diff --git a/src/AcDream.UI.Abstractions/Panels/Chat/ChatInputParser.cs b/src/AcDream.UI.Abstractions/Panels/Chat/ChatInputParser.cs
index 2e41997..82aaf76 100644
--- a/src/AcDream.UI.Abstractions/Panels/Chat/ChatInputParser.cs
+++ b/src/AcDream.UI.Abstractions/Panels/Chat/ChatInputParser.cs
@@ -304,4 +304,21 @@ public static class ChatInputParser
         foreach (var (v, _) in ChannelVerbs) set.Add(v);
         return set;
     }
+
+    /// <summary>
+    /// Returns true if <paramref name="verb"/> (with leading
+    /// <c>/</c>) is one this parser routes — used by callers that
+    /// need to distinguish "unknown slash command" from "known
+    /// verb with bad arguments" without reproducing the alias
+    /// tables. <c>@</c>-prefixed verbs need to be normalized to
+    /// <c>/</c> before passing.
+    /// </summary>
+    public static bool IsKnownVerb(string verb) => AllKnownVerbs.Contains(verb);
+
+    /// <summary>
+    /// Pull the first whitespace-separated token (the command verb)
+    /// from <paramref name="command"/>. Returns the entire string if
+    /// there is no whitespace.
+    /// </summary>
+    public static string GetVerbToken(string command) => ExtractVerb(command);
 }
diff --git a/src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs b/src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs
index c60524b..fe8939f 100644
--- a/src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs
+++ b/src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs
@@ -131,6 +131,27 @@ public sealed class ChatPanel : IPanel
                 return;
             }
 
+            // Phase J Tier 4: any /-prefixed input that ISN'T one of our
+            // known verbs gets a local "Unknown command" message instead
+            // of being broadcast to the server as plain speech. The
+            // user reported "/ls" / "/mp /path" leaking out as chat —
+            // a / prefix is a command, never speech. (@-prefixed unknown
+            // verbs still pass through to ACE because ACE's
+            // CommandManager intercepts @ server-side and replies with
+            // its own "Unknown command" / valid command output.)
+            if (trimmed.Length > 0 && trimmed[0] == '/')
+            {
+                string verb = ChatInputParser.GetVerbToken(trimmed);
+                if (!ChatInputParser.IsKnownVerb(verb))
+                {
+                    _vm.ShowSystemMessage(
+                        $"Unknown command: {verb}. Type /help for the list of supported commands.");
+                    _input = string.Empty;
+                    renderer.End();
+                    return;
+                }
+            }
+
             var parsed = ChatInputParser.Parse(
                 trimmed,
                 ChatChannelKind.Say,
diff --git a/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatInputParserTests.cs b/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatInputParserTests.cs
index 2ac4c2c..052aa6b 100644
--- a/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatInputParserTests.cs
+++ b/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatInputParserTests.cs
@@ -228,8 +228,11 @@ public sealed class ChatInputParserTests
         // Phase J added long-form aliases (/general, /allegiance,
         // /patron, etc.). The exact-token rule still applies — a
         // verb prefix that ISN'T one of the listed aliases falls
-        // through to the default channel. "/genio" is not /g, /general,
-        // or /gen — must stay as Say carrying the literal text.
+        // through. The Parse-level behaviour for unknown /-verbs is
+        // still "Say with the literal text" (matches holtburger);
+        // the ChatPanel layer is what catches unknowns and shows the
+        // local "Unknown command" line. ChatPanelInputTests cover
+        // that end-to-end behaviour.
         var parsed = ChatInputParser.Parse("/genio public", ChatChannelKind.Say, lastTellSender: null);
 
         Assert.NotNull(parsed);
@@ -237,6 +240,32 @@ public sealed class ChatInputParserTests
         Assert.Equal("/genio public", parsed.Value.Text);
     }
 
+    [Theory]
+    [InlineData("/g",          true)]
+    [InlineData("/say",        true)]
+    [InlineData("/tell",       true)]
+    [InlineData("/retell",     true)]
+    [InlineData("/allegiance", true)]
+    [InlineData("/lookingforgroup", true)]
+    [InlineData("/genio",      false)]
+    [InlineData("/ls",         false)]
+    [InlineData("/foo",        false)]
+    [InlineData("/",           false)]
+    public void IsKnownVerb_ChecksAgainstAliasTables(string verb, bool expected)
+    {
+        Assert.Equal(expected, ChatInputParser.IsKnownVerb(verb));
+    }
+
+    [Theory]
+    [InlineData("/g hello", "/g")]
+    [InlineData("/tell Bob hi", "/tell")]
+    [InlineData("/foo", "/foo")]
+    [InlineData("/", "/")]
+    public void GetVerbToken_PullsFirstWhitespaceToken(string command, string expected)
+    {
+        Assert.Equal(expected, ChatInputParser.GetVerbToken(command));
+    }
+
     [Theory]
     [InlineData("/general what's the deal",     ChatChannelKind.General,    "what's the deal")]
     [InlineData("/allegiance recall",            ChatChannelKind.Allegiance, "recall")]
diff --git a/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatPanelInputTests.cs b/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatPanelInputTests.cs
index 3496a42..07b3bf5 100644
--- a/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatPanelInputTests.cs
+++ b/tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatPanelInputTests.cs
@@ -112,6 +112,38 @@ public sealed class ChatPanelInputTests
         Assert.Contains("(10.0, 20.0, 30.0)", entry.Text);
     }
 
+    [Theory]
+    [InlineData("/foo")]
+    [InlineData("/ls")]
+    [InlineData("/mp /tools/script.py")]
+    [InlineData("/genio public")]
+    [InlineData("/")]
+    public void Submit_UnknownSlashCommand_ShowsUnknownAndDoesNotPublish(string raw)
+    {
+        // Phase J Tier 4: /-prefixed text is NEVER broadcast as plain
+        // speech. Filed after a 2026-04-25 trace where typing /ls (a
+        // command-style request the user wanted) was getting echoed by
+        // the server as "You say, \"/ls\"". Now we intercept and show
+        // a local "Unknown command" line; nothing goes on the wire.
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = raw,
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+        var entry = Assert.Single(log.Snapshot());
+        Assert.Equal(ChatKind.System, entry.Kind);
+        Assert.Contains("Unknown command", entry.Text);
+        Assert.Contains("/help", entry.Text);
+    }
+
     [Fact]
     public void Submit_AtAcehelp_PassesThroughToSayWithAtIntact()
     {