diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index b890f68649e..316bc154fa4 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -2342,10 +2342,12 @@ def _validate_messaging_env_value(platform_id: str, key: str, value: str) -> Non
         )
     if key == "SLACK_ALLOWED_USERS":
         user_ids = [part.strip() for part in value.split(",")]
+        # "*" is the gateway's allow-all wildcard (see gateway/platforms/slack.py),
+        # so accept it as a valid entry alongside Slack member IDs (U.../W...).
         invalid = [
             user_id
             for user_id in user_ids
-            if not user_id or not re.fullmatch(r"[UW][A-Z0-9]{2,}", user_id)
+            if user_id != "*" and (not user_id or not re.fullmatch(r"[UW][A-Z0-9]{2,}", user_id))
         ]
         if invalid:
             raise HTTPException(
diff --git a/tests/hermes_cli/test_web_server.py b/tests/hermes_cli/test_web_server.py
index d44c789b3e3..d7a4dbcbbf9 100644
--- a/tests/hermes_cli/test_web_server.py
+++ b/tests/hermes_cli/test_web_server.py
@@ -1687,6 +1687,19 @@ class TestWebServerEndpoints:
         assert resp.status_code == 400
         assert "member IDs" in resp.json()["detail"]
 
+    def test_update_messaging_platform_accepts_slack_allowed_users_wildcard(self):
+        # "*" is the gateway's allow-all wildcard (gateway/platforms/slack.py),
+        # so the dashboard must accept it rather than rejecting it as malformed.
+        from hermes_cli.config import load_env
+
+        resp = self.client.put(
+            "/api/messaging/platforms/slack",
+            json={"env": {"SLACK_ALLOWED_USERS": "*"}},
+        )
+
+        assert resp.status_code == 200
+        assert load_env()["SLACK_ALLOWED_USERS"] == "*"
+
     def test_messaging_platform_test_reports_missing_required_setup(self):
         resp = self.client.put("/api/messaging/platforms/discord", json={"enabled": True})
         assert resp.status_code == 200
diff --git a/web/src/pages/ChannelsPage.tsx b/web/src/pages/ChannelsPage.tsx
index 84791738a25..db56beb1925 100644
--- a/web/src/pages/ChannelsPage.tsx
+++ b/web/src/pages/ChannelsPage.tsx
@@ -76,7 +76,7 @@ function validateMessagingEnvField(field: MessagingPlatformEnvVar, value: string
     if (parts.some((part) => !part)) {
       return "Slack member IDs must be comma-separated without empty entries.";
     }
-    const invalid = parts.find((part) => !SLACK_MEMBER_ID_RE.test(part));
+    const invalid = parts.find((part) => part !== "*" && !SLACK_MEMBER_ID_RE.test(part));
     if (invalid) {
       return `${invalid} does not look like a Slack member ID. Use IDs like U01ABC2DEF3.`;
     }