diff --git a/manager/README.md b/manager/README.md
index a103a7b..3e856fb 100644
--- a/manager/README.md
+++ b/manager/README.md
@@ -46,6 +46,19 @@ python3 -m manager doctor      # diagnose problems
   before the optional accelerators are import-tested and again at the end.
   Requirements files must never pin a torch dependency below what torch needs
   (torch 2.13 wants `setuptools>=77.0.3`).
+- **`ui/package-lock.json` is never modified by installing.** A plain
+  `npm install` re-derives the lockfile for whichever machine runs it — on
+  Windows it strips the `libc` fields off the Linux-only optional binaries
+  (`@next/swc-linux-*`, rollup, lightningcss), on Linux it adds them back — so
+  with the install baked into `npm run build_and_start` every user got a dirty
+  tree on every launch, which then blocks `manager update` (a dirty tree aborts
+  the pull). `nodejs.ensure_ui_deps` owns the install instead: `npm install
+  --no-save`, gated on a hash of `ui/package.json` + `ui/package-lock.json`
+  (stored in the venv state), with the lockfile bytes snapshotted and restored
+  either way. `manager launch` therefore runs `npm run db_build_start`, not
+  `build_and_start`; the latter still exists for the manual
+  `cd ui && npm run build_and_start` flow in the README and calls the same
+  non-writing install via `npm run install_deps`.
 - **Nothing global is ever installed.** FFmpeg (shared builds — the libs
   torchcodec dlopens) goes to `.ffmpeg/` ([ffmpeg.py](ffmpeg.py)), Node
   (when the system lacks >= 20) to `.node/` ([nodejs.py](nodejs.py)), the uv
diff --git a/manager/env.py b/manager/env.py
index f376e49..cd0d380 100644
--- a/manager/env.py
+++ b/manager/env.py
@@ -544,6 +544,7 @@ def sync(spec, detection, dry_run=False, force=False):
     ensure_requirements(spec, dry_run=dry_run, force=force or changed_torch)
     ffmpeg.ensure_ffmpeg(detection, dry_run=dry_run)
     nodejs.ensure_node(detection, dry_run=dry_run)
+    nodejs.ensure_ui_deps(dry_run=dry_run)
     write_sitecustomize(dry_run=dry_run)
     migrations.run_pending(dry_run=dry_run)
     ok("Environment is up to date.")
diff --git a/manager/launch.py b/manager/launch.py
index 68ca4c6..d94aca8 100644
--- a/manager/launch.py
+++ b/manager/launch.py
@@ -1,4 +1,4 @@
-"""Launch the AI Toolkit web UI (ui/ -> npm run build_and_start)."""
+"""Launch the AI Toolkit web UI (ui/ -> npm run db_build_start)."""
 
 import os
 import subprocess
@@ -45,12 +45,6 @@ def build_env():
     return env
 
 
-def _find_npm(env):
-    import shutil
-
-    return shutil.which("npm.cmd" if IS_WINDOWS else "npm", path=env.get("PATH"))
-
-
 def _headless():
     return IS_LINUX and not (
         os.environ.get("DISPLAY") or os.environ.get("WAYLAND_DISPLAY")
@@ -79,7 +73,7 @@ def launch_ui(open_browser=True):
         die("No Python environment found. Run: python3 -m manager install")
 
     env = build_env()
-    npm = _find_npm(env)
+    npm = nodejs.find_npm(env)
     if not npm:
         die(
             "Node.js was not found. Run `python3 -m manager sync` to install a "
@@ -93,6 +87,10 @@ def launch_ui(open_browser=True):
             % (major, nodejs.MIN_NODE_MAJOR)
         )
 
+    # deps are installed here rather than by the npm script so the lockfile is
+    # never rewritten (see nodejs.ensure_ui_deps); a no-op once they're in sync
+    nodejs.ensure_ui_deps(env=env)
+
     info("Starting AI Toolkit UI (%s) ..." % UI_URL)
     stop_event = threading.Event()
     if open_browser and not _headless():
@@ -101,7 +99,7 @@ def launch_ui(open_browser=True):
         ).start()
 
     proc = subprocess.Popen(
-        [npm, "run", "build_and_start"], cwd=os.path.join(REPO_ROOT, "ui"), env=env
+        [npm, "run", "db_build_start"], cwd=os.path.join(REPO_ROOT, "ui"), env=env
     )
     try:
         return proc.wait()
diff --git a/manager/nodejs.py b/manager/nodejs.py
index 6eb8b93..011f1e9 100644
--- a/manager/nodejs.py
+++ b/manager/nodejs.py
@@ -14,10 +14,13 @@ from .util import (
     IS_MAC,
     IS_WINDOWS,
     REPO_ROOT,
+    clean_env,
     download,
     extract_archive,
+    file_hash,
     info,
     ok,
+    run,
     warn,
     which,
 )
@@ -27,6 +30,9 @@ NODE_DIR = os.path.join(REPO_ROOT, ".node")
 NODE_VERSION = "24.11.1"
 MIN_NODE_MAJOR = 20
 
