diff --git a/src/dev/seed.test.ts b/src/dev/seed.test.ts
index 9341836..b63b8a7 100644
--- a/src/dev/seed.test.ts
+++ b/src/dev/seed.test.ts
@@ -48,16 +48,17 @@ describe('seedDevData', () => {
       supportAddress: SUPPORT_ADDRESS,
     })
 
-    expect(result.conversationCount).toBe(6)
+    expect(result.conversationCount).toBe(7)
 
     // Real sends went through the injected sender for every reply EXCEPT the
     // one deliberately routed through the seed's own failing sender — see
-    // seed.ts's "failed" demo.
+    // seed.ts's "failed" demo. The HTML demo is inbound-only (no reply), so
+    // it adds a conversation without adding a send.
     expect(sent.length).toBe(4)
 
     const open = await store.listConversations({ limit: 50, folder: 'open' })
     const closed = await store.listConversations({ limit: 50, folder: 'closed' })
-    expect(open).toHaveLength(5)
+    expect(open).toHaveLength(6)
     expect(closed).toHaveLength(1)
 
     const allThreads = await Promise.all(
@@ -83,5 +84,19 @@ describe('seedDevData', () => {
       .find((t) => t.deliveryStatus === 'pending')
     expect(pendingThread).toBeDefined()
     expect(Date.now() - (pendingThread?.createdAt.getTime() ?? 0)).toBeGreaterThan(5 * 60_000)
+
+    // The HTML demo exists to exercise the inbox UI's sanitized-HTML path, so
+    // its inbound thread must actually carry a bodyHtml, stored verbatim
+    // (spec §5: the store returns untrusted HTML as-is; the renderer sanitizes)
+    // alongside a plain-text alternative. Assert the four things the UI must
+    // handle survived storage: formatting, a link, a remote <img>, a <script>.
+    const htmlThread = allThreads.flatMap((c) => c?.threads ?? []).find((t) => t.bodyHtml !== null)
+    expect(htmlThread).toBeDefined()
+    expect(htmlThread?.direction).toBe('inbound')
+    expect(htmlThread?.bodyText).not.toBeNull()
+    expect(htmlThread?.bodyHtml).toContain('<strong>')
+    expect(htmlThread?.bodyHtml).toContain('<a href=')
+    expect(htmlThread?.bodyHtml).toContain('<img ')
+    expect(htmlThread?.bodyHtml).toContain('<script>')
   })
 })
diff --git a/src/dev/seed.ts b/src/dev/seed.ts
index 7cae61a..a71b432 100644
--- a/src/dev/seed.ts
+++ b/src/dev/seed.ts
@@ -3,8 +3,10 @@
  * boot for the upcoming Agent Inbox UI (HT-23) to exercise every state it
  * must render: an inbound-only conversation, a threaded back-and-forth, one
  * outbound thread in each delivery state (`sent`/`failed`/stale `pending`),
- * and a closed conversation. Every name and message below is invented for
- * this seed — never real customer data (CLAUDE.md).
+ * a closed conversation, and one inbound message with a rich HTML body (so
+ * the UI's sanitized-HTML path — spec §5's stored-XSS contract — is exercised
+ * against real data). Every name and message below is invented for this
+ * seed — never real customer data (CLAUDE.md).
  *
  * Reuses the real engine paths wherever practical rather than raw SQL, so
  * seeding itself exercises the store and the send pipeline:
@@ -272,5 +274,41 @@ export async function seedDevData(deps: SeedDevDataDeps): Promise<SeedDevDataRes
   }
   await store.setConversationStatus(closedDemo.conversationId, 'closed')
 
+  // --- 7. Inbound-only with a rich HTML body. --------------------------------
+  // The one demo that exercises the inbox UI's sanitized-HTML path (spec
+  // §5's stored-XSS contract). The parser stores inbound HTML verbatim —
+  // `<script>` and all (specs/mail/threading.md §5, fixtures/mail/observed/
+  // html-body.json) — so this `bodyHtml` deliberately carries the four
+  // things the UI's sanitizer, its "HTML email · sanitized · external images
+  // blocked" caption, and its Show-original modal must all handle: formatting,
+  // a link, a remote `<img>` (a tracking pixel), and a `<script>`. The store
+  // returns it untouched (safe as JSON); sanitization is the renderer's job.
+  // `bodyText` is the plain-text alternative the same mail would carry.
+  await store.createConversation({
+    subject: 'Unexpected charge on my March invoice',
+    customerEmail: 'noah.feldman@example.test',
+    firstMessage: {
+      direction: 'inbound',
+      messageId: '<inbound-1@noah-feldman.example.test>',
+      fromAddress: 'noah.feldman@example.test',
+      bodyText:
+        'Hi there,\n\n' +
+        "My March invoice shows a charge I don't recognize — a line item for " +
+        '"Priority Support" that I never signed up for.\n\n' +
+        'Here is the invoice in question: https://billing.example.com/invoices/48213\n\n' +
+        'Could you take a look and let me know? Thanks,\nNoah',
+      bodyHtml:
+        '<p>Hi there,</p>' +
+        '<p>My <strong>March invoice</strong> shows a charge I don&rsquo;t recognize &mdash; ' +
+        'a line item for <em>&ldquo;Priority Support&rdquo;</em> that I never signed up for.</p>' +
+        '<p>Here is the invoice in question: ' +
+        '<a href="https://billing.example.com/invoices/48213">billing.example.com/invoices/48213</a></p>' +
+        '<p>Could you take a look and let me know? Thanks,<br>Noah</p>' +
+        '<img src="https://tracker.example.com/o.gif?u=48213" width="1" height="1" alt="">' +
+        '<script>document.title = "pwned"</script>',
+    },
+  })
+  conversationCount++
+
   return { conversationCount }
 }