diff --git a/README.md b/README.md
index dcab97c..db9ac16 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,7 @@ Orchestrator: `scripts/build-vite.js` (flags: `--local`, `--headless`,
 ## The registry: `apps.json`
 
 Each entry registers one doc app. A `slug` is required; the source is one of
-`repo` (+ optional `version`), `localPath`, or `prebuilt`:
+`repo` (+ optional `version`), `localPath`, `prebuilt`, or an `iframe` URL:
 
 ```jsonc
 [
@@ -97,6 +97,29 @@ Each entry registers one doc app. A `slug` is required; the source is one of
 ]
 ```
 
+### iframe onboarding (temporary)
+
+Teams that already host their docs elsewhere and can't yet produce a headless
+package can be listed immediately with an **iframe** entry — no `repo`,
+`marketplace.json`, or artifact needed. It renders as a full-viewport `<iframe>`
+inside the marketplace chrome and shows an **External** badge in the catalogue.
+
+```jsonc
+{
+  "type": "iframe",
+  "url": "https://my-team.example.com/docs",
+  "slug": "my-team",
+  "name": "My Team Docs",
+  "description": "...",
+  "icon": "book-open",
+  "tags": ["my-team"],
+  "temporary": true          // stopgap — migrate to a headless package when ready
+}
+```
+
+The external site must permit embedding (its CSP `frame-ancestors` /
+`X-Frame-Options` must not block the marketplace origin). See issue #10.
+
 The repo ships an `apps.json` that registers the **vendored docs-example fixture**
 twice (`user-guide`, `guide-mirror`) so the build and tests are hermetic out of
 the box. Replace it with your own apps for a real deployment.
diff --git a/apps.json b/apps.json
index 941dbc3..9e242fb 100644
--- a/apps.json
+++ b/apps.json
@@ -20,5 +20,17 @@
       "cross-app"
     ],
     "prebuilt": "tests/fixtures/docs-example.dist.tar.gz"
+  },
+  {
+    "slug": "external-docs",
+    "name": "External Docs",
+    "description": "Iframe-onboarded external documentation site (temporary integration path).",
+    "icon": "book-open",
+    "tags": [
+      "external"
+    ],
+    "type": "iframe",
+    "url": "https://example.com/docs",
+    "temporary": true
   }
 ]
diff --git a/contract/HEADLESS_RULES.md b/contract/HEADLESS_RULES.md
index c56cf38..201469f 100644
--- a/contract/HEADLESS_RULES.md
+++ b/contract/HEADLESS_RULES.md
@@ -4,6 +4,15 @@ Doc apps included in the marketplace must produce **headless HTML output** — p
 contain content (sidebar + prose) but **do not** include a site-level top navigation bar.
 The marketplace injects its own persistent 56 px chrome at the top of every page.
 
+> **Temporary alternative — iframe onboarding.** Teams that already host their docs
+> elsewhere and cannot yet produce a headless package can be listed immediately with an
+> `apps.json` entry of `"type": "iframe"` + a `"url"` (no `marketplace.json`, no release
+> artifact, no headless build). The page is rendered as a full-viewport `<iframe>` inside
+> the marketplace chrome. This is an **explicit stopgap** — such entries should carry
+> `"temporary": true` and be migrated to a proper headless package when possible. The
+> external site must allow embedding (its CSP `frame-ancestors` / `X-Frame-Options` must
+> not block the marketplace origin). See issue #10.
+
 ---
 
 ## Required: Headless build flag
diff --git a/scripts/build-vite.js b/scripts/build-vite.js
index fa15af5..1eb3a37 100644
--- a/scripts/build-vite.js
+++ b/scripts/build-vite.js
@@ -100,14 +100,32 @@ async function build() {
 
   const registeredApps = JSON.parse(readFileSync(join(ROOT, 'apps.json'), 'utf8'));
 
