diff --git a/ctl.sh b/ctl.sh
index bba8c58f..127fdf93 100755
--- a/ctl.sh
+++ b/ctl.sh
@@ -7,6 +7,7 @@ PID_FILE="${HERMES_WEBUI_PID_FILE:-${HERMES_HOME}/webui.pid}"
 LOG_FILE="${HERMES_WEBUI_LOG_FILE:-${HERMES_HOME}/webui.log}"
 STATE_FILE="${HERMES_WEBUI_CTL_STATE_FILE:-${HERMES_HOME}/webui.ctl.env}"
 DEFAULT_STATE_DIR="${HERMES_WEBUI_STATE_DIR:-${HERMES_HOME}/webui}"
+DEFAULT_LAUNCHD_LABEL="${HERMES_WEBUI_LAUNCHD_LABEL:-com.parantoux.hermes-webui}"
 
 usage() {
   cat <<'EOF'
@@ -197,6 +198,24 @@ _clear_stale_pid() {
   fi
 }
 
+_launchd_webui_pid() {
+  [[ "${HERMES_WEBUI_CTL_ALLOW_LAUNCHD_CONFLICT:-0}" == "1" ]] && return 1
+  command -v launchctl >/dev/null 2>&1 || return 1
+  local label="${HERMES_WEBUI_LAUNCHD_LABEL:-${DEFAULT_LAUNCHD_LABEL}}"
+  [[ -n "${label}" ]] || return 1
+  local uid launchd_out pid
+  uid="$(id -u)"
+  launchd_out="$(launchctl print "gui/${uid}/${label}" 2>/dev/null)" || return 1
+  pid="$(printf '%s\n' "${launchd_out}" | awk '/^[[:space:]]*pid = / {print $3; exit}')"
+  [[ "${pid}" =~ ^[0-9]+$ ]] || return 1
+  (( pid > 0 )) || return 1
+  if _is_alive "${pid}"; then
+    printf '%s\n' "${pid}"
+    return 0
+  fi
+  return 1
+}
+
 start_cmd() {
   ensure_home
   _load_repo_dotenv_preserving_env
@@ -212,6 +231,12 @@ start_cmd() {
     echo "[ctl] Hermes WebUI is already running (PID ${existing_pid})"
     return 0
   fi
+  local launchd_pid
+  if launchd_pid="$(_launchd_webui_pid 2>/dev/null)"; then
+    echo "[ctl] Refusing to start a second Hermes WebUI while launchd job ${HERMES_WEBUI_LAUNCHD_LABEL:-${DEFAULT_LAUNCHD_LABEL}} is running (PID ${launchd_pid})." >&2
+    echo "[ctl] Use launchctl kickstart -k gui/$(id -u)/${HERMES_WEBUI_LAUNCHD_LABEL:-${DEFAULT_LAUNCHD_LABEL}} or disable the launchd job before using ctl.sh start." >&2
+    return 2
+  fi
   _clear_stale_pid >/dev/null 2>&1 || true
 
   local python_exe pid
diff --git a/docs/supervisor.md b/docs/supervisor.md
index 85821a69..b3f2ad5f 100644
--- a/docs/supervisor.md
+++ b/docs/supervisor.md
@@ -16,6 +16,8 @@ Or set ``HERMES_WEBUI_FOREGROUND=1`` in the environment. The Web UI will
 auto-detect launchd / systemd / supervisord even without the flag, but being
 explicit is safer.
 
+**Important (launchd on macOS):** if the ``com.parantoux.hermes-webui`` LaunchAgent is enabled, treat launchd as the single source of truth for WebUI lifecycle. Do **not** also run ``./ctl.sh start``, ``bash start.sh``, ``python bootstrap.py``, or ``python server.py`` against the same state dir/port, or you can create a second WebUI instance and trigger port-8787 restart churn.
+
 ## Why ``--foreground`` matters
 
 Without it, ``bootstrap.py`` does this:
diff --git a/tests/test_ctl_script.py b/tests/test_ctl_script.py
index 526d5ded..cd878e00 100644
--- a/tests/test_ctl_script.py
+++ b/tests/test_ctl_script.py
@@ -110,6 +110,7 @@ def test_start_writes_pid_under_hermes_home_runs_foreground_no_browser_and_logs(
             "FAKE_PYTHON_LOG": str(fake_log),
             "HERMES_WEBUI_HOST": "0.0.0.0",
             "HERMES_WEBUI_PORT": "18991",
+            "HERMES_WEBUI_CTL_ALLOW_LAUNCHD_CONFLICT": "1",
         },
     )
 
@@ -166,6 +167,7 @@ def test_start_loads_dotenv_but_inline_overrides_win(tmp_path):
             "HERMES_WEBUI_PYTHON": str(fake_python),
             "FAKE_PYTHON_LOG": str(fake_log),
             "HERMES_WEBUI_HOST": "0.0.0.0",
+            "HERMES_WEBUI_CTL_ALLOW_LAUNCHD_CONFLICT": "1",
         },
         repo_root=repo_root,
     )
@@ -201,6 +203,49 @@ def test_stale_pid_file_is_removed_without_killing_unrelated_process(tmp_path):
             sleeper.kill()
 
 
+def test_start_refuses_second_instance_when_launchd_job_is_running(tmp_path):
+    fake_bin = tmp_path / "bin"
+    fake_bin.mkdir()
+
+    sleeper = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(30)"])
+    launchctl = fake_bin / "launchctl"
+    launchctl.write_text(
+        textwrap.dedent(
+            f"""
+            #!/usr/bin/env bash
+            if [[ "$1" == "print" ]]; then
+              printf '\tpid = {sleeper.pid}\\n'
+              exit 0
+            fi
+            exit 1
+            """
+        ).lstrip(),
+        encoding="utf-8",
+    )
+    launchctl.chmod(0o755)
+
+    try:
+        result = run_ctl(
+            tmp_path,
+            "start",
+            env={
+                "PATH": f"{fake_bin}:{os.environ.get('PATH', '')}",
+                "HERMES_WEBUI_LAUNCHD_LABEL": "com.parantoux.hermes-webui",
+            },
+        )
+        assert result.returncode == 2
+        combined = result.stdout + result.stderr
+        assert "Refusing to start a second Hermes WebUI" in combined
+        assert "launchctl kickstart -k" in combined
+        assert not (tmp_path / ".hermes" / "webui.pid").exists()
+    finally:
+        sleeper.terminate()
+        try:
+            sleeper.wait(timeout=3)
+        except subprocess.TimeoutExpired:
+            sleeper.kill()
+
+
 def test_logs_supports_non_following_line_count(tmp_path):
     hermes_home = tmp_path / ".hermes"
     hermes_home.mkdir()