import SlackChannel
 
@@ -2067,13 +2072,9 @@ class TestSlackAllowedUsers:
             "ts": "1710000000.000100",
         }
 
-        def submit_coro(coro, loop):
-            coro.close()
-            return MagicMock()
-
         with patch(
             "app.channels.slack.asyncio.run_coroutine_threadsafe",
-            side_effect=submit_coro,
+            side_effect=self._submit_coro,
         ) as submit:
             channel._handle_message_event(event)
 
@@ -2085,6 +2086,74 @@ class TestSlackAllowedUsers:
         assert inbound.chat_id == "C123"
         assert inbound.text == "hello from slack"
 
+    def test_string_allowed_users_match_event_user_id(self):
+        from app.channels.slack import SlackChannel
+
+        bus = MessageBus()
+        bus.publish_inbound = AsyncMock()
+        channel = SlackChannel(
+            bus=bus,
+            config={"allowed_users": "U123456"},
+        )
+        channel._loop = MagicMock()
+        channel._loop.is_running.return_value = True
+        channel._add_reaction = MagicMock()
+        channel._send_running_reply = MagicMock()
+
+        event = {
+            "user": "U123456",
+            "text": "hello from slack",
+            "channel": "C123",
+            "ts": "1710000000.000100",
+        }
+
+        with patch(
+            "app.channels.slack.asyncio.run_coroutine_threadsafe",
+            side_effect=self._submit_coro,
+        ) as submit:
+            channel._handle_message_event(event)
+
+        channel._add_reaction.assert_called_once_with("C123", "1710000000.000100", "eyes")
+        channel._send_running_reply.assert_called_once_with("C123", "1710000000.000100")
+        submit.assert_called_once()
+        inbound = bus.publish_inbound.call_args.args[0]
+        assert inbound.user_id == "U123456"
+        assert inbound.chat_id == "C123"
+        assert inbound.text == "hello from slack"
+
+    def test_scalar_allowed_users_warns_and_matches_stringified_event_user_id(self, caplog):
+        from app.channels.slack import SlackChannel
+
+        bus = MessageBus()
+        bus.publish_inbound = AsyncMock()
+        with caplog.at_level("WARNING"):
+            channel = SlackChannel(
+                bus=bus,
+                config={"allowed_users": 123456},
+            )
+        channel._loop = MagicMock()
+        channel._loop.is_running.return_value = True
+        channel._add_reaction = MagicMock()
+        channel._send_running_reply = MagicMock()
+
+        event = {
+            "user": "123456",
+            "text": "hello from slack",
+            "channel": "C123",
+            "ts": "1710000000.000100",
+        }
+
+        with patch(
+            "app.channels.slack.asyncio.run_coroutine_threadsafe",
+            side_effect=self._submit_coro,
+        ) as submit:
+            channel._handle_message_event(event)
+
+        assert "Slack allowed_users should be a list" in caplog.text
+        submit.assert_called_once()
+        inbound = bus.publish_inbound.call_args.args[0]
+        assert inbound.user_id == "123456"
+
     def test_raises_after_all_retries_exhausted(self):
         from app.channels.slack import SlackChannel
 
diff --git a/config.example.yaml b/config.example.yaml
index 32a94105..b9f7a963 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -867,7 +867,7 @@ checkpointer:
 #     enabled: false
 #     bot_token: $SLACK_BOT_TOKEN     # xoxb-...
 #     app_token: $SLACK_APP_TOKEN     # xapp-... (Socket Mode)
-#     allowed_users: []               # empty = allow all
+#     allowed_users: []               # empty = allow all; can also be a single Slack user ID string, e.g. U123456, but list form is recommended
 #
 #   telegram:
 #     enabled: false