diff --git a/e2e/specs/mock/steering-escalation.spec.ts b/e2e/specs/mock/steering-escalation.spec.ts
new file mode 100644
index 0000000000..7223f58cc3
--- /dev/null
+++ b/e2e/specs/mock/steering-escalation.spec.ts
@@ -0,0 +1,225 @@
+import { expect, test } from '@playwright/test';
+import type { Page, Response } from '@playwright/test';
+import {
+  MOCK_ENDPOINTS,
+  NEW_CHAT_PATH,
+  messagesView,
+  replyPrompt,
+  replyText,
+  selectMockEndpoint,
+  sendMessage,
+} from './helpers';
+
+/** Last chunk streamed by the fake model's slow replies (160 chunks, 0-indexed). */
+const SLOW_REPLY_LAST_CHUNK = 'chunk-159';
+
+const uniqueLabel = (prefix: string) =>
+  `${prefix}-${Date.now()}-${Math.floor(Math.random() * 1e4)}`;
+
+const messageInput = (page: Page) => page.getByRole('textbox', { name: 'Message input' });
+const duringRunSendButton = (page: Page) => page.getByTestId('during-run-send-button');
+const queuedRows = (page: Page) => page.getByTestId('queued-message-row');
+const messageTurns = (page: Page) => messagesView(page).locator('.message-render');
+const inFlightSteers = (page: Page) => page.getByTestId('in-flight-steer');
+const appliedSteerParts = (page: Page) => messagesView(page).getByTestId('steer-part');
+
+function isSteerRequest(response: Response) {
+  return (
+    response.request().method() === 'POST' &&
+    new URL(response.url()).pathname === '/api/agents/chat/steer'
+  );
+}
+
+function isArmRequest(response: Response) {
+  return (
+    response.request().method() === 'POST' &&
+    new URL(response.url()).pathname === '/api/agents/chat/steer/arm'
+  );
+}
+
+/** Establish a real conversation with a fast first turn so during-run actions
+ *  target a persisted conversation id instead of racing new-convo creation. */
+async function establishConversation(page: Page, label: string) {
+  const setup = await sendMessage(page, replyPrompt(label));
+  expect(setup.ok()).toBeTruthy();
+  await expect(messagesView(page).getByText(replyText(label))).toBeVisible({ timeout: 30000 });
+  await expect(page).toHaveURL(/\/c\/[0-9a-fA-F-]{36}$/, { timeout: 15000 });
+}
+
+/** Fill the composer mid-run: the during-run send button must take the
+ *  send/stop slot (it becomes the form submit target for Enter). */
+async function typeDuringRun(page: Page, text: string) {
+  const input = messageInput(page);
+  await input.click();
+  await input.fill(text);
+  await expect(duringRunSendButton(page)).toBeVisible({ timeout: 5000 });
+}
+
+/**
+ * Escalation of WAITING messages (PR: interrupt-steer escalation controls).
+ * `E2E_SLOW_REPLY` streams pure text with no tool boundary, so nothing here
+ * can inject the ordinary way — an in-thread steer part can only come from a
+ * mid-stream seal, which makes it the behavioral proof that escalation armed
+ * a real interrupt rather than relabelling a chip.
+ */
+test.describe('escalating waiting messages to an interrupt', () => {
+  /** `steerInterruptsByDefault` is a localStorage preference; the toggle test
+   *  flips it, and a mid-test failure must not leak preempt-by-default into
+   *  the rest of the serial suite. */
+  test.afterEach(async ({ page }) => {
+    await page.evaluate(() => window.localStorage.removeItem('steerInterruptsByDefault'));
+  });
+
+  test('queued row escalates as an interrupt: the message seals mid-stream instead of waiting for run end', async ({
+    page,
+  }) => {
+    test.setTimeout(150000);
+    const label = uniqueLabel('queue-escalate');
+    const queueText = `Escalated queued message ${label}`;
+
+    await page.goto(NEW_CHAT_PATH, { timeout: 10000 });
+    await selectMockEndpoint(page, MOCK_ENDPOINTS[0]);
+    await establishConversation(page, `queue-escalate-setup-${label}`);
+
+    const run = await sendMessage(page, `E2E_SLOW_REPLY:${label}`);
+    expect(run.ok()).toBeTruthy();
+    await expect(messagesView(page).getByText('chunk-010')).toBeVisible({ timeout: 15000 });
+
+    // Queue the message (Ctrl/Cmd+Enter routes to the non-default action).
