diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 214a1855b30..3c027e908c5 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -5688,12 +5688,29 @@ def _build_web_ui(web_dir: Path, *, fatal: bool = False) -> bool:
             print("Install Node.js, then run:  cd web && npm install && npm run build")
         return not fatal
     print("→ Building web UI...")
+
+    def _relay(result: "subprocess.CompletedProcess") -> None:
+        """Print captured npm output so users can see *why* a step failed.
+
+        Windows users hitting `rm -rf` / `cp -r` errors (or any other
+        sync-assets / Vite failure) would otherwise see only ``Web UI
+        build failed`` with no hint of the underlying cause, because
+        the npm calls run with ``capture_output=True``.
+        """
+        for blob in (result.stdout, result.stderr):
+            if not blob:
+                continue
+            text = blob.decode("utf-8", errors="replace").rstrip() if isinstance(blob, bytes) else blob.rstrip()
+            if text:
+                print(text)
+
     r1 = _run_npm_install_deterministic(npm, web_dir, extra_args=("--silent",))
     if r1.returncode != 0:
         print(
             f"  {'✗' if fatal else '⚠'} Web UI npm install failed"
             + ("" if fatal else " (hermes web will not be available)")
         )
+        _relay(r1)
         if fatal:
             print("  Run manually:  cd web && npm install && npm run build")
         return False
@@ -5739,8 +5756,7 @@ def _build_web_ui(web_dir: Path, *, fatal: bool = False) -> bool:
             f"  {'✗' if fatal else '⚠'} Web UI build failed"
             + ("" if fatal else " (hermes web will not be available)")
         )
-        if stderr_tail:
-            print(f"  Build error:\n  {stderr_tail}")
+        _relay(r2)
         if fatal:
             print("  Run manually:  cd web && npm install && npm run build")
         return False
diff --git a/web/package.json b/web/package.json
index 56262ff2a82..50456076b64 100644
--- a/web/package.json
+++ b/web/package.json
@@ -4,7 +4,7 @@
   "version": "0.0.0",
   "type": "module",
   "scripts": {
-    "sync-assets": "node -e \"const fs=require('fs');fs.rmSync('public/fonts',{recursive:true,force:true});fs.rmSync('public/ds-assets',{recursive:true,force:true});fs.cpSync('node_modules/@nous-research/ui/dist/fonts','public/fonts',{recursive:true});fs.cpSync('node_modules/@nous-research/ui/dist/assets','public/ds-assets',{recursive:true});\"",
+    "sync-assets": "node scripts/sync-assets.mjs",
     "predev": "npm run sync-assets",
     "prebuild": "npm run sync-assets",
     "dev": "vite",
diff --git a/web/scripts/sync-assets.mjs b/web/scripts/sync-assets.mjs
new file mode 100644
index 00000000000..19b0bafb6aa
--- /dev/null
+++ b/web/scripts/sync-assets.mjs
@@ -0,0 +1,27 @@
+#!/usr/bin/env node
+// Cross-platform replacement for the previous shell pipeline:
+//
+//   rm -rf public/fonts public/ds-assets
+//   && cp -r node_modules/@nous-research/ui/dist/fonts public/fonts
+//   && cp -r node_modules/@nous-research/ui/dist/assets public/ds-assets
+//
+// `rm -rf` / `cp -r` don't exist on Windows cmd.exe, so `npm run build`
+// (invoked from Python via subprocess → cmd.exe) failed before Vite ran.
+// Using Node's stdlib fs keeps this dependency-free and platform-neutral.
+
+import { cpSync, rmSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const webRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const uiDist = resolve(webRoot, "node_modules", "@nous-research", "ui", "dist");
+
+const targets = [
+  { from: resolve(uiDist, "fonts"), to: resolve(webRoot, "public", "fonts") },
+  { from: resolve(uiDist, "assets"), to: resolve(webRoot, "public", "ds-assets") },
+];
+
+for (const { from, to } of targets) {
+  rmSync(to, { recursive: true, force: true });
+  cpSync(from, to, { recursive: true });
+}