diff --git a/plugins/platforms/photon/adapter.py b/plugins/platforms/photon/adapter.py
index 6ecd67bdde2f..4d49ccbdacfd 100644
--- a/plugins/platforms/photon/adapter.py
+++ b/plugins/platforms/photon/adapter.py
@@ -1199,7 +1199,14 @@ class PhotonAdapter(BasePlatformAdapter):
         from hermes_cli._subprocess_compat import windows_hide_flags
 
         try:
-            patch = subprocess.run(  # noqa: S603
+            # Off the event loop, for the same reason the dep reinstall above
+            # hops to a thread: this spawns node and *waits* for it (up to 10s).
+            # Run inline it holds the shared gateway loop for that whole window,
+            # so every other platform's traffic stalls — and _start_sidecar runs
+            # on every reconnect (connect(is_reconnect=True)), not just startup,
+            # so the stall recurs on a live gateway.
+            patch = await asyncio.to_thread(
+                subprocess.run,  # noqa: S603
                 [
                     self._node_bin,
                     str(_SIDECAR_DIR / "patch-spectrum-mixed-attachments.mjs"),
diff --git a/tests/plugins/platforms/photon/test_sidecar_lifecycle.py b/tests/plugins/platforms/photon/test_sidecar_lifecycle.py
index 5049736f9b57..717efddeb7a3 100644
--- a/tests/plugins/platforms/photon/test_sidecar_lifecycle.py
+++ b/tests/plugins/platforms/photon/test_sidecar_lifecycle.py
@@ -253,3 +253,70 @@ async def test_reap_inspects_listeners_off_the_event_loop(
             "must be dispatched via asyncio.to_thread so a 5s lsof/ps spawn "
             "can't freeze every other platform on the gateway loop"
         )
+
+
+@pytest.mark.asyncio
+async def test_spectrum_patch_runs_off_the_event_loop(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The node patch run must not block the shared gateway event loop.
+
+    ``_start_sidecar`` spawns the Spectrum patch script and *waits* for it
+    (``timeout=10``). Run inline it holds the loop for that whole window, so
+    every other platform's traffic stalls — and ``_start_sidecar`` runs on
+    every reconnect (``connect(is_reconnect=True)``), not just startup, so the
+    stall recurs on a live gateway. The dep reinstall a few lines above already
+    hops to a thread for exactly this reason; the patch run must too.
+    """
+    import threading
+
+    adapter = _make_adapter(monkeypatch)
+    main_thread = threading.current_thread()
+    seen: Dict[str, Any] = {}
+
+    # node_modules present + deps fresh, so we reach the patch run.
+    monkeypatch.setattr(photon_adapter.Path, "exists", lambda self: True)
+    monkeypatch.setattr(photon_adapter, "_sidecar_deps_stale", lambda: False)
+
+    async def _no_reap() -> None:
+        return None
+
+    monkeypatch.setattr(adapter, "_reap_stale_sidecar", _no_reap)
+
+    def _fake_run(*a: Any, **k: Any) -> Any:
+        seen["thread"] = threading.current_thread()
+
+        class _Done:
+            returncode = 0
+            stdout = ""
+            stderr = ""
+
+        return _Done()
+
+    monkeypatch.setattr(photon_adapter.subprocess, "run", _fake_run)
+
+    class _FakeProc:
+        pid = 4242
+        stdin = None
+        stdout = None
+
+        def poll(self) -> None:
+            return None
+
+    monkeypatch.setattr(
+        photon_adapter.subprocess, "Popen", lambda *a, **k: _FakeProc()
+    )
+
+    try:
+        await adapter._start_sidecar()
+    except Exception:
+        # Readiness/handshake past the patch run may fail under the fakes —
+        # irrelevant here; we only assert where the patch run executed.
+        pass
+
+    assert seen.get("thread") is not None, "patch run never executed"
+    assert seen["thread"] is not main_thread, (
+        "Spectrum patch subprocess ran on the event-loop thread; it must be "
+        "dispatched via asyncio.to_thread so a 10s node spawn can't freeze "
+        "every other platform on the gateway loop"
+    )