diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 4a3f7183c..40c9778d1 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -8567,6 +8567,48 @@ def _venv_scripts_dir() -> Path | None:
     return scripts if scripts.is_dir() else None
 
 
+def _wait_for_interpreter_venv_ready(*, timeout: float = 15.0) -> bool:
+    """Ensure the venv hosting ``sys.executable`` has an intact ``pyvenv.cfg``.
+
+    During ``hermes update`` the managed-uv path can rebuild the project venv
+    (``rebuild_venv`` → ``shutil.rmtree`` + ``uv venv``) before the
+    desktop-rebuild and profile-skills-sync steps run. Both of those steps
+    spawn a child process with ``sys.executable``. If they fire while the venv
+    is mid-rewrite, the interpreter launcher finds the venv directory but no
+    ``pyvenv.cfg`` yet and aborts with the bare stderr line
+    ``No pyvenv.cfg file`` — surfacing as a spurious "Desktop build failed" /
+    "sync failed" on an update that otherwise succeeded.
+
+    A venv's ``pyvenv.cfg`` sits one level up from the interpreter's ``bin`` /
+    ``Scripts`` dir. If ``sys.executable`` is NOT a venv interpreter (no
+    sibling marker dir, e.g. a system Python on PATH), there is nothing to
+    wait for and we return True immediately. Otherwise we poll briefly for the
+    marker to (re)appear — the rewrite window is short — and return whether
+    it's present. Best-effort: never raises, callers proceed regardless.
+    """
+    try:
+        exe = Path(sys.executable).resolve()
+    except Exception:
+        return True
+
+    venv_dir = exe.parent.parent  # .../venv/{bin,Scripts}/python -> .../venv
+    bin_dir = venv_dir / ("Scripts" if _is_windows() else "bin")
+    if not bin_dir.is_dir():
+        # Not a venv-hosted interpreter — pyvenv.cfg is irrelevant.
+        return True
+
+    cfg = venv_dir / "pyvenv.cfg"
+    if cfg.is_file():
+        return True
+
+    deadline = _time.monotonic() + max(0.0, timeout)
+    while _time.monotonic() < deadline:
+        if cfg.is_file():
+            return True
+        _time.sleep(0.25)
+    return cfg.is_file()
+
+
 def _hermes_exe_shims(scripts_dir: Path) -> list[Path]:
     """Entry-point shims that uv may try to rewrite during ``pip install -e .``.
 
@@ -10260,11 +10302,19 @@ def _cmd_update_impl(args, gateway_mode: bool):
         has_desktop_app = _desktop_packaged_executable(desktop_dir) is not None or _desktop_dist_exists(desktop_dir)
         if (desktop_dir / "package.json").exists() and shutil.which("npm") and has_desktop_app:
             print("→ Checking if desktop app needs rebuilding...")
-            build_result = subprocess.run(
-                [sys.executable, "-m", "hermes_cli.main", "desktop", "--build-only"],
-                cwd=PROJECT_ROOT,
-                check=False,
-            )
+            # The Python-dependency step above may have rebuilt the venv that
+            # hosts sys.executable. Wait for its pyvenv.cfg to settle before
+            # spawning, or the child interpreter aborts with "No pyvenv.cfg
+            # file" and the rebuild spuriously "fails" on a successful update.
+            _wait_for_interpreter_venv_ready()
+            _desktop_build_cmd = [sys.executable, "-m", "hermes_cli.main", "desktop", "--build-only"]
+            # Stream the build output live (long Electron builds otherwise
+            # look hung). On the rare nonzero exit, retry once after waiting
+            # again for the venv — this covers a still-settling rebuild window
+            # the first wait didn't fully catch.
+            build_result = subprocess.run(_desktop_build_cmd, cwd=PROJECT_ROOT, check=False)
+            if build_result.returncode != 0 and _wait_for_interpreter_venv_ready():
+                build_result = subprocess.run(_desktop_build_cmd, cwd=PROJECT_ROOT, check=False)
             if build_result.returncode != 0:
                 print("  ⚠ Desktop build failed (non-fatal; run `hermes desktop` to retry)")
 
