Throw a clear error when an Execute command contains whitespace

src/StackExchange.Redis/ExceptionFactory.cs

@@ -30,6 +30,9 @@ internal static Exception CommandDisabled(string command)
         internal static Exception TooManyArgs(string command, int argCount)
             => new RedisCommandException($"This operation would involve too many arguments ({argCount + 1} vs the redis limit of {MessageWriter.REDIS_MAX_ARGS}): {command}");
 
+        internal static Exception CommandHasWhitespace(string command)
+            => new RedisCommandException($"The command '{command}' contains whitespace and would be sent as a single unknown token; pass each word as a separate argument, for example Execute(\"ACL\", \"SETUSER\", \"x\") rather than Execute(\"ACL SETUSER x\").");
+
         internal static Exception ConnectionFailure(bool includeDetail, ConnectionFailureType failureType, string message, ServerEndPoint? server)
         {
             var ex = new RedisConnectionException(failureType, message);

src/StackExchange.Redis/RedisDatabase.cs

@@ -5549,6 +5549,11 @@ public ExecuteMessage(CommandMap? map, int db, CommandFlags flags, string comman
                     throw ExceptionFactory.TooManyArgs(command, args.Count);
                 }
 
+                // a redis command token never contains space, so a command like
+                // "ACL SETUSER x" is always a caller mistake (it gets sent as one unknown token
+                // and the server replies with an opaque error); fail fast with actionable guidance
+                if (command.IndexOf(' ') >= 0) throw ExceptionFactory.CommandHasWhitespace(command);
+
                 map ??= CommandMap.Default;
                 _unknownCommand = "";
                 if (Command is RedisCommand.UNKNOWN)

tests/StackExchange.Redis.Tests/RoundTripUnitTests/AdhocMessageRoundTrip.cs

@@ -48,6 +48,26 @@ public async Task EchoRoundTripTest(MapMode mode, string payload, string request
         }
     }
 
+    [Theory(Timeout = 1000)]
+    [InlineData("ACL SETUSER x")]
+    [InlineData("get key")]
+    public void CommandWithWhitespaceThrows(string command)
+    {
+        object[] args = [];
+        var ex = Assert.Throws<RedisCommandException>(
+            () => new RedisDatabase.ExecuteMessage(CommandMap.Default, -1, CommandFlags.None, command, args));
+        Assert.Contains("whitespace", ex.Message);
+    }
+
+    [Fact(Timeout = 1000)]
+    public void SingleTokenCommandDoesNotThrow()
+    {
+        // the correct token-per-argument form must still be accepted unchanged
+        object[] args = ["SETUSER", "x"];
+        var msg = new RedisDatabase.ExecuteMessage(CommandMap.Default, -1, CommandFlags.None, "ACL", args);
+        Assert.Equal("ACL", msg.CommandString);
+    }
+
     private static CommandMap? GetMap(MapMode mode) => mode switch
     {
         MapMode.Null => null,