diff --git a/REUSE.toml b/REUSE.toml
index f3ccbd90d..40648d946 100644
--- a/REUSE.toml
+++ b/REUSE.toml
@@ -16,3 +16,11 @@ SPDX-FileCopyrightText = "asciinema contributors"
 SPDX-License-Identifier = "Apache-2.0"
 # NOTE: This file must be placed before deploying tutorials that use ::cast directives.
 # Download from https://github.com/asciinema/asciinema-player/releases
+
+[[annotations]]
+path = "hugo/static/js/vendor/asciinema-player.css"
+SPDX-FileCopyrightText = "asciinema contributors"
+SPDX-License-Identifier = "Apache-2.0"
+# NOTE: This file must be placed before deploying tutorials that use ::cast directives.
+# Download from https://github.com/asciinema/asciinema-player/releases
+
diff --git a/hugo/assets/js/__tests__/cast.test.ts b/hugo/assets/js/__tests__/cast.test.ts
new file mode 100644
index 000000000..ddf52fd9d
--- /dev/null
+++ b/hugo/assets/js/__tests__/cast.test.ts
@@ -0,0 +1,66 @@
+// @vitest-environment happy-dom
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Provide a minimal AsciinemaPlayer stub on window.
+const mockCreate = vi.fn();
+beforeEach(() => {
+  vi.resetModules();
+  document.body.innerHTML = '';
+  (window as unknown as Record<string, unknown>)['AsciinemaPlayer'] = { create: mockCreate };
+  mockCreate.mockClear();
+});
+
+it('mounts player on .tut-cast elements in viewport', async () => {
+  document.body.innerHTML = `
+    <div class="tut-cast" data-tut-cast="https://example.com/demo.cast"
+         data-rows="24" data-speed="1.5"></div>`;
+  const { initCast } = await import('../cast.js');
+  await initCast({ skipObserver: true });  // bypass IO for unit test
+  expect(mockCreate).toHaveBeenCalledOnce();
+  const [src, el, opts] = mockCreate.mock.calls[0];
+  expect(src).toBe('https://example.com/demo.cast');
+  expect(el).toBeInstanceOf(HTMLElement);
+  expect(opts.rows).toBe(24);
+  expect(opts.speed).toBe(1.5);
+});
+
+it('does not double-mount an already mounted element', async () => {
+  document.body.innerHTML = `
+    <div class="tut-cast" data-tut-cast="https://example.com/demo.cast"
+         data-cast-mounted="true"></div>`;
+  const { initCast } = await import('../cast.js');
+  await initCast({ skipObserver: true });
+  expect(mockCreate).not.toHaveBeenCalled();
+});
+
+it('falls back to immediate mount when IntersectionObserver absent', async () => {
+  document.body.innerHTML = `
+    <div class="tut-cast" data-tut-cast="https://example.com/demo.cast"></div>`;
+  // Remove IO from window to simulate absent support.
+  const origIO = (window as unknown as Record<string, unknown>)['IntersectionObserver'];
+  delete (window as unknown as Record<string, unknown>)['IntersectionObserver'];
+  const { initCast } = await import('../cast.js');
+  await initCast({});
+  expect(mockCreate).toHaveBeenCalledOnce();
+  (window as unknown as Record<string, unknown>)['IntersectionObserver'] = origIO;
+});
+
+it('passes idle-limit as idleTimeLimit to player', async () => {
+  document.body.innerHTML = `
+    <div class="tut-cast" data-tut-cast="https://example.com/demo.cast"
+         data-idle-limit="2"></div>`;
+  const { initCast } = await import('../cast.js');
+  await initCast({ skipObserver: true });
+  const [, , opts] = mockCreate.mock.calls[0];
+  expect(opts.idleTimeLimit).toBe(2);
+});
+
+it('removes the placeholder pre after mounting', async () => {
+  document.body.innerHTML = `
+    <div class="tut-cast" data-tut-cast="https://example.com/demo.cast">
+      <pre class="tut-cast__placeholder">Loading…</pre>
+    </div>`;
+  const { initCast } = await import('../cast.js');
+  await initCast({ skipObserver: true });
+  expect(document.querySelector('.tut-cast__placeholder')).toBeNull();
+});
diff --git a/hugo/assets/js/cast.ts b/hugo/assets/js/cast.ts
index cb0ff5c3b..941f6a12a 100644
--- a/hugo/assets/js/cast.ts
+++ b/hugo/assets/js/cast.ts
@@ -1 +1,117 @@
-export {};
+// cast.ts — lazy-mount asciinema player on .tut-cast[data-tut-cast] elements.