+  // Validate registry entries up front (fail fast with a clear message).
+  for (const app of registeredApps) {
+    if (!app.slug) fail('apps.json: an entry is missing "slug"');
+    if (app.type === 'iframe') {
+      if (!app.url) fail(app.slug + ': iframe entry requires a "url"');
+      if (app.temporary) warn(app.slug + ': temporary iframe entry — migrate to a headless package when ready');
+    } else if (!app.prebuilt && !app.localPath && !app.repo) {
+      fail(app.slug + ': packaged entry needs one of "repo", "localPath", or "prebuilt"');
+    }
+  }
+
+  // iframe entries have no artifact to fetch/build — they render a single route
+  // (see src/pages/[...path].astro). Only packaged apps go through the pipeline.
+  const iframeApps   = registeredApps.filter(a => a.type === 'iframe');
+  const packagedApps = registeredApps.filter(a => a.type !== 'iframe');
+
   // 1. Prepare apps/ directory
   step('1/3  Preparing sub-app artifacts → apps/');
   mkdirSync(APPS_DIR, { recursive: true });
 
+  for (const app of iframeApps) ok(app.slug + ' registered (iframe → ' + app.url + ')');
+
   // Apps shipping a `prebuilt` artifact are prepared locally regardless of mode
   // (hermetic — no network, no per-app build). The rest go through fetch/local.
-  const prebuiltApps  = registeredApps.filter(a => a.prebuilt);
-  const remainingApps = registeredApps.filter(a => !a.prebuilt);
+  const prebuiltApps  = packagedApps.filter(a => a.prebuilt);
+  const remainingApps = packagedApps.filter(a => !a.prebuilt);
 
   for (const app of prebuiltApps) preparePrebuilt(app);
 
@@ -140,7 +158,7 @@ async function build() {
   mkdirSync(PUBLIC_ROOT, { recursive: true });
 
   // Clean only known slug dirs so user-owned public/ files (favicon, robots.txt…) are preserved
-  for (const app of registeredApps) {
+  for (const app of packagedApps) {
     const slugDir = join(PUBLIC_ROOT, app.slug);
     if (existsSync(slugDir)) rmSync(slugDir, { recursive: true });
   }
@@ -155,7 +173,7 @@ async function build() {
     }
   }
 
-  for (const app of registeredApps) {
+  for (const app of packagedApps) {
     const srcDir = join(APPS_DIR, app.slug);
     if (!existsSync(srcDir)) { warn(app.slug + ': apps/ dir missing, skipping asset copy'); continue; }
     copyAssets(srcDir, join(PUBLIC_ROOT, app.slug));
@@ -194,7 +212,7 @@ async function build() {
   const elapsed = ((Date.now() - startMs) / 1000).toFixed(1);
   const slugs = readdirSync(APPS_DIR).filter(d => statSync(join(APPS_DIR, d)).isDirectory());
   const appList = slugs.map(s => '    \u2022 \x1b[36m' + s + '\x1b[0m → dist/' + s + '/').join('\n');
-  console.log('\n\x1b[32m✓\x1b[0m \x1b[1m' + slugs.length + ' app(s) integrated\x1b[0m in ' + elapsed + 's\n\n  Apps:\n' + appList + '\n  Prefix: \x1b[33m' + PATH_PREFIX + '\x1b[0m\n');
+  console.log('\n\x1b[32m✓\x1b[0m \x1b[1m' + (slugs.length + iframeApps.length) + ' app(s) integrated\x1b[0m in ' + elapsed + 's\n\n  Apps:\n' + appList + '\n  Prefix: \x1b[33m' + PATH_PREFIX + '\x1b[0m\n');
 }
 
 build().catch(err => {
diff --git a/scripts/setup-test-apps.mjs b/scripts/setup-test-apps.mjs
index ceb6ce5..8664300 100644
--- a/scripts/setup-test-apps.mjs
+++ b/scripts/setup-test-apps.mjs
@@ -56,6 +56,17 @@ const apps = [
     tags: ['mirror', 'cross-app'],
     prebuilt: artifact,
   },
+  {
+    // iframe onboarding mode (issue #10): no artifact, renders an <iframe>.
+    slug: 'external-docs',
+    name: 'External Docs',
+    description: 'Iframe-onboarded external documentation site (temporary integration path).',
+    icon: 'book-open',
+    tags: ['external'],
+    type: 'iframe',
+    url: 'https://example.com/docs',
+    temporary: true,
+  },
 ];
 