+    await typeDuringRun(page, queueText);
+    await messageInput(page).press('ControlOrMeta+Enter');
+    const row = queuedRows(page).filter({ hasText: queueText });
+    await expect(row).toBeVisible({ timeout: 10000 });
+
+    // Escalate it: the row's ZapOff button submits the queued text as an
+    // interrupt steer (a preempt-armed POST /chat/steer).
+    const [steerResponse] = await Promise.all([
+      page.waitForResponse(isSteerRequest, { timeout: 15000 }),
+      row.getByTestId('queued-interrupt-now').click(),
+    ]);
+    expect(steerResponse.status()).toBe(202);
+    expect(((await steerResponse.json()) as { preempt?: boolean }).preempt).toBe(true);
+    await expect(row).toHaveCount(0, { timeout: 10000 });
+
+    // Injected in-thread with no tool boundary available — only a mid-stream
+    // seal can put a steer part here. Without escalation this message would
+    // have waited for run end and auto-sent as its own follow-up turn.
+    await expect(appliedSteerParts(page).filter({ hasText: queueText })).toHaveCount(1, {
+      timeout: 90000,
+    });
+    await expect(inFlightSteers(page)).toHaveCount(0);
+
+    // Sealed, not run to completion, and the pre-seal text survives.
+    await expect(messagesView(page).getByText(SLOW_REPLY_LAST_CHUNK)).toHaveCount(0);
+    await expect(messagesView(page).getByText('chunk-010')).toBeVisible();
+
+    // Stayed INSIDE the response: no auto-sent follow-up pair.
+    await expect(messageTurns(page)).toHaveCount(4);
+    await expect(queuedRows(page)).toHaveCount(0);
+  });
+
+  test('waiting steer bubble arms in place via POST /chat/steer/arm and seals mid-stream', async ({
+    page,
+  }) => {
+    test.setTimeout(150000);
+    const label = uniqueLabel('bubble-arm');
+    const steerText = `Armed waiting steer ${label}`;
+
+    await page.goto(NEW_CHAT_PATH, { timeout: 10000 });
+    await selectMockEndpoint(page, MOCK_ENDPOINTS[0]);
+    await establishConversation(page, `bubble-arm-setup-${label}`);
+
+    const run = await sendMessage(page, `E2E_SLOW_REPLY:${label}`);
+    expect(run.ok()).toBeTruthy();
+    await expect(messagesView(page).getByText('chunk-010')).toBeVisible({ timeout: 15000 });
+
+    // An ORDINARY steer (plain Enter, preference off): with no tool boundary
+    // in this stream it stays acknowledged-and-waiting as a bubble.
+    await typeDuringRun(page, steerText);
+    const [steerResponse] = await Promise.all([
+      page.waitForResponse(isSteerRequest, { timeout: 15000 }),
+      messageInput(page).press('Enter'),
+    ]);
+    expect(steerResponse.status()).toBe(202);
+    expect(((await steerResponse.json()) as { preempt?: boolean }).preempt).toBeFalsy();
+    const bubble = inFlightSteers(page).filter({ hasText: steerText });
+    await expect(bubble).toBeVisible({ timeout: 10000 });
+
+    // Escalate from the bubble's overflow menu: ONE atomic in-place arm.
