diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 4c0c733449b..58043229893 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -15738,6 +15738,21 @@ Examples:
         action="store_true",
         help="List running hermes dashboard processes and exit",
     )
+    # Backward-compat shim: older Hermes desktop app shells (<= 0.15.x) spawn the
+    # backend as `hermes dashboard --no-open --tui --host ... --port ...`. The
+    # `--tui` flag was removed from this subcommand in cae6b5486 (embedded chat is
+    # always on now). When a user's CLI updates past that commit but their desktop
+    # app binary has not, argparse used to hard-error with "unrecognized arguments:
+    # --tui" and exit(2) — the backend died before becoming ready and the GUI just
+    # showed "Hermes couldn't start" with no actionable cause. Accept and silently
+    # ignore the flag so an old app + new CLI degrades gracefully instead of
+    # bricking. Hidden from --help; safe to delete once the floor app version is
+    # well past 0.16.0.
+    dashboard_parser.add_argument(
+        "--tui",
+        action="store_true",
+        help=argparse.SUPPRESS,
+    )
     dashboard_parser.set_defaults(func=cmd_dashboard)
 
     # `hermes dashboard register` — register a self-hosted dashboard OAuth
diff --git a/tests/hermes_cli/test_dashboard_tui_backcompat.py b/tests/hermes_cli/test_dashboard_tui_backcompat.py
new file mode 100644
index 00000000000..677b1f4c867
--- /dev/null
+++ b/tests/hermes_cli/test_dashboard_tui_backcompat.py
@@ -0,0 +1,82 @@
+"""Regression test: `hermes dashboard --tui` must not hard-crash.
+
+Older Hermes desktop app shells (<= 0.15.x) spawn the backend as::
+
+    hermes dashboard --no-open --tui --host 127.0.0.1 --port <PORT>
+
+The ``--tui`` flag was removed from the ``dashboard`` subcommand in cae6b5486
+(embedded chat is always on now). When a user's CLI updates past that commit
+but their desktop app binary has not, argparse used to reject the unknown flag
+with ``error: unrecognized arguments: --tui`` and ``exit(2)`` — the backend
+died before it became ready and the desktop GUI showed only "Hermes couldn't
+start" with no actionable cause.
+
+The fix adds a hidden, deprecated, accepted-and-ignored ``--tui`` flag to the
+dashboard subparser so an old app shell + new CLI degrades gracefully instead
+of bricking. These tests pin that contract.
+"""
+
+import os
+import subprocess
+import sys
+
+import pytest
+
+REPO_ROOT = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), os.pardir, os.pardir)
+)
+
+
+def _run_cli(args, timeout=60):
+    """Invoke the real hermes_cli.main parser in a subprocess.
+
+    Uses ``--status`` so the dashboard command exits immediately after parsing
+    (it scans the process table and returns) instead of starting a server.
+    Returns the CompletedProcess.
+    """
+    env = dict(os.environ)
+    env["PYTHONPATH"] = REPO_ROOT + os.pathsep + env.get("PYTHONPATH", "")
+    return subprocess.run(
+        [sys.executable, "-m", "hermes_cli.main", *args],
+        cwd=REPO_ROOT,
+        env=env,
+        capture_output=True,
+        text=True,
+        timeout=timeout,
+    )
+
+
+def test_dashboard_tui_flag_is_accepted_not_rejected():
+    """The exact argv an old desktop app sends must parse without argparse error."""
+    result = _run_cli(
+        ["dashboard", "--no-open", "--tui", "--host", "127.0.0.1",
+         "--port", "39997", "--status"]
+    )
+    combined = (result.stdout or "") + (result.stderr or "")
+    # The pre-fix failure signature.
+    assert "unrecognized arguments" not in combined, combined
+    assert "--tui" not in (result.stderr or ""), result.stderr
+    # argparse usage errors exit 2; the parse itself must not be that error.
+    assert result.returncode != 2, combined
+
+
+def test_dashboard_tui_flag_is_hidden_from_help():
+    """The deprecated shim must not re-advertise a removed feature in --help."""
+    result = _run_cli(["dashboard", "--help"])
+    combined = (result.stdout or "") + (result.stderr or "")
+    assert result.returncode == 0, combined
+    assert "--tui" not in combined, (
+        "dashboard --tui is a deprecated back-compat shim and must stay "
+        "hidden via argparse.SUPPRESS:\n" + combined
+    )
+
+
+def test_dashboard_without_tui_still_parses():
+    """Sanity: the modern (no --tui) invocation is unaffected by the shim."""
+    result = _run_cli(
+        ["dashboard", "--no-open", "--host", "127.0.0.1",
+         "--port", "39996", "--status"]
+    )
+    combined = (result.stdout or "") + (result.stderr or "")
+    assert "unrecognized arguments" not in combined, combined
+    assert result.returncode != 2, combined