 const out = join(ROOT, 'apps.json');
diff --git a/src/components/AppCard.astro b/src/components/AppCard.astro
index 9fe61ec..fb1c0cf 100644
--- a/src/components/AppCard.astro
+++ b/src/components/AppCard.astro
@@ -9,6 +9,7 @@ interface Props {
     description: string;
     icon?: string;
     tags?: string[];
+    type?: string;
   };
   delay?: number;
   base?: string;
@@ -26,6 +27,7 @@ const delayClass = delay > 0 && delay <= 4 ? ` delay-${delay}` : '';
   <h2 class="text-[15px] font-semibold mb-1.5 leading-snug" style="color:var(--text-heading)">{app.name}</h2>
   <p class="text-[13px] leading-relaxed flex-1 mb-4" style="color:var(--text-body)">{app.description}</p>
   <div class="flex flex-wrap gap-1.5 mt-auto">
+    {app.type === 'iframe' && <span class="mp-tag mp-tag-external" title="Embedded via iframe (external site)">External</span>}
     {(app.tags || []).map(tag => <span class="mp-tag">{tag}</span>)}
   </div>
 </a>
diff --git a/src/pages/[...path].astro b/src/pages/[...path].astro
index 03b4afa..9925e92 100644
--- a/src/pages/[...path].astro
+++ b/src/pages/[...path].astro
@@ -1,15 +1,16 @@
 ---
 // src/pages/[...path].astro
 //
-// Catchall route that serves every sub-app HTML file as a static Astro page.
-// getStaticPaths enumerates all HTML files under apps/{slug}/ for every entry
-// in apps.json, so `astro build` outputs them to dist/{slug}/...
-// In `astro dev` the same function runs on demand — no custom middleware needed.
+// Catchall route that serves every sub-app page as a static Astro page.
+// getStaticPaths enumerates all HTML files under apps/{slug}/ for every packaged
+// entry in apps.json, plus a single route per iframe-type entry (issue #10).
 
 import { readFileSync } from 'node:fs';
 import { ClientRouter } from 'astro:transitions';
 import { getAppPages } from '../utils/apps.js';
 import { transformSubAppHtml } from '../utils/transform.js';
+import Base from '../layouts/Base.astro';
+import Chrome from '../components/Chrome.astro';
 
 const PREFIX = 'knowledge-base';
 
@@ -21,16 +22,39 @@ export function getStaticPaths() {
   }));
 }
 
-const { file, slug, fileRelDir, appHeadless, apps, title, section } = Astro.props;
-const raw  = readFileSync(file, 'utf-8');
-const html = transformSubAppHtml(raw, apps, slug, fileRelDir, PREFIX, appHeadless);
+const props = Astro.props;
+const { slug, appHeadless, apps, title } = props;
 
