fix(tui): fix invisible slash-command overlays and FormattedText display serialization

tests/test_tui_gateway_server.py

@@ -730,6 +730,41 @@ def test_complete_slash_surfaces_completer_error(monkeypatch):
     assert "no completer" in resp["error"]["message"]
 
 
+def test_complete_slash_display_is_plain_string(monkeypatch):
+    """complete.slash must convert prompt_toolkit FormattedText display values
+    to plain strings before sending them over RPC.
+
+    Regression: c.display is a FormattedText object, not a str.  Passing it
+    raw caused JSON serialization failures and broken rendering in the TUI
+    completion dropdown.
+    """
+    from prompt_toolkit.formatted_text import FormattedText
+
+    class _FakeCompletion:
+        text = "/resume"
+        display = FormattedText([("class:command", "/resume")])
+        display_meta = FormattedText([("", "Resume a session")])
+
+    with patch("hermes_cli.commands.SlashCommandCompleter") as MockCompleter:
+        instance = MockCompleter.return_value
+        instance.get_completions.return_value = [_FakeCompletion()]
+
+        resp = server.handle_request(
+            {"id": "1", "method": "complete.slash", "params": {"text": "/res"}}
+        )
+
+    assert "result" in resp, resp
+    items = resp["result"]["items"]
+    assert len(items) >= 1
+    item = next(i for i in items if i["text"] == "/resume")
+    assert isinstance(item["display"], str), (
+        "display must be a plain str — FormattedText breaks JSON serialization"
+    )
+    assert item["display"] == "/resume"
+    assert isinstance(item["meta"], str)
+    assert item["meta"] == "Resume a session"
+
+
 def test_input_detect_drop_attaches_image(monkeypatch):
     fake_cli = types.ModuleType("cli")
     fake_cli._detect_file_drop = lambda raw: {