diff --git a/src/web/public/index.html b/src/web/public/index.html
index 5e9741c3..7f87cc52 100644
--- a/src/web/public/index.html
+++ b/src/web/public/index.html
@@ -114,7 +114,7 @@
           </div>
         </div>
         <button class="btn-icon-header btn-response-viewer-header btn-response-viewer-header--hidden" onclick="app.toggleResponseViewer()" title="View last response" aria-label="View last response"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z"/><circle cx="12" cy="12" r="3"/></svg></button>
-        <button class="btn-icon-header btn-attachments-history" id="attachmentsHistoryBtn" onclick="app.toggleAttachmentHistory()" title="Attachments" aria-label="Open attachment history" aria-expanded="false">
+        <button class="btn-icon-header btn-attachments-history btn-attachments-history--hidden" id="attachmentsHistoryBtn" onclick="app.toggleAttachmentHistory()" title="Attachments" aria-label="Open attachment history" 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="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 class="attachment-history-badge" id="attachmentHistoryBadge" style="display:none;">0</span>
         </button>
@@ -1016,6 +1016,13 @@
                   <span class="slider"></span>
                 </label>
               </div>
+              <div class="settings-item" title="Show the attachments button in header (opens the attachment history drawer)">
+                <span class="settings-item-label">Attachments Button</span>
+                <label class="switch switch-sm">
+                  <input type="checkbox" id="appSettingsShowAttachmentsButton">
+                  <span class="slider"></span>
+                </label>
+              </div>
               <div class="settings-item" title="Show the multi-monitor button in the header (opens Codeman spanned across all displays)">
                 <span class="settings-item-label">Multi-monitor Button</span>
                 <label class="switch switch-sm">
diff --git a/src/web/public/mobile.css b/src/web/public/mobile.css
index 9ace1aaf..b12c16e1 100644
--- a/src/web/public/mobile.css
+++ b/src/web/public/mobile.css
@@ -435,11 +435,10 @@ html.mobile-init .file-browser-panel {
   }
 
   /* Hide header settings gear and lifecycle log on mobile - settings moved to toolbar.
-     Also hide the attachment-history button: the header is kept minimal on phones and
-     the drawer is a desktop-oriented panel. */
+     (The attachments button is opt-in / default-hidden everywhere via its own
+     --hidden marker, so it needs no mobile-specific rule here.) */
   .btn-icon-header.btn-settings,
-  .btn-icon-header.btn-lifecycle-log,
-  .btn-icon-header.btn-attachments-history {
+  .btn-icon-header.btn-lifecycle-log {
     display: none !important;
   }
 
diff --git a/src/web/public/settings-ui.js b/src/web/public/settings-ui.js
index 6b92055a..a4ceb53f 100644
--- a/src/web/public/settings-ui.js
+++ b/src/web/public/settings-ui.js
@@ -307,6 +307,7 @@ Object.assign(CodemanApp.prototype, {
     document.getElementById('appSettingsShowSystemStats').checked = settings.showSystemStats ?? defaults.showSystemStats ?? true;
     document.getElementById('appSettingsShowLifecycleLog').checked = settings.showLifecycleLog ?? defaults.showLifecycleLog ?? true;
     document.getElementById('appSettingsShowResponseViewer').checked = settings.showResponseViewer ?? defaults.showResponseViewer ?? false;
+    document.getElementById('appSettingsShowAttachmentsButton').checked = settings.showAttachmentsButton ?? defaults.showAttachmentsButton ?? false;
     document.getElementById('appSettingsSkin').value = settings.skin ?? defaults.skin ?? 'daylight-blue';
     document.getElementById('appSettingsShowMonitor').checked = settings.showMonitor ?? defaults.showMonitor ?? false;
     document.getElementById('appSettingsShowProjectInsights').checked = settings.showProjectInsights ?? defaults.showProjectInsights ?? false;
@@ -1362,6 +1363,7 @@ Object.assign(CodemanApp.prototype, {
       showSystemStats: document.getElementById('appSettingsShowSystemStats').checked,
       showLifecycleLog: document.getElementById('appSettingsShowLifecycleLog').checked,
       showResponseViewer: document.getElementById('appSettingsShowResponseViewer').checked,
+      showAttachmentsButton: document.getElementById('appSettingsShowAttachmentsButton').checked,
       showMonitor: document.getElementById('appSettingsShowMonitor').checked,
       showProjectInsights: document.getElementById('appSettingsShowProjectInsights').checked,
       showFileBrowser: document.getElementById('appSettingsShowFileBrowser').checked,
@@ -1528,6 +1530,7 @@ Object.assign(CodemanApp.prototype, {
       extendedKeyboardBar: _ekb,
       skin: _skin,
       showPlanUsageLimits: _pul,
+      showAttachmentsButton: _ahb,
       ...serverSettings
     } = settings;
     try {
@@ -1681,6 +1684,7 @@ Object.assign(CodemanApp.prototype, {
         showSubagents: false,
         showMultiMonitorButton: false,
         showPlanUsageLimits: false,
+        showAttachmentsButton: false,
         // Input
         gestureControlEnabled: false,
         // Feature toggles - keep tracking on even on mobile
@@ -1783,6 +1787,14 @@ Object.assign(CodemanApp.prototype, {
       responseViewerBtn.classList.toggle('btn-response-viewer-header--hidden', !showResponseViewer);
     }
 