-// Split at </head> so we can inject <ClientRouter /> before it closes.
-// This preserves all of transformSubAppHtml's output (URL rewrites, chrome,
-// fragment router, etc.) while making every sub-app page participate in
-// Astro's client-side routing for cross-page transitions.
-const headEnd = html.indexOf('</head>');
-const beforeHeadEnd = headEnd >= 0 ? html.slice(0, headEnd) : html;
-const afterHeadEnd  = headEnd >= 0 ? html.slice(headEnd)   : '';
+// Packaged apps: read the sub-app HTML file and transform it. Split at </head>
+// so <ClientRouter /> is injected for Astro's client-side transitions.
+let beforeHeadEnd = '';
+let afterHeadEnd = '';
+if (!props.iframe) {
+  const raw  = readFileSync(props.file, 'utf-8');
+  const html = transformSubAppHtml(raw, apps, slug, props.fileRelDir, PREFIX, appHeadless);
+  const headEnd = html.indexOf('</head>');
+  beforeHeadEnd = headEnd >= 0 ? html.slice(0, headEnd) : html;
+  afterHeadEnd  = headEnd >= 0 ? html.slice(headEnd)   : '';
+}
+
+// iFrame entries: fill the viewport (below the 56px chrome when standalone; full
+// height when headless/embedded, where the host supplies its own chrome).
+const iframeHeight = appHeadless ? '100vh' : 'calc(100vh - var(--mp-chrome-h, 56px))';
 ---
-<Fragment set:html={beforeHeadEnd} /><ClientRouter /><Fragment set:html={afterHeadEnd} />
+{props.iframe ? (
+  <Base headless={appHeadless} title={title}>
+    {!appHeadless && <Chrome apps={apps} activeSlug={slug} base={`/${PREFIX}`} />}
+    <main style="display:block;flex:1 1 auto;min-height:0">
+      <iframe
+        src={props.url}
+        title={title}
+        style={`width:100%;height:${iframeHeight};border:0;display:block`}
+        loading="lazy"
+        referrerpolicy="no-referrer-when-downgrade"
+        sandbox="allow-scripts allow-same-origin allow-popups allow-forms allow-downloads"
+      ></iframe>
+    </main>
+  </Base>
+) : (
+  <Fragment set:html={beforeHeadEnd} /><ClientRouter /><Fragment set:html={afterHeadEnd} />
+)}
diff --git a/src/styles/marketplace.css b/src/styles/marketplace.css
index 8b436a7..fb91551 100644
--- a/src/styles/marketplace.css
+++ b/src/styles/marketplace.css
@@ -222,3 +222,11 @@ wf-body.mp-wrapped #sidebar { top: var(--mp-chrome-h) !important; }
   color: var(--text-muted);
   border: 1px solid var(--border);
 }
+
+/* iframe-onboarding entries (issue #10) — flags an externally embedded app */
+.mp-tag-external {
+  background: var(--color-kb-50);
+  color: var(--color-kb-600);
+  border-color: var(--color-kb-100);
+  font-weight: 600;
+}
diff --git a/src/utils/apps.js b/src/utils/apps.js
index 1d5d129..cda9a19 100644
--- a/src/utils/apps.js
+++ b/src/utils/apps.js
@@ -39,6 +39,24 @@ export function getAppPages(cwd, headless) {
     const appDir      = join(appsDir, app.slug);
     const appHeadless = app.headless ?? headless;
 
+    if (app.type === 'iframe') {
+      // iFrame onboarding mode: no artifact on disk — emit a single route that
+      // renders a full-viewport <iframe> for the external URL. See issue #10.
+      pages.push({
+        routePath:   app.slug,
+        file:        null,
+        slug:        app.slug,
+        fileRelDir:  '',
+        appHeadless,
+        apps,
+        title:       app.name ?? app.slug,
+        section:     null,
+        iframe:      true,
+        url:         app.url,
+      });
+      continue;
+    }
+
     if (Array.isArray(app.pages) && app.pages.length > 0) {
       // Manifest-driven routing: use the pages array from marketplace.json
       for (const page of app.pages) {
diff --git a/tests/build-integrity.spec.js b/tests/build-integrity.spec.js
index 368249f..5f65e80 100644
--- a/tests/build-integrity.spec.js
+++ b/tests/build-integrity.spec.js
@@ -74,3 +74,29 @@ test.describe('Build integrity', () => {
     expect(html).toContain('astro-view-transitions-enabled');
   });
 });
+
+// ── iframe onboarding mode (issue #10) ──────────────────────────────────────
+test.describe('iframe onboarding', () => {
+  test('renders a single route with a full-viewport iframe to the external URL', () => {
+    expect(existsSync(join(DIST, 'external-docs/index.html')), 'iframe app page missing').toBe(true);
+    const html = read('external-docs/index.html');
+    expect(html).toMatch(/<iframe[^>]*src="https:\/\/example\.com\/docs"/);
+    expect(html).toContain('height:100vh'); // headless build → full viewport
+  });
+
+  test('iframe entry does not produce packaged sub-app pages', () => {
+    // No artifact was fetched/built — only the single index route exists.
+    expect(existsSync(join(DIST, 'external-docs/docs')), 'unexpected packaged pages for iframe entry').toBe(false);
+  });
+
+  test('landing shows the iframe app card with an External badge', () => {
+    const html = read('index.html');
+    expect(html).toContain('External Docs');
+    expect(html).toContain('mp-tag-external');
+  });
+
+  test('packaged apps are unaffected by the iframe entry', () => {
+    expect(existsSync(join(DIST, 'user-guide/docs/index.html'))).toBe(true);
+    expect(existsSync(join(DIST, 'guide-mirror/docs/index.html'))).toBe(true);
+  });
+});