@@ -10320,6 +10370,10 @@ def _cmd_update_impl(args, gateway_mode: bool):
             if all_profiles:
                 print()
                 print("→ Syncing bundled skills to all profiles...")
+                # seed_profile_skills spawns sys.executable; if the venv was
+                # just rebuilt above, wait for pyvenv.cfg before the loop so
+                # the children don't abort with "No pyvenv.cfg file".
+                _wait_for_interpreter_venv_ready()
                 for p in all_profiles:
                     try:
                         r = seed_profile_skills(p.path, quiet=True)
diff --git a/tests/hermes_cli/test_update_venv_ready.py b/tests/hermes_cli/test_update_venv_ready.py
new file mode 100644
index 000000000..69cf1f4f6
--- /dev/null
+++ b/tests/hermes_cli/test_update_venv_ready.py
@@ -0,0 +1,83 @@
+"""Tests for ``_wait_for_interpreter_venv_ready`` in ``hermes_cli/main.py``.
+
+During ``hermes update`` the managed-uv path can rebuild the project venv
+(rmtree + ``uv venv``) before the desktop-rebuild and profile-skills-sync
+steps spawn ``sys.executable``. If those children fire while the venv is
+mid-rewrite, the interpreter launcher aborts with ``No pyvenv.cfg file`` and
+the step spuriously "fails" on an otherwise-successful update. The helper
+waits for the marker to settle first.
+"""
+
+from __future__ import annotations
+
+import os
+import threading
+import time
+from pathlib import Path
+
+from hermes_cli.main import _wait_for_interpreter_venv_ready
+
+
+def _make_fake_venv(tmp_path: Path, *, with_cfg: bool) -> Path:
+    """Create a venv-shaped dir and return the interpreter path inside it."""
+    bin_name = "Scripts" if os.name == "nt" else "bin"
+    bin_dir = tmp_path / bin_name
+    bin_dir.mkdir(parents=True)
+    py = bin_dir / ("python.exe" if os.name == "nt" else "python")
+    py.write_text("#!/bin/sh\n")
+    if with_cfg:
+        (tmp_path / "pyvenv.cfg").write_text("home = /usr\n")
+    return py
+
+
+class TestWaitForInterpreterVenvReady:
+    def test_intact_venv_returns_immediately(self, tmp_path, monkeypatch):
+        py = _make_fake_venv(tmp_path, with_cfg=True)
+        monkeypatch.setattr("sys.executable", str(py))
+        t0 = time.monotonic()
+        assert _wait_for_interpreter_venv_ready(timeout=5) is True
+        assert time.monotonic() - t0 < 0.5
+
+    def test_non_venv_interpreter_returns_immediately(self, tmp_path, monkeypatch):
+        # A bare interpreter whose parent.parent has no bin/Scripts marker
+        # dir is not venv-hosted; pyvenv.cfg is irrelevant.
+        sys_py = tmp_path / "usr" / "bin" / "python"
+        sys_py.parent.mkdir(parents=True)
+        sys_py.write_text("#!/bin/sh\n")
+        # Ensure parent.parent (tmp_path/usr) has no bin sibling shaped like a venv
+        monkeypatch.setattr("sys.executable", str(sys_py))
+        # parent.parent == tmp_path/usr; its "bin" child IS tmp_path/usr/bin
+        # which exists — so this would look venv-ish. Use a deeper layout
+        # where parent.parent has no bin marker:
+        deep = tmp_path / "opt" / "py3" / "real" / "python"
+        deep.parent.mkdir(parents=True)
+        deep.write_text("#!/bin/sh\n")
+        monkeypatch.setattr("sys.executable", str(deep))
+        t0 = time.monotonic()
+        assert _wait_for_interpreter_venv_ready(timeout=5) is True
+        assert time.monotonic() - t0 < 0.5
+
+    def test_waits_for_cfg_to_appear(self, tmp_path, monkeypatch):
+        py = _make_fake_venv(tmp_path, with_cfg=False)
+        monkeypatch.setattr("sys.executable", str(py))
+
+        def _write_cfg_later():
+            time.sleep(0.6)
+            (tmp_path / "pyvenv.cfg").write_text("home = /usr\n")
+
+        th = threading.Thread(target=_write_cfg_later)
+        th.start()
+        try:
+            t0 = time.monotonic()
+            assert _wait_for_interpreter_venv_ready(timeout=5) is True
+            elapsed = time.monotonic() - t0
+        finally:
+            th.join()
+        assert 0.5 < elapsed < 2.0
+
+    def test_returns_false_when_cfg_never_appears(self, tmp_path, monkeypatch):
+        py = _make_fake_venv(tmp_path, with_cfg=False)
+        monkeypatch.setattr("sys.executable", str(py))
+        t0 = time.monotonic()
+        assert _wait_for_interpreter_venv_ready(timeout=1) is False
+        assert 0.9 < time.monotonic() - t0 < 1.6