+    // Hide the attachments (history) button when disabled. Opt-in, default OFF —
+    // marker class, base is display:inline-flex !important.
+    const showAttachmentsButton = settings.showAttachmentsButton ?? defaults.showAttachmentsButton ?? false;
+    const attachmentsBtn = document.getElementById('attachmentsHistoryBtn');
+    if (attachmentsBtn) {
+      attachmentsBtn.classList.toggle('btn-attachments-history--hidden', !showAttachmentsButton);
+    }
+
     // Multi-monitor button — hidden by default (App Settings → Display → "Header
     // Displays"). The server renders the correct initial state on every reload;
     // this handles a live toggle from a settings save (no reload). Toggle the
@@ -2018,7 +2030,7 @@ Object.assign(CodemanApp.prototype, {
           'showLifecycleLog', 'showResponseViewer',
           'showMonitor', 'showProjectInsights', 'showFileBrowser', 'showSubagents',
           'subagentActiveTabOnly', 'tabTwoRows', 'localEchoEnabled', 'cjkInputEnabled', 'extendedKeyboardBar',
-          'skin', 'showPlanUsageLimits',
+          'skin', 'showPlanUsageLimits', 'showAttachmentsButton',
         ]);
         // The plan-usage chip is a PER-DEVICE display setting (default OFF): desktop
         // can show it while mobile stays hidden. It used to sync, so an older
diff --git a/src/web/public/styles.css b/src/web/public/styles.css
index 0e5a51df..1e469bc1 100644
--- a/src/web/public/styles.css
+++ b/src/web/public/styles.css
@@ -8328,6 +8328,16 @@ kbd {
   display: none !important;
 }
 
+/* "Attachments" header button — opt-in (App Settings → Display), hidden by
+   default. Same pattern as the response viewer: a base inline-flex !important so
+   an inline style can't override it, and a more-specific marker rule to hide. */
+.btn-attachments-history {
+  display: inline-flex !important;
+}
+.btn-attachments-history.btn-attachments-history--hidden {
+  display: none !important;
+}
+
 /* Plan-usage chip (App Settings → Display → "Plan Usage Limits"). Shows the
    live 5-hour + weekly plan limits parsed from the Claude statusline. Hidden by
    default via the marker class below; the server strips it at render when the
diff --git a/test/mobile-header-buttons-policy.test.ts b/test/mobile-header-buttons-policy.test.ts
index 133014a2..e060e456 100644
--- a/test/mobile-header-buttons-policy.test.ts
+++ b/test/mobile-header-buttons-policy.test.ts
@@ -35,8 +35,10 @@ const PHONE_WIDTH = 393;
 const MOBILE_VISIBLE_ALLOWLIST = new Set<string>([]);
 
 // Buttons we expect to STAY hidden on phones — an explicit lock so a future edit
-// that removes a hide rule fails loudly (not silently).
-const KNOWN_PHONE_HIDDEN = ['btn-attachments-history', 'btn-settings', 'btn-lifecycle-log'];
+// that removes a hide rule fails loudly (not silently). The attachments button is
+// NOT here: it's opt-in (default-hidden everywhere via its own --hidden marker), so
+// it's excluded from the default-visible enumeration rather than mobile-hidden.
+const KNOWN_PHONE_HIDDEN = ['btn-settings', 'btn-lifecycle-log'];
 
