diff --git a/apps/desktop/e2e/queue-turn-boundary.spec.ts b/apps/desktop/e2e/queue-turn-boundary.spec.ts
new file mode 100644
index 000000000000..834a5fe949e4
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+++ b/apps/desktop/e2e/queue-turn-boundary.spec.ts
@@ -0,0 +1,69 @@
+/**
+ * A queued prompt must remain local until the current inference turn settles.
+ *
+ * Hold the first streamed reply open after its first token. This gives the
+ * composer a live, busy turn while the user queues a follow-up, then lets us
+ * assert against the mock provider's real request log before and after the
+ * held turn completes.
+ */
+
+import { expect, test, type Page } from './test'
+
+import { type MockBackendFixture, setupMockBackend, waitForAppReady } from './fixtures'
+import { MOCK_REPLY } from './mock-server'
+
+const ACTIVE_PROMPT = 'E2E_QUEUE_TURN_BOUNDARY_ACTIVE'
+const QUEUED_PROMPT = 'E2E_QUEUE_TURN_BOUNDARY_QUEUED'
+
+async function send(page: Page, text: string): Promise<void> {
+  const composer = page.locator('[contenteditable="true"]').first()
+  await composer.waitFor({ state: 'visible', timeout: 15_000 })
+  await composer.click()
+  await composer.type(text, { delay: 5 })
+  await page.keyboard.press('Enter')
+}
+
+test.describe('queued prompt turn boundary', () => {
+  let fixture: MockBackendFixture | null = null
+
+  test.beforeEach(async () => {
+    fixture = await setupMockBackend({
+      mockServer: { holdFirstStreamForPrompt: ACTIVE_PROMPT }
+    })
+    await waitForAppReady(fixture, 120_000)
+  })
+
+  test.afterEach(async () => {
+    await fixture?.cleanup()
+    fixture = null
+  })
+
+  test('submits a queued prompt only after the active turn completes', async () => {
+    const { mock, page } = fixture!
+    const composer = page.locator('[contenteditable="true"]').first()
+    const queue = page.locator('[data-slot="composer-root"] button[aria-label="Queue message"]')
+
+    await send(page, ACTIVE_PROMPT)
+    await mock.waitForHeldStream()
+
+    await composer.click()
+    await composer.type(QUEUED_PROMPT, { delay: 5 })
+    await queue.click()
+    await expect(page.getByText('1 Queued')).toBeVisible()
+
+    // The mock keeps the active SSE stream open, so a queued prompt has no
+    // completed-turn boundary that could legitimately drain it. Wait past the
+    // queue retry interval and assert the provider saw only the active turn.
+    await page.waitForTimeout(1_000)
+    expect(mock.receivedPrompts.filter(prompt => prompt === QUEUED_PROMPT)).toHaveLength(0)
+    await expect(page.locator('[data-slot="aui_thread-viewport"]')).not.toContainText(QUEUED_PROMPT)
+
+    mock.releaseHeldStream()
+    await page.waitForFunction(
+      expected => (document.querySelector('[data-slot="aui_thread-viewport"]')?.textContent ?? '').includes(expected),
+      MOCK_REPLY,
+      { timeout: 60_000 }
+    )
+    await expect.poll(() => mock.receivedPrompts.filter(prompt => prompt === QUEUED_PROMPT)).toHaveLength(1)
+  })
+})
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