diff --git a/scripts/prepare-test-vendor.mjs b/scripts/prepare-test-vendor.mjs
index 66717b31..8f9e41c9 100644
--- a/scripts/prepare-test-vendor.mjs
+++ b/scripts/prepare-test-vendor.mjs
@@ -4,22 +4,42 @@
  *
  * The mobile suite (test/mobile/**) drives a real browser against a WebServer
  * started from TypeScript source, so fastify-static serves
- * `join(__dirname, 'public')` = `src/web/public` — NOT `dist/web/public`, where
- * `npm run build` puts the vendor bundles. Every `/vendor/xterm*` request 404s,
- * so `Terminal` is never defined, `initTerminal()` never runs, and every test
- * touching `app.terminal` dies with `Cannot read properties of null`.
+ * `join(__dirname, 'public')` = `src/web/public`, NOT `dist/web/public`, where
+ * `npm run build` puts the vendor bundles. Without them every `/vendor/xterm*`
+ * request 404s, so `Terminal` is never defined, `initTerminal()` never runs, and
+ * every test touching `app.terminal` dies with `Cannot read properties of null`.
  *
  * That stayed invisible because config/vitest.ci.config.ts excludes
  * `test/mobile/**`, so CI never ran the suite.
  *
- * Mirrors the vendor steps in scripts/build.mjs, targeting the source tree.
- * Same inputs and output names, so the page markup needs no test-only branch.
+ * ⚠️ scripts/postinstall.js:238-303 already writes these same 7 outputs (same
+ * names, same alias tail), so a plain `npm install` leaves the suite working. What
+ * this script adds is FRESHNESS and independence from install time: a checkout
+ * installed with `--ignore-scripts`, or one borrowing another tree's
+ * `node_modules`, never ran postinstall, and an edit to the zerolag package after
+ * install leaves the bundle stale. It runs as `pretest:mobile`.
+ *
+ * Mirrors the vendor steps in scripts/build.mjs, targeting the source tree. Same
+ * inputs and output names, so the page markup needs no test-only branch. That
+ * makes THREE hand-synced copies of this asset table (here, build.mjs:45-51,
+ * postinstall.js:255-303); keep them in step or a missing entry becomes a 404 that
+ * silently disables the terminal.
  * `src/web/public/vendor/` is gitignored, so these stay build artifacts.
  *
- * Idempotent: skips outputs already newer than their source.
+ * Idempotent: skips outputs newer than every input they derive from.
  */
 import { execFileSync } from 'node:child_process';
-import { appendFileSync, copyFileSync, existsSync, mkdirSync, statSync } from 'node:fs';
+import {
+  appendFileSync,
+  copyFileSync,
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  readdirSync,
+  renameSync,
+  rmSync,
+  statSync,
+} from 'node:fs';
 import { dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
 
@@ -54,14 +74,45 @@ const ASSETS = [
     out: 'xterm-zerolag-input.js',
     mode: 'bundle',
     globalName: 'XtermZerolagInput',
+    // The alias tail appended below. Its absence means the output is a partial
+    // write from an older version of this script, whatever its mtime says.
+    mustContain: 'window.LocalEchoOverlay',
   },
 ];
 
