why not

Content.Server/Chat/Systems/ChatSystem.cs

@@ -754,7 +754,7 @@ public sealed partial class ChatSystem : SharedChatSystem
 
         var playerName = changeling.HiveName;
 
-        message = $"{char.ToUpper(message[0])}{message[1..]}";
+        message = OopsConcat(char.ToUpper(message[0]).ToString(), message[1..]);
 
         var wrappedMessage = Loc.GetString("chat-manager-send-changeling-chat-wrap-message",
             ("player", playerName),
@@ -1135,6 +1135,12 @@ public sealed partial class ChatSystem : SharedChatSystem
         return sb.ToString();
     }
 
+    private static string OopsConcat(string a, string b)
+    {
+        // This exists to prevent Roslyn being clever and compiling something that fails sandbox checks.
+        return a + b;
+    }
+
     #endregion
 }