+UI_DIR = os.path.join(REPO_ROOT, "ui")
+UI_STATE_KEY = "ui_deps_hash"
+
 
 def node_bin_dir():
     # windows zips have node.exe/npm.cmd at the archive root; unix under bin/
@@ -119,3 +125,93 @@ def ensure_node(detection, dry_run=False):
         return True
     finally:
         shutil.rmtree(tmp, ignore_errors=True)
+
+
+# ---------------------------------------------------------------- ui deps
+
+
+def npm_env():
+    """Scrubbed env with the local .node/ ahead of PATH."""
+    env = clean_env()
+    if os.path.isdir(node_bin_dir()):
+        env["PATH"] = node_bin_dir() + os.pathsep + env.get("PATH", "")
+    return env
+
+
+def find_npm(env=None):
+    """npm from the local .node/ copy when we installed one, else the system."""
+    path = (env or npm_env()).get("PATH")
+    return shutil.which("npm.cmd" if IS_WINDOWS else "npm", path=path)
+
+
+def _ui_deps_hash():
+    return file_hash(
+        [
+            os.path.join(UI_DIR, "package-lock.json"),
+            os.path.join(UI_DIR, "package.json"),
+        ]
+    )
+
+
+def ensure_ui_deps(env=None, dry_run=False):
+    """Install ui/node_modules without ever rewriting ui/package-lock.json.
+
+    A plain `npm install` treats the lockfile as writable and re-derives it for
+    the machine doing the install: on Windows it strips the `libc` fields off
+    the Linux-only optional binaries (@next/swc-linux-*, rollup, lightningcss),
+    on Linux it puts them back. Since the UI installs on every launch, everyone
+    ends up with a modified lockfile they never asked for — which then blocks
+    `manager update`, because a dirty tree aborts the pull.
+
+    `--no-save` keeps the resolution but stops the write-back, and stays an
+    incremental install (seconds) rather than the full wipe-and-refetch of
+    `npm ci`. The lockfile is snapshotted and restored regardless, so this
+    holds even if a future npm changes what `--no-save` covers.
+
+    Gated on a hash of the manifests so steady-state launches skip npm.
+    """
+    from . import env as env_mod
+
+    lock = os.path.join(UI_DIR, "package-lock.json")
+    if not os.path.isfile(lock):
+        warn("ui/package-lock.json is missing — skipping UI dependency install.")
+        return False
+    want = _ui_deps_hash()
+    if env_mod.venv_exists() and env_mod.load_state().get(UI_STATE_KEY) == want:
+        if os.path.isdir(os.path.join(UI_DIR, "node_modules")):
+            ok("UI dependencies already installed.")
+            return False
+    if dry_run:
+        info("[dry-run] would run: npm install --no-save (in %s)" % UI_DIR)
+        return False
+    env = env or npm_env()
+    npm = find_npm(env)
+    if npm is None:
+        warn("npm not found — skipping UI dependency install.")
+        return False
+
+    info("Installing UI dependencies...")
+    with open(lock, "rb") as f:
+        before = f.read()
+    code, _ = run(
+        [npm, "install", "--no-save", "--no-audit", "--no-fund"],
+        cwd=UI_DIR,
+        env=env,
+        stream=True,
+        check=False,
+    )
+    with open(lock, "rb") as f:
+        after = f.read()
+    if after != before:
+        with open(lock, "wb") as f:
+            f.write(before)
+        info("Reverted npm's edit to ui/package-lock.json (managed by the repo).")
+    if code != 0:
+        warn("UI dependency install failed — the UI may not start.")
+        return False
+    if env_mod.venv_exists():
+        state = env_mod.load_state()
+        state[UI_STATE_KEY] = want
+        env_mod.save_state(state)
+    ok("UI dependencies ready.")
+    return True
diff --git a/ui/package.json b/ui/package.json
index b799bba..0b72a0b 100644
--- a/ui/package.json
+++ b/ui/package.json
@@ -6,7 +6,9 @@
     "dev": "concurrently -k -n WORKER,UI \"ts-node-dev --project tsconfig.worker.json --respawn --watch cron --transpile-only cron/worker.ts\" \"ts-node-dev --project tsconfig.worker.json --respawn --watch cron --transpile-only cron/fileServer.ts dev --port 3000\"",
     "build": "tsc -p tsconfig.worker.json && next build",
     "start": "concurrently --restart-tries -1 --restart-after 1000 -n WORKER,UI \"node dist/cron/worker.js\" \"node dist/cron/fileServer.js start --port 8675\"",
-    "build_and_start": "npm install && npm run update_db && npm run build && npm run start",
+    "install_deps": "npm install --no-save --no-audit --no-fund",
+    "db_build_start": "npm run update_db && npm run build && npm run start",
+    "build_and_start": "npm run install_deps && npm run db_build_start",
     "lint": "next lint",
     "update_db": "npx prisma generate && npx prisma db push",
     "format": "prettier --write \"**/*.{js,jsx,ts,tsx,css,scss}\""