+// The asciinema-player is vendored as an IIFE (sets window.AsciinemaPlayer),
+// so we inject a <script> tag at runtime rather than using import().
+
+declare const AsciinemaPlayer:
+  | { create(src: string, el: Element, opts: Record<string, unknown>): void }
+  | undefined;
+
+const PLAYER_JS = '/js/vendor/asciinema-player.min.js';
+const PLAYER_CSS = '/js/vendor/asciinema-player.css';
+
+function loadPlayerResources(): Promise<void> {
+  return new Promise<void>((resolve) => {
+    // If the player global already exists, nothing to load.
+    if (typeof (window as unknown as Record<string, unknown>)['AsciinemaPlayer'] !== 'undefined') {
+      resolve();
+      return;
+    }
+
+    // Inject CSS link if not already present.
+    if (!document.querySelector(`link[href="${PLAYER_CSS}"]`)) {
+      const link = document.createElement('link');
+      link.rel = 'stylesheet';
+      link.href = PLAYER_CSS;
+      document.head.appendChild(link);
+    }
+
+    // Inject script tag if not already present.
+    if (!document.querySelector(`script[src="${PLAYER_JS}"]`)) {
+      const script = document.createElement('script');
+      script.src = PLAYER_JS;
+      script.onload = () => resolve();
+      script.onerror = () => resolve(); // fail-open: resolve anyway
+      document.head.appendChild(script);
+    } else {
+      // Script tag exists but global not yet set — wait for it.
+      const existing = document.querySelector(`script[src="${PLAYER_JS}"]`) as HTMLScriptElement;
+      existing.addEventListener('load', () => resolve(), { once: true });
+    }
+  });
+}
+
+function mount(el: HTMLElement): void {
+  const src = el.dataset['tutCast'];
+  if (!src) return;
+
+  // Remove placeholder if present.
+  const placeholder = el.querySelector('.tut-cast__placeholder');
+  if (placeholder) placeholder.remove();
+
+  // Build options from data attributes.
+  const opts: Record<string, unknown> = {};
+  const rows = el.dataset['rows'];
+  const cols = el.dataset['cols'];
+  const speed = el.dataset['speed'];
+  const poster = el.dataset['poster'];
+  const idleLimit = el.dataset['idleLimit'];
+
+  if (rows !== undefined) opts['rows'] = Number(rows);
+  if (cols !== undefined) opts['cols'] = Number(cols);
+  if (speed !== undefined) opts['speed'] = Number(speed);
+  if (poster !== undefined) opts['poster'] = poster;
+  if (idleLimit !== undefined) opts['idleTimeLimit'] = Number(idleLimit);
+
+  // Mark as mounted to guard against double-mount.
+  el.dataset['castMounted'] = 'true';
+
+  // AsciinemaPlayer is available as a global from the IIFE.
+  (AsciinemaPlayer as NonNullable<typeof AsciinemaPlayer>).create(src, el, opts);
+}
+
+export async function initCast(opts?: { skipObserver?: boolean }): Promise<void> {
+  const elements = Array.from(
+    document.querySelectorAll<HTMLElement>('.tut-cast[data-tut-cast]:not([data-cast-mounted])')
+  );
+
+  if (elements.length === 0) return;
+
+  const skipIO = opts?.skipObserver === true;
+
+  if (skipIO || !('IntersectionObserver' in window)) {
+    // Bypass IntersectionObserver — mount immediately (test seam + IO-absent fallback).
+    await loadPlayerResources();
+    for (const el of elements) {
+      mount(el);
+    }
+    return;
+  }
+
+  // Lazy-mount via IntersectionObserver with a 200px root margin so the
+  // player loads slightly before scrolling into view.
+  const observer = new IntersectionObserver(
+    (entries) => {
+      for (const entry of entries) {
+        if (!entry.isIntersecting) continue;
+        observer.unobserve(entry.target);
+        const target = entry.target as HTMLElement;
+        void loadPlayerResources().then(() => mount(target));
+      }
+    },
+    { rootMargin: '200px' }
+  );
+
+  for (const el of elements) {
+    observer.observe(el);
+  }
+}
+
+// Auto-init when the script is loaded by the page.
+if (document.readyState === 'loading') {
+  document.addEventListener('DOMContentLoaded', () => void initCast({}));
+} else {
+  void initCast({});
+}
+
+// Test seam — exported for unit tests only.
