diff --git a/src/web/public/index.html b/src/web/public/index.html
index bb53fef7..139b035a 100644
--- a/src/web/public/index.html
+++ b/src/web/public/index.html
@@ -191,6 +191,7 @@
         </button>
         <button class="btn-icon-header btn-file-viewer" onclick="app.toggleFileBrowserButton()" title="File Viewer" aria-label="Open file viewer" aria-expanded="false"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg></button>
         <button class="btn-icon-header btn-multimonitor btn-multimonitor--hidden" onclick="app.launchMultiMonitor()" title="Open Codeman across all displays" aria-label="Open Codeman across all displays"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg></button>
+        <button class="btn-icon-header btn-split btn-split--hidden" onclick="app.openSplitPicker()" title="Split: open a second session beside this one" aria-label="Split: open a second session beside this one"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg></button>
         <button class="btn-icon-header btn-ultracode-agents btn-ultracode-agents--hidden" onclick="app.toggleUltracodeAgentsPanel()" title="Ultracode / Workflow agents" aria-label="Open ultracode workflow agents"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="6" cy="6" r="2.5"/><circle cx="6" cy="18" r="2.5"/><circle cx="18" cy="12" r="2.5"/><path d="M8.2 7.2 15.6 11M8.2 16.8 15.6 13"/></svg></button>
         <div class="header-plan-usage header-plan-usage--hidden" id="planUsageChip" title="Claude and Codex plan usage limits">—</div>
         <button class="btn-icon-header btn-notifications" onclick="app.toggleNotifications()" title="Notifications" aria-label="Toggle notifications" style="display:none;">
@@ -1924,6 +1925,7 @@
                     <label class="set-chip" data-preview="header" data-preview-order="9"><input type="checkbox" id="appSettingsShowAttachmentsButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg><span>Attachments</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="10"><input type="checkbox" id="appSettingsShowFileViewerButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg><span>File Viewer</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="11"><input type="checkbox" id="appSettingsShowMultiMonitorButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg><span>Multi-monitor</span></label>
+                    <label class="set-chip" data-preview="header" data-preview-order="12"><input type="checkbox" id="appSettingsShowSplitButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg><span>Split</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="13" data-preview-text="42%"><input type="checkbox" id="appSettingsShowPlanUsageLimits"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M4 18a8 8 0 1 1 16 0"/><path d="M12 18l4.5-5"/></svg><span>Plan Usage</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="14"><input type="checkbox" id="appSettingsShowLifecycleLog"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/></svg><span>Lifecycle Log</span></label>
                   </div>
diff --git a/src/web/public/settings-ui.js b/src/web/public/settings-ui.js
index 1ade54a4..6518a6a5 100644
--- a/src/web/public/settings-ui.js
+++ b/src/web/public/settings-ui.js
@@ -407,6 +407,7 @@ Object.assign(CodemanApp.prototype, {
     document.getElementById('appSettingsUltracodeFloatingWindows').checked =
       settings.ultracodeFloatingWindows ?? defaults.ultracodeFloatingWindows ?? false;
     document.getElementById('appSettingsShowMultiMonitorButton').checked = settings.showMultiMonitorButton ?? defaults.showMultiMonitorButton ?? false;
+    document.getElementById('appSettingsShowSplitButton').checked = settings.showSplitButton ?? defaults.showSplitButton ?? false;
     document.getElementById('appSettingsShowPlanUsageLimits').checked = this.planUsageChipEnabled(settings);
     document.getElementById('appSettingsShowRedrawButton').checked = settings.showRedrawButton ?? defaults.showRedrawButton ?? false;
     // Phone overview home screen: only meaningful under 600px, so the row is
@@ -2120,6 +2121,7 @@ Object.assign(CodemanApp.prototype, {
       readMyMindEnabled: document.getElementById('appSettingsReadMyMind').checked,
       ultracodeFloatingWindows: document.getElementById('appSettingsUltracodeFloatingWindows').checked,
       showMultiMonitorButton: document.getElementById('appSettingsShowMultiMonitorButton').checked,
+      showSplitButton: document.getElementById('appSettingsShowSplitButton').checked,
       showPlanUsageLimits: document.getElementById('appSettingsShowPlanUsageLimits').checked,
       showRedrawButton: document.getElementById('appSettingsShowRedrawButton').checked,
       mobileOverviewEnabled: document.getElementById('appSettingsMobileOverview').checked,
@@ -2742,6 +2744,7 @@ Object.assign(CodemanApp.prototype, {
         showUltracodeAgents: false,
         ultracodeFloatingWindows: false,
         showMultiMonitorButton: false,
+        showSplitButton: false,
         // Desktop defaults this ON (see planUsageChipEnabled); handhelds keep it
         // OFF so the phone header stays minimal and the mobile-header-buttons
         // policy guard keeps passing.