+    await bubble.getByRole('button', { name: 'More options' }).click();
+    const [armResponse] = await Promise.all([
+      page.waitForResponse(isArmRequest, { timeout: 15000 }),
+      page.getByRole('menuitem', { name: 'Interrupt & steer now' }).click(),
+    ]);
+    expect(armResponse.status()).toBe(200);
+    expect(((await armResponse.json()) as { armed?: boolean }).armed).toBe(true);
+
+    // Relabelled IN PLACE: still exactly one bubble with the same text, and
+    // an interrupting steer no longer offers escalation on reopen.
+    await expect(inFlightSteers(page)).toHaveCount(1);
+    await bubble.getByRole('button', { name: 'More options' }).click();
+    await expect(page.getByRole('menuitem', { name: 'Cancel steering message' })).toBeVisible({
+      timeout: 5000,
+    });
+    await expect(page.getByRole('menuitem', { name: 'Interrupt & steer now' })).toHaveCount(0);
+    await page.keyboard.press('Escape');
+
+    // The armed steer seals mid-stream and injects with no tool boundary.
+    await expect(appliedSteerParts(page).filter({ hasText: steerText })).toHaveCount(1, {
+      timeout: 90000,
+    });
+    await expect(inFlightSteers(page)).toHaveCount(0);
+    await expect(messagesView(page).getByText(SLOW_REPLY_LAST_CHUNK)).toHaveCount(0);
+    await expect(messagesView(page).getByText('chunk-010')).toBeVisible();
+    await expect(messageTurns(page)).toHaveCount(4);
+  });
+
+  test('always-interrupt toggle in a waiting row menu makes plain Enter preempt', async ({
+    page,
+  }) => {
+    test.setTimeout(150000);
+    const label = uniqueLabel('toggle');
+    const queueText = `Queued while toggling ${label}`;
+    const steerText = `Enter now interrupts ${label}`;
+
+    await page.goto(NEW_CHAT_PATH, { timeout: 10000 });
+    await selectMockEndpoint(page, MOCK_ENDPOINTS[0]);
+    await establishConversation(page, `toggle-setup-${label}`);
+
+    const run = await sendMessage(page, `E2E_SLOW_REPLY:${label}`);
+    expect(run.ok()).toBeTruthy();
+    await expect(messagesView(page).getByText('chunk-010')).toBeVisible({ timeout: 15000 });
+
+    // A queued row hosts the overflow menu carrying the preference toggle.
+    await typeDuringRun(page, queueText);
+    await messageInput(page).press('ControlOrMeta+Enter');
+    const row = queuedRows(page).filter({ hasText: queueText });
+    await expect(row).toBeVisible({ timeout: 10000 });
+
+    await row.getByRole('button', { name: 'More options' }).click();
+    await page.getByRole('menuitem', { name: 'Always interrupt instead' }).click();
+
+    // The toggle is live for the SAME run: plain Enter now routes the default
+    // steer through the preempt path (the 202 carries the armed flag).
+    await typeDuringRun(page, steerText);
+    const [steerResponse] = await Promise.all([
+      page.waitForResponse(isSteerRequest, { timeout: 15000 }),
+      messageInput(page).press('Enter'),
+    ]);
+    expect(steerResponse.status()).toBe(202);
+    expect(((await steerResponse.json()) as { preempt?: boolean }).preempt).toBe(true);
+
+    // And the seal proves it end to end: injected with no boundary available.
+    await expect(appliedSteerParts(page).filter({ hasText: steerText })).toHaveCount(1, {
+      timeout: 90000,
+    });
+    await expect(messagesView(page).getByText(SLOW_REPLY_LAST_CHUNK)).toHaveCount(0);
+
+    // The menu now offers the way back (label flipped), on the queued row
+    // that is still waiting for run end.
+    await row.getByRole('button', { name: 'More options' }).click();
+    await expect(page.getByRole('menuitem', { name: 'Wait for tool steps instead' })).toBeVisible({
+      timeout: 5000,
+    });
+  });
+});