+export const __test__ = { loadPlayerResources, mount };
diff --git a/package.json b/package.json
index 5f48ac342..41f57b5c1 100644
--- a/package.json
+++ b/package.json
@@ -14,7 +14,7 @@
     "start:approuter": "cd approuter && NODE_ENV=development npm start",
     "bind:setup": "node scripts/setup-hybrid-env.js",
     "seed:thirdparty": "echo 'RETIRED: HomepageShelves (incl. third-party links) is now seeded idempotently at boot by srv/lib/homepage/seed-homepage-shelves.js from srv/lib/homepage/homepage-shelves-defaults.js. Edit that module or use /admin-ui/#homepage; no manual seed step needed.' && exit 1",
-    "setup": "npm --prefix hugo-apps install --no-audit --no-fund && npm --prefix app/explore install --no-audit --no-fund && npm --prefix app/channel-atlas install --no-audit --no-fund && sh scripts/install-git-hooks.sh",
+    "setup": "npm --prefix hugo-apps install --no-audit --no-fund && npm --prefix app/explore install --no-audit --no-fund && npm --prefix app/channel-atlas install --no-audit --no-fund && sh scripts/install-git-hooks.sh && npm run vendor:asciinema",
     "build:cds": "cds build --production",
     "fetch-tutorials": "tsx scripts/fetch-tutorials.ts --target hugo",
     "fetch-homepage-shelves": "tsx scripts/fetch-homepage-shelves.ts",
@@ -83,8 +83,9 @@
     "vendor:mediapipe": "node scripts/vendor-mediapipe.cjs",
     "vendor:imgly": "node scripts/vendor-imgly.cjs",
     "vendor:animegan": "node scripts/vendor-animegan.mjs",
+    "vendor:asciinema": "node scripts/vendor-asciinema.mjs",
     "clean:island-bundles": "node scripts/clean-island-bundles.cjs",
-    "build:apps": "npm run vendor:mediapipe && npm run vendor:imgly && npm run vendor:animegan && npm run clean:island-bundles && npm --prefix hugo-apps run build",
+    "build:apps": "npm run vendor:asciinema && npm run vendor:mediapipe && npm run vendor:imgly && npm run vendor:animegan && npm run clean:island-bundles && npm --prefix hugo-apps run build",
     "build:island-manifest": "node scripts/build-island-manifest.cjs",
     "check:ui5-single-copy": "node scripts/check-ui5-single-copy.cjs",
     "retain:assets": "node scripts/retain-asset-bundles.cjs --js-dir hugo/public/js --css-dir hugo/public/css --manifest-out hugo/public/_retained-assets.json",
diff --git a/scripts/__tests__/vendor-asciinema.test.mjs b/scripts/__tests__/vendor-asciinema.test.mjs
new file mode 100644
index 000000000..8aa70d05f
--- /dev/null
+++ b/scripts/__tests__/vendor-asciinema.test.mjs
@@ -0,0 +1,32 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import fs from 'node:fs';
+
+// Uses vi.mock to stub fetch and fs so the test runs offline.
+// NOTE: vi.stubGlobal returns the `vi` object for chaining (not the mock),
+// so we capture the mock fn separately before passing it to stubGlobal.
+describe('vendor-asciinema', () => {
+  afterEach(() => { vi.restoreAllMocks(); });
+
+  it('exits 1 and prints error when fetch returns non-ok', async () => {
+    // Ensure sentinel check passes (returns false = not present) so we reach the fetch path.
+    vi.spyOn(fs, 'existsSync').mockReturnValue(false);
+    vi.stubGlobal('fetch', async () => ({ ok: false, status: 404, statusText: 'Not Found' }));
+    const exitSpy = vi.spyOn(process, 'exit').mockImplementation(() => { throw new Error('exit'); });
+    const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const { main } = await import('../vendor-asciinema.mjs?v=' + Date.now());
+    await expect(main()).rejects.toThrow('exit');
+    expect(exitSpy).toHaveBeenCalledWith(1);
+    expect(consoleSpy.mock.calls.some(args => args[0].toString().includes('404'))).toBe(true);
+  });
+
+  it('skips download when sentinel file already present', async () => {
+    const existsSpy = vi.spyOn(fs, 'existsSync').mockReturnValue(true);
+    const mockFetch = vi.fn();
+    vi.stubGlobal('fetch', mockFetch);
+    const { main } = await import('../vendor-asciinema.mjs?v=' + Date.now());
+    await main();
+    expect(mockFetch).not.toHaveBeenCalled();
+    existsSpy.mockRestore();
+  });
+});
+
diff --git a/scripts/parsers/directives/__tests__/cast-diagram.test.ts b/scripts/parsers/directives/__tests__/cast-diagram.test.ts
index 1e607675a..642de9a18 100644
--- a/scripts/parsers/directives/__tests__/cast-diagram.test.ts
+++ b/scripts/parsers/directives/__tests__/cast-diagram.test.ts
@@ -22,6 +22,15 @@ describe('cast directive', () => {
     const node = { name: 'cast', kind: 'leaf' as const, label: 'demo.cast', attrs: {}, classes: [], line: 1, column: 0 };
     expect(def.toText(node, {})).toContain('demo.cast');
   });
+
+  it('cast shortcode HTML has data-tut-cast and placeholder pre', () => {
+    // This test documents the attribute contract cast.ts relies on.