-function isFresh(src, dest) {
+/**
+ * Every input an asset is derived from. For the bundle that is the whole package
+ * source dir, not just the entry: esbuild pulls in the entry's siblings, so
+ * comparing against the entry alone reports "up to date" after an edit to
+ * overlay-renderer.ts and the suite then tests a stale overlay. Editing those
+ * siblings is exactly the single-source workflow CLAUDE.md mandates.
+ */
+function sourcesOf(asset) {
+  if (asset.mode !== 'bundle') return [asset.src];
+  const dir = dirname(asset.src);
+  try {
+    return readdirSync(dir)
+      .filter((f) => f.endsWith('.ts'))
+      .map((f) => join(dir, f));
+  } catch {
+    return [asset.src];
+  }
+}
+
+function isFresh(asset, dest) {
   if (!existsSync(dest)) return false;
   try {
-    return statSync(dest).mtimeMs >= statSync(src).mtimeMs;
+    // Content check before the mtime check, because mtime cannot see a WRONG file.
+    // The atomic rename below stops this script from ever publishing a half-written
+    // bundle, but it cannot repair one already on disk: anyone who ran an earlier
+    // version that appended the aliases in place has a complete-looking file with a
+    // current mtime and no alias tail, and a pure mtime cache calls that "up to
+    // date" forever while the suite dies on `LocalEchoOverlay is not defined`.
+    if (asset.mustContain && !readFileSync(dest, 'utf-8').includes(asset.mustContain)) return false;
+    const destMs = statSync(dest).mtimeMs;
+    return sourcesOf(asset).every((src) => destMs >= statSync(src).mtimeMs);
   } catch {
+    // an unreadable or vanished input: rebuild rather than trust the cache
     return false;
   }
 }
@@ -76,28 +127,43 @@ for (const asset of ASSETS) {
     console.error(`[test-vendor] missing input: ${asset.src}\n  run \`npm install\` first`);
     process.exit(1);
   }
-  if (isFresh(asset.src, dest)) {
+  if (isFresh(asset, dest)) {
     skipped += 1;
     continue;
   }
-  if (asset.mode === 'copy') {
-    copyFileSync(asset.src, dest);
-  } else if (asset.mode === 'minify') {
-    execFileSync('npx', ['esbuild', asset.src, '--minify', `--outfile=${dest}`], { stdio: 'inherit' });
-  } else {
-    execFileSync(
-      'npx',
-      [
+  // Build into a temp path and rename into place at the very end. The zerolag
+  // bundle is finished by a SECOND step (the alias append below), so writing
+  // `dest` directly leaves a window where a complete-looking file with a current
+  // mtime is missing its tail: `isFresh` then reports "up to date" forever and the
+  // suite dies on `LocalEchoOverlay is not defined`, which is the exact failure
+  // this script exists to prevent. An interrupted esbuild or copy poisons the
+  // cache the same way. rename(2) is atomic within a directory, so a reader sees
+  // either the old file or the finished new one, never a half-written one.
+  const tmp = `${dest}.tmp`;
+  rmSync(tmp, { force: true });
+  // cwd: ROOT so `npx` resolves the repo's pinned esbuild. Without it a run from
+  // another directory misses the local install and fetches an unpinned one.
+  const run = (args) => execFileSync('npx', args, { stdio: 'inherit', cwd: ROOT });
+  try {
+    if (asset.mode === 'copy') {
+      copyFileSync(asset.src, tmp);
+    } else if (asset.mode === 'minify') {
+      run(['esbuild', asset.src, '--minify', `--outfile=${tmp}`]);
+    } else {
+      run([
         'esbuild',
         asset.src,
         '--bundle',
         '--minify',
         '--format=iife',
         `--global-name=${asset.globalName}`,
-        `--outfile=${dest}`,
-      ],
-      { stdio: 'inherit' }
-    );
+        `--outfile=${tmp}`,
+      ]);
+    }
+  } catch (err) {
+    rmSync(tmp, { force: true });
+    console.error(`[test-vendor] failed to build ${asset.out} from ${asset.src}\n  ${err.message}`);
+    process.exit(1);
   }
   built += 1;
 
@@ -108,7 +174,7 @@ for (const asset of ASSETS) {
   // every later step (including the mobile touch handlers) silently never runs.
   if (asset.out === 'xterm-zerolag-input.js') {
     appendFileSync(
-      dest,
+      tmp,
       '\n// Global aliases for browser usage\n' +
         'if(typeof window!=="undefined"){' +
         'window.ZerolagInputAddon=XtermZerolagInput.ZerolagInputAddon;' +
@@ -121,6 +187,9 @@ for (const asset of ASSETS) {
         '}\n'
     );
   }
+
+  // Only now is the output complete, so publish it.
+  renameSync(tmp, dest);
 }
 
 console.log(`[test-vendor] ${built} built, ${skipped} up to date -> src/web/public/vendor/`);
diff --git a/test/mobile/README.md b/test/mobile/README.md
index 65d89ea0..18483f7c 100644
--- a/test/mobile/README.md
+++ b/test/mobile/README.md
@@ -16,22 +16,30 @@ Validates Codeman's mobile UI across 136 devices, covering:
 
 ## Quick Start
 
+⚠️ Go through `npm run test:mobile`, not `npx vitest` directly. The suite serves the
+page from `src/web/public`, but `npm run build` puts the xterm vendor bundles in
+`dist/web/public`, so without them every `/vendor/xterm*` request 404s, `Terminal` is
+never defined and every test touching `app.terminal` fails on a null. The
+`pretest:mobile` hook (`scripts/prepare-test-vendor.mjs`) is what puts them in place,
+and npm only fires it for `npm run test:mobile`. Run the prepare script by hand first
+if you really need a bare `npx vitest`.
+
 ```bash
 # Run all mobile tests
-npx vitest run --config test/mobile/vitest.config.ts
+npm run test:mobile
 
 # Run a single test file
-npx vitest run --config test/mobile/vitest.config.ts test/mobile/keyboard.test.ts
+npm run test:mobile -- test/mobile/keyboard.test.ts
 
-# Quick mode — 6 representative devices, skip full matrix
-CI_QUICK=1 npx vitest run --config test/mobile/vitest.config.ts
+# Quick mode: 6 representative devices, skip full matrix
+CI_QUICK=1 npm run test:mobile
 
 # Full device matrix only (136 devices)
-npx vitest run --config test/mobile/vitest.config.ts test/mobile/device-matrix.test.ts
+npm run test:mobile -- test/mobile/device-matrix.test.ts
 
 # Update visual baselines (delete old baselines, re-run)
 rm -rf test/mobile/snapshots/*.png
-npx vitest run --config test/mobile/vitest.config.ts test/mobile/visual-regression.test.ts
+npm run test:mobile -- test/mobile/visual-regression.test.ts
 ```
 
 ## Test Files