diff --git a/src/entrypoints/desktop_cli.py b/src/entrypoints/desktop_cli.py
index 439ebeb6..69518069 100644
--- a/src/entrypoints/desktop_cli.py
+++ b/src/entrypoints/desktop_cli.py
@@ -54,6 +54,27 @@ def build_launch_plan(app_dir: Path, *, install: bool, dev: bool) -> list[list[s
     return plan
 
 
+# The dev renderer binds this fixed port (ui-desktop package.json dev:renderer).
+DEV_RENDERER_PORT = 5174
+
+
+def dev_port_busy(port: int | None = None, host: str = "127.0.0.1") -> bool:
+    """True when something already listens on the dev renderer port.
+
+    A second `clawcodex desktop` would otherwise die mid-boot on vite's raw
+    "Port 5174 is already in use" stack trace — the app is almost certainly
+    just already running. The port resolves at call time so tests (and a
+    future config override) can repoint DEV_RENDERER_PORT.
+    """
+    import socket
+
+    if port is None:
+        port = DEV_RENDERER_PORT
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as probe:
+        probe.settimeout(0.5)
+        return probe.connect_ex((host, port)) == 0
+
+
 def run_desktop_subcommand(argv: list[str]) -> int:
     parser = argparse.ArgumentParser(
         prog="clawcodex desktop",
@@ -77,6 +98,11 @@ def run_desktop_subcommand(argv: list[str]) -> int:
         print("desktop: npm not found on PATH — install Node.js 22+ first.",
               file=sys.stderr)
         return 2
+    if not args.no_dev and dev_port_busy():
+        print("desktop: ClawCodex Desktop appears to be already running "
+              f"(port {DEV_RENDERER_PORT} is in use). Switch to its window, "
+              "or quit it and run this again.", file=sys.stderr)
+        return 1
 
     env = launch_env(root)
     for cmd in build_launch_plan(app_dir, install=args.install, dev=not args.no_dev):
diff --git a/tests/server/test_desktop_sessions.py b/tests/server/test_desktop_sessions.py
index c7fd368a..19c226f6 100644
--- a/tests/server/test_desktop_sessions.py
+++ b/tests/server/test_desktop_sessions.py
@@ -251,3 +251,34 @@ def test_desktop_subcommand_refuses_without_app(tmp_path: Path,
 
     monkeypatch.setattr(mod, "repo_root", lambda: tmp_path)
     assert mod.run_desktop_subcommand([]) == 2
+
+
+def test_desktop_port_guard_detects_running_instance(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture
+) -> None:
+    import socket
+
+    import src.entrypoints.desktop_cli as mod
+
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as listener:
+        listener.bind(("127.0.0.1", 0))
+        # Backlog > 1: each dev_port_busy probe completes a handshake that
+        # sits unaccepted in the queue; a backlog of 1 makes the second probe
+        # fail spuriously.
+        listener.listen(8)
+        port = listener.getsockname()[1]
+
+        assert mod.dev_port_busy(port) is True
+
+        # The subcommand refuses cleanly when the dev port is held.
+        app_dir = tmp_path / "ui-desktop"
+        app_dir.mkdir()
+        (app_dir / "package.json").write_text("{}", encoding="utf-8")
+        monkeypatch.setattr(mod, "repo_root", lambda: tmp_path)
+        monkeypatch.setattr(mod, "DEV_RENDERER_PORT", port)
+        monkeypatch.setattr(mod.shutil, "which", lambda n: "/usr/bin/npm")
+        assert mod.run_desktop_subcommand([]) == 1
+        assert "already running" in capsys.readouterr().err
+
+    # Listener closed — the guard clears.
+    assert mod.dev_port_busy(port) is False