+    // The shortcode is Hugo-rendered — test the *directive output* which feeds Hugo.
+    const out = expandDirectives('::cast[demo.cast]{rows=24 idle-limit=2}', { slug: 's', repo: 'r', branch: 'b' });
+    // The output is a Hugo shortcode call (not HTML), so we verify the shortcode call params.
+    expect(out).toContain('rows="24"');
+    expect(out).toContain('idle-limit="2"');
+  });
 });
 
 describe('diagram directive', () => {
diff --git a/scripts/vendor-asciinema.mjs b/scripts/vendor-asciinema.mjs
new file mode 100644
index 000000000..14e439699
--- /dev/null
+++ b/scripts/vendor-asciinema.mjs
@@ -0,0 +1,46 @@
+#!/usr/bin/env node
+// scripts/vendor-asciinema.mjs
+// Downloads asciinema-player v3.18.0 static assets from GitHub releases.
+// Both files are placed in hugo/static/js/vendor/ (gitignored).
+// Idempotent: skips if both files and sentinel are already present at the
+// expected version.
+//
+// License: asciinema-player is Apache-2.0. REUSE.toml records both files.
+
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+const DEST_DIR = path.resolve(__dirname, '../hugo/static/js/vendor');
+const VERSION = '3.18.0';
+const SENTINEL = path.join(DEST_DIR, `.asciinema-vendored-${VERSION}`);
+
+export async function main() {
+  if (fs.existsSync(SENTINEL)) {
+    console.log(`skipped (already v${VERSION})`);
+    return;
+  }
+
+  const files = ['asciinema-player.min.js', 'asciinema-player.css'];
+
+  for (const name of files) {
+    const url = `https://github.com/asciinema/asciinema-player/releases/download/v${VERSION}/${name}`;
+    const res = await fetch(url);
+    if (!res.ok) {
+      console.error(`Failed to download ${name}: ${res.status} ${res.statusText}`);
+      process.exit(1);
+    }
+    const buf = Buffer.from(await res.arrayBuffer());
+    fs.writeFileSync(path.join(DEST_DIR, name), buf);
+  }
+
+  fs.writeFileSync(SENTINEL, VERSION);
+  console.log(`vendored asciinema-player v${VERSION}`);
+}
+
+// Only run when invoked directly (not when imported by tests)
+if (process.argv[1] && fileURLToPath(import.meta.url) === path.resolve(process.argv[1])) {
+  main().catch(e => { console.error(e); process.exit(1); });
+}
diff --git a/vitest.config.ts b/vitest.config.ts
index a09d26259..5b7ee3c95 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -41,7 +41,7 @@ export default defineConfig({
           name: 'unit',
           globals: true,
           environment: 'node',
-          include: ['test/**/*.test.{js,ts}', 'scripts/__tests__/**/*.test.{js,ts}', 'scripts/**/__tests__/**/*.test.{js,ts}', 'srv/**/__tests__/**/*.test.{js,ts}', 'app/analytics-explorer/src/**/__tests__/**/*.test.ts', 'app/explore/src/**/__tests__/**/*.test.ts', 'app/channel-atlas/src/**/__tests__/**/*.test.ts', 'hugo-apps/src/**/*.{test,spec}.{js,ts}', '.claude/skills/**/__tests__/**/*.test.{js,ts}', 'hugo/assets/js/__tests__/**/*.test.{js,ts}'],
+          include: ['test/**/*.test.{js,mjs,ts}', 'scripts/__tests__/**/*.test.{js,mjs,ts}', 'scripts/**/__tests__/**/*.test.{js,mjs,ts}', 'srv/**/__tests__/**/*.test.{js,mjs,ts}', 'app/analytics-explorer/src/**/__tests__/**/*.test.ts', 'app/explore/src/**/__tests__/**/*.test.ts', 'app/channel-atlas/src/**/__tests__/**/*.test.ts', 'hugo-apps/src/**/*.{test,spec}.{js,ts}', '.claude/skills/**/__tests__/**/*.test.{js,mjs,ts}', 'hugo/assets/js/__tests__/**/*.test.{js,mjs,ts}'],
           // test/e2e/** is the Playwright-driven `e2e` project (below). Its
           // specs launch a real browser against a DEPLOYED approuter and would
           // hang the unit tier (which has no BASE_URL); the broad