diff --git a/package.json b/package.json
index 59ee931d..df56e048 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,8 @@
     "test:watch": "vitest --config config/vitest.config.ts",
     "test:coverage": "vitest run --config config/vitest.config.ts --coverage",
     "test:ci": "vitest run --config config/vitest.ci.config.ts",
+    "pretest:mobile": "node scripts/prepare-test-vendor.mjs",
+    "test:mobile": "vitest run --config test/mobile/vitest.config.ts",
     "check:frontend-syntax": "node scripts/check-frontend-syntax.mjs",
     "fix:node-pty": "node scripts/fix-node-pty.mjs",
     "typecheck": "tsc --noEmit",
diff --git a/scripts/prepare-test-vendor.mjs b/scripts/prepare-test-vendor.mjs
new file mode 100644
index 00000000..66717b31
--- /dev/null
+++ b/scripts/prepare-test-vendor.mjs
@@ -0,0 +1,126 @@
+#!/usr/bin/env node
+/**
+ * Populate `src/web/public/vendor/` with the browser bundles the mobile tests need.
+ *
+ * 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`.
+ *
+ * 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.
+ * `src/web/public/vendor/` is gitignored, so these stay build artifacts.
+ *
+ * Idempotent: skips outputs already newer than their source.
+ */
+import { execFileSync } from 'node:child_process';
+import { appendFileSync, copyFileSync, existsSync, mkdirSync, statSync } from 'node:fs';
+import { dirname, join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const ROOT = resolve(dirname(fileURLToPath(import.meta.url)), '..');
+const OUT = join(ROOT, 'src', 'web', 'public', 'vendor');
+const NM = join(ROOT, 'node_modules');
+
+/**
+ * Every `vendor/` asset index.html requests, minus the two already committed
+ * (dompurify, marked). Kept in sync with scripts/build.mjs steps 3-4 — a missing
+ * entry here is a 404 that silently disables the terminal in tests.
+ *
+ * mode: 'copy' | 'minify' | 'bundle'
+ */
+const ASSETS = [
+  { src: join(NM, '@xterm/xterm/css/xterm.css'), out: 'xterm.css', mode: 'copy' },
+  { src: join(NM, '@xterm/xterm/lib/xterm.js'), out: 'xterm.min.js', mode: 'minify' },
+  { src: join(NM, '@xterm/addon-fit/lib/addon-fit.js'), out: 'xterm-addon-fit.min.js', mode: 'minify' },
+  {
+    src: join(NM, '@xterm/addon-serialize/lib/addon-serialize.js'),
+    out: 'xterm-addon-serialize.min.js',
+    mode: 'minify',
+  },
+  {
+    src: join(NM, '@xterm/addon-unicode11/lib/addon-unicode11.js'),
+    out: 'xterm-addon-unicode11.min.js',
+    mode: 'minify',
+  },
+  { src: join(NM, '@xterm/addon-webgl/lib/addon-webgl.js'), out: 'xterm-addon-webgl.min.js', mode: 'copy' },
+  {
+    src: join(ROOT, 'packages/xterm-zerolag-input/src/zerolag-input-addon.ts'),
+    out: 'xterm-zerolag-input.js',
+    mode: 'bundle',
+    globalName: 'XtermZerolagInput',
+  },
+];
+
+function isFresh(src, dest) {
+  if (!existsSync(dest)) return false;
+  try {
+    return statSync(dest).mtimeMs >= statSync(src).mtimeMs;
+  } catch {
+    return false;
+  }
+}
+
+mkdirSync(OUT, { recursive: true });
+
+let built = 0;
+let skipped = 0;
+for (const asset of ASSETS) {
+  const dest = join(OUT, asset.out);
+  if (!existsSync(asset.src)) {
+    console.error(`[test-vendor] missing input: ${asset.src}\n  run \`npm install\` first`);
+    process.exit(1);
+  }
+  if (isFresh(asset.src, dest)) {
+    skipped += 1;
+    continue;
+  }
+  if (asset.mode === 'copy') {
+    copyFileSync(asset.src, dest);
+  } else if (asset.mode === 'minify') {
+    execFileSync('npx', ['esbuild', asset.src, '--minify', `--outfile=${dest}`], { stdio: 'inherit' });
+  } else {
+    execFileSync(
+      'npx',
+      [
+        'esbuild',
+        asset.src,
+        '--bundle',
+        '--minify',
+        '--format=iife',
+        `--global-name=${asset.globalName}`,
+        `--outfile=${dest}`,
+      ],
+      { stdio: 'inherit' }
+    );
+  }
+  built += 1;
+
+  // The zerolag bundle exports only `XtermZerolagInput`. app.js constructs
+  // `new LocalEchoOverlay(terminal)` directly, so scripts/build.mjs appends
+  // global aliases after esbuild — without them initTerminal() throws
+  // `LocalEchoOverlay is not defined` at the point it builds the overlay, and
+  // every later step (including the mobile touch handlers) silently never runs.
+  if (asset.out === 'xterm-zerolag-input.js') {
+    appendFileSync(
+      dest,
+      '\n// Global aliases for browser usage\n' +
+        'if(typeof window!=="undefined"){' +
+        'window.ZerolagInputAddon=XtermZerolagInput.ZerolagInputAddon;' +
+        'window.LocalEchoOverlay=class extends XtermZerolagInput.ZerolagInputAddon{' +
+        'constructor(terminal){' +
+        'super({prompt:{type:"character",char:"\\u276f",offset:2}});' +
+        'this.activate(terminal);' +
+        '}' +
+        '};' +
+        '}\n'
+    );
+  }
+}
+
+console.log(`[test-vendor] ${built} built, ${skipped} up to date -> src/web/public/vendor/`);