@@ -2947,6 +2950,13 @@ Object.assign(CodemanApp.prototype, {
       multiMonitorBtn.classList.toggle('btn-multimonitor--hidden', !showMultiMonitorButton);
     }
 
+    // Split button — hidden by default
+    const showSplitButton = settings.showSplitButton ?? defaults.showSplitButton ?? false;
+    const splitBtn = document.querySelector('.btn-split');
+    if (splitBtn) {
+      splitBtn.classList.toggle('btn-split--hidden', !showSplitButton);
+    }
+
     // Ultracode/Workflow agents launcher — hidden by default; reveal when enabled.
     // Marker class only (base is display:inline-flex !important) so it's auto-excluded
     // from the mobile-header-buttons-policy guard.
diff --git a/src/web/schemas.ts b/src/web/schemas.ts
index f1b1fc22..485c046b 100644
--- a/src/web/schemas.ts
+++ b/src/web/schemas.ts
@@ -1376,6 +1376,7 @@ export const SettingsUpdateSchema = z
     showFileBrowser: z.boolean().optional(),
     showSubagents: z.boolean().optional(),
     showMultiMonitorButton: z.boolean().optional(),
+    showSplitButton: z.boolean().optional(),
     // Doubles as the plan-usage telemetry COLLECTION switch, read fresh from
     // disk by readPlanUsageTelemetryEnabled() (hooks-config.ts) at every claude
     // session create/respawn — not just the chip's DISPLAY preference. See that
diff --git a/test/routes/system-routes-split-button-setting.test.ts b/test/routes/system-routes-split-button-setting.test.ts
new file mode 100644
index 00000000..0f9fe069
--- /dev/null
+++ b/test/routes/system-routes-split-button-setting.test.ts
@@ -0,0 +1,69 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { createRouteTestHarness, type RouteTestHarness } from './_route-test-utils.js';
+import { registerSystemRoutes } from '../../src/web/routes/system-routes.js';
+
+const { subagentWatcher, imageWatcher, workflowRunWatcher, fileSystem } = vi.hoisted(() => {
+  const makeWatcher = () => ({
+    isRunning: vi.fn(() => false),
+    start: vi.fn(),
+    stop: vi.fn(),
+    getStats: vi.fn(() => ({})),
+    watchSession: vi.fn(),
+    getRecentRunSummaries: vi.fn(() => []),
+  });
+
+  // Simulate in-memory file storage
+  let storedSettings: Record<string, unknown> = {};
+
+  return {
+    subagentWatcher: makeWatcher(),
+    imageWatcher: makeWatcher(),
+    workflowRunWatcher: makeWatcher(),
+    fileSystem: {
+      storedSettings,
+    },
+  };
+});
+
+vi.mock('node:fs/promises', () => ({
+  default: {
+    readFile: vi.fn(async () => JSON.stringify(fileSystem.storedSettings)),
+    writeFile: vi.fn(async (_path: string, content: string) => {
+      fileSystem.storedSettings = JSON.parse(content);
+    }),
+  },
+}));
+
+vi.mock('node:fs', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('node:fs')>();
+  return { ...actual, existsSync: vi.fn(() => true), mkdirSync: vi.fn(), readdirSync: vi.fn(() => []) };
+});
+
+vi.mock('../../src/subagent-watcher.js', () => ({ subagentWatcher }));
+vi.mock('../../src/image-watcher.js', () => ({ imageWatcher }));
+vi.mock('../../src/workflow-run-watcher.js', () => ({ workflowRunWatcher }));
+
+describe('showSplitButton setting', () => {
+  let harness: RouteTestHarness;
+
+  beforeEach(async () => {
+    harness = await createRouteTestHarness(registerSystemRoutes);
+  });
+
+  afterEach(async () => {
+    await harness.app.close();
+  });
+
+  it('round-trips through PUT and GET /api/settings', async () => {
+    const putRes = await harness.app.inject({
+      method: 'PUT',
+      url: '/api/settings',
+      payload: { showSplitButton: true },
+    });
+    expect(putRes.statusCode).toBe(200);
+
+    const getRes = await harness.app.inject({ method: 'GET', url: '/api/settings' });
+    const body = JSON.parse(getRes.body);
+    expect(body.showSplitButton).toBe(true);
+  });
+});