diff --git a/config/test-suites.ts b/config/test-suites.ts
index f0a3fb1f..9f3ed295 100644
--- a/config/test-suites.ts
+++ b/config/test-suites.ts
@@ -32,6 +32,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/codex-predictive-echo.test.ts', // also needs a real codex binary
   'test/split-pane-terminal.browser.test.ts',
   'test/split-pane-orchestration.browser.test.ts',
+  'test/split-pane-auto-collapse.browser.test.ts',
 ];
 
 /**
diff --git a/src/web/public/terminal-split.js b/src/web/public/terminal-split.js
index 6af60796..6ba51427 100644
--- a/src/web/public/terminal-split.js
+++ b/src/web/public/terminal-split.js
@@ -241,3 +241,17 @@ Object.assign(CodemanApp.prototype, {
     });
   },
 });
+
+const _originalOnSessionDeleted = CodemanApp.prototype._onSessionDeleted;
+CodemanApp.prototype._onSessionDeleted = function (data) {
+  if (this._splitSessionId === data.id) {
+    this.closeSplitPane();
+  } else if (this._splitPane && this.activeSessionId === data.id) {
+    // Pane A's session ended: promote Pane B by closing the split and
+    // selecting its session as the new (single) active pane.
+    const promoted = this._splitSessionId;
+    this.closeSplitPane();
+    if (promoted) this.selectSession(promoted);
+  }
+  return _originalOnSessionDeleted.call(this, data);
+};
diff --git a/test/split-pane-auto-collapse.browser.test.ts b/test/split-pane-auto-collapse.browser.test.ts
new file mode 100644
index 00000000..c321df56
--- /dev/null
+++ b/test/split-pane-auto-collapse.browser.test.ts
@@ -0,0 +1,76 @@
+// test/split-pane-auto-collapse.browser.test.ts
+/** @fileoverview Real Chromium coverage for split auto-collapse when either session ends (Task 6). */
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { chromium, type Browser, type Page } from 'playwright';
+import { WebServer } from '../src/web/server.js';
+
+const PORT = 3177;
+const BASE_URL = `http://localhost:${PORT}`;
+
+describe('split-pane auto-collapse in a real browser', () => {
+  let server: WebServer;
+  let browser: Browser;
+  let page: Page;
+
+  beforeAll(async () => {
+    server = new WebServer(PORT, false, true);
+    await server.start();
+    browser = await chromium.launch({ headless: true });
+    page = await browser.newPage();
+    await page.goto(BASE_URL, { waitUntil: 'domcontentloaded' });
+    await page.waitForFunction(() => (window as any).app?.terminal, null, { timeout: 30000 });
+  }, 90000);
+
+  afterAll(async () => {
+    if (browser) await browser.close();
+    if (server) await server.stop();
+  }, 60000);
+
+  async function createShellSession(): Promise<string> {
+    return page.evaluate(async () => {
+      const res = await fetch('/api/sessions', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ workingDir: '/tmp', mode: 'shell' }),
+      });
+      // POST /api/sessions nests the session under data.session, and mode:'shell'
+      // does not spawn a PTY on creation alone (pid: null, no pane) — an explicit
+      // POST .../shell is what actually starts it (both found and fixed by Task 4's
+      // implementer against this exact pattern; carried forward here so this task
+      // does not rediscover the same two bugs).
+      const id = (await res.json()).data.session.id;
+      await fetch(`/api/sessions/${id}/shell`, { method: 'POST' });
+      return id;
+    });
+  }
+
+  it('deleting the Pane B session auto-collapses the split', async () => {
+    const idA = await createShellSession();
+    const idB = await createShellSession();
+
+    await page.evaluate((id) => (window as any).app.selectSession(id), idA);
+    await page.waitForFunction((id) => (window as any).app.activeSessionId === id, idA, { timeout: 10000 });
+    await page.evaluate((id) => (window as any).app.openSplitPane(id), idB);
+    await page.waitForSelector('.terminal-pane-b', { timeout: 10000 });
+
+    // Delete Pane B's session from "outside" (simulating the SSE event another
+    // client's delete would produce, by hitting the DELETE route directly).
+    await page.evaluate(async (id) => {
+      await fetch(`/api/sessions/${id}`, { method: 'DELETE' });
+    }, idB);
+    await page.waitForFunction(() => document.querySelector('.terminal-split-container') === null, null, {
+      timeout: 10000,
+    });
+
+    const collapsed = await page.evaluate(() => ({
+      hasContainer: document.querySelector('.terminal-split-container') === null,
+      splitPaneNulled: (window as any).app._splitPane === null,
+    }));
+    expect(collapsed.hasContainer).toBe(true);
+    expect(collapsed.splitPaneNulled).toBe(true);
+
+    await page.evaluate(async (id) => {
+      await fetch(`/api/sessions/${id}`, { method: 'DELETE' });
+    }, idA);
+  });
+});