 function attrOf(openTag: string, name: string): string {
   const m = openTag.match(new RegExp(`${name}="([^"]*)"`));
diff --git a/test/mobile/header-buttons.test.ts b/test/mobile/header-buttons.test.ts
index 753c25a7..c84ba8b5 100644
--- a/test/mobile/header-buttons.test.ts
+++ b/test/mobile/header-buttons.test.ts
@@ -1,10 +1,12 @@
-// Port 3208 - Mobile header button visibility (real-browser E2E).
+// Port 3208 - Header button visibility (real-browser E2E).
 //
 // Companion to the CI static guard (test/mobile-header-buttons-policy.test.ts).
-// This one renders the real app in an emulated phone vs. a desktop-class tablet
-// and asserts the ACTUAL computed visibility — catching CSS/layout regressions
-// the static parser can't see. Regression history: the COD-39 attachments button
-// (and earlier the plan-usage chip) shipped visible on the cramped phone header.
+// Renders the real app and asserts ACTUAL computed visibility, catching CSS/layout
+// regressions the static parser can't see. Two behaviours are covered:
+//   1. The minimal phone header hides the settings gear + lifecycle log.
+//   2. The attachments button (COD-39) is OPT-IN — default-hidden everywhere, shown
+//      only when App Settings → Display → "Attachments Button" is enabled. It first
+//      shipped always-visible (mobile, then desktop), which is the regression here.
 import { describe, it, beforeAll, afterAll } from 'vitest';
 import type { WebServer } from '../../src/web/server.js';
 import { createTestServer, stopTestServer } from './helpers/server.js';
@@ -16,15 +18,7 @@ import { PORTS, WAIT } from './helpers/constants.js';
 const PORT = PORTS.HEADER_BUTTONS;
 const BASE_URL = `http://localhost:${PORT}`;
 
-// Secondary header buttons that must NOT clutter the minimal phone header
-// (settings/case controls live in the mobile toolbar instead).
-const PHONE_HIDDEN = [
-  '#attachmentsHistoryBtn', // COD-39 — the reported regression
-  '.btn-icon-header.btn-settings',
-  '.btn-icon-header.btn-lifecycle-log',
-];
-
-describe('Mobile header button visibility (E2E)', () => {
+describe('Header button visibility (E2E)', () => {
   let server: WebServer;
 
   beforeAll(async () => {
@@ -36,19 +30,29 @@ describe('Mobile header button visibility (E2E)', () => {
     await stopTestServer(server);
   });
 
-  it('hides secondary header buttons on a standard phone (iPhone 14 Pro, 393px)', async () => {
+  it('hides the settings gear + lifecycle log on a standard phone (iPhone 14 Pro)', async () => {
     const { page } = await createDevicePage(REPRESENTATIVE_DEVICES['standard-phone'], BASE_URL);
     await page.waitForTimeout(WAIT.PAGE_SETTLE);
-    for (const sel of PHONE_HIDDEN) {
-      await assertHidden(page, sel);
-    }
+    await assertHidden(page, '.btn-icon-header.btn-settings');
+    await assertHidden(page, '.btn-icon-header.btn-lifecycle-log');
   });
 
-  it('keeps the attachments button visible on a desktop-class tablet', async () => {
-    // assertHidden also passes when an element is ABSENT, so prove the selector is
-    // real by asserting the same button IS visible where the phone rule doesn't apply.
+  it('keeps the opt-in attachments button HIDDEN by default on a desktop-class viewport', async () => {
     const { page } = await createDevicePage(REPRESENTATIVE_DEVICES['large-tablet'], BASE_URL);
     await page.waitForTimeout(WAIT.PAGE_SETTLE);
+    await assertHidden(page, '#attachmentsHistoryBtn');
+  });
+
+  it('shows the attachments button once the setting is enabled', async () => {
+    const { page } = await createDevicePage(REPRESENTATIVE_DEVICES['large-tablet'], BASE_URL);
+    // Desktop-class devices use the non-mobile settings blob.
+    await page.evaluate(() => {
+      const cur = JSON.parse(localStorage.getItem('codeman-app-settings') || '{}');
+      cur.showAttachmentsButton = true;
+      localStorage.setItem('codeman-app-settings', JSON.stringify(cur));
+    });
+    await page.reload({ waitUntil: 'domcontentloaded' });
+    await page.waitForTimeout(WAIT.PAGE_SETTLE);
     await assertVisible(page, '#attachmentsHistoryBtn');
   });
 });