diff --git a/src/content/content.js b/src/content/content.js
index ec9b49f..0f25436 100644
--- a/src/content/content.js
+++ b/src/content/content.js
@@ -1134,9 +1134,17 @@
         });
         await setStorageData('ss_mappings', currentMappings);
 
-        // Update local mappings
+        // Update local mappings so the fetch interceptor uses them immediately
         mappings = currentMappings;
 
+        // Replace the PPI value in the current input right now
+        if (inputEl) {
+          replaceInInput(inputEl, real, fake);
+          // Re-scan — will dismiss warning if no more PPI remains
+          if (inputScanTimer) clearTimeout(inputScanTimer);
+          inputScanTimer = setTimeout(() => scanInputForPPI(inputEl), 150);
+        }
+
         // Visual feedback
         btn.textContent = '\u2714';
         btn.style.color = '#4ade80';
@@ -1145,6 +1153,27 @@
     });
   }
 
+  // Replace all occurrences of `real` with `fake` in an input or contenteditable element
+  function replaceInInput(el, real, fake) {
+    if (!el) return;
+    const re = new RegExp(real.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'), 'gi');
+    if (el.value !== undefined) {
+      // <textarea> or <input>
+      el.value = el.value.replace(re, fake);
+    } else if (el.isContentEditable) {
+      // contenteditable div — walk text nodes to avoid breaking inner HTML
+      const walker = document.createTreeWalker(el, NodeFilter.SHOW_TEXT);
+      let node;
+      while ((node = walker.nextNode())) {
+        re.lastIndex = 0;
+        if (re.test(node.textContent)) {
+          re.lastIndex = 0;
+          node.textContent = node.textContent.replace(re, fake);
+        }
+      }
+    }
+  }
+
   // Storage helpers for page world (uses postMessage to injector)
   function getStorageData(key) {
     return new Promise(resolve => {
diff --git a/src/lib/sync.js b/src/lib/sync.js
index ccdea73..e2a1540 100644
--- a/src/lib/sync.js
+++ b/src/lib/sync.js
@@ -171,6 +171,61 @@ const SilentSendSync = {
       return [];
     }
   },
+
+  // ----------------------------------------------------------------
+  // File System Access API helpers — folder-based sync
+  // The directory handle is stored in IndexedDB so the user only
+  // needs to grant access once per browser session.
+  // ----------------------------------------------------------------
+
+  _dbPromise: null,
+
+  _openDB() {
+    if (this._dbPromise) return this._dbPromise;
+    this._dbPromise = new Promise((resolve, reject) => {
+      const req = indexedDB.open('ss_sync_handles', 1);
+      req.onupgradeneeded = () => req.result.createObjectStore('handles');
+      req.onsuccess = () => resolve(req.result);
+      req.onerror = () => reject(req.error);
+    });
+    return this._dbPromise;
+  },
+
+  async saveSyncDirHandle(handle) {
+    const db = await this._openDB();
+    return new Promise((resolve, reject) => {
+      const tx = db.transaction('handles', 'readwrite');
+      tx.objectStore('handles').put(handle, 'syncDir');
+      tx.oncomplete = resolve;
+      tx.onerror = () => reject(tx.error);
+    });
+  },
+
+  async loadSyncDirHandle() {
+    try {
+      const db = await this._openDB();
+      return new Promise((resolve) => {
+        const tx = db.transaction('handles', 'readonly');
+        const req = tx.objectStore('handles').get('syncDir');
+        req.onsuccess = () => resolve(req.result || null);
+        req.onerror = () => resolve(null);
+      });
+    } catch {
+      return null;
+    }
+  },
+
+  async clearSyncDirHandle() {
+    try {
+      const db = await this._openDB();
+      return new Promise((resolve) => {
+        const tx = db.transaction('handles', 'readwrite');
+        tx.objectStore('handles').delete('syncDir');
+        tx.oncomplete = resolve;
+        tx.onerror = resolve;
+      });
+    } catch { /* ignore */ }
+  },
 };
 
 export default SilentSendSync;
diff --git a/src/options/options.html b/src/options/options.html
index 4589afa..e283434 100644
--- a/src/options/options.html
+++ b/src/options/options.html
@@ -140,6 +140,23 @@
 
         <div id="syncStatus" style="margin-top:8px;font-size:12px;min-height:16px"></div>
       </div>
+
+      <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
+        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Auto-Sync Folder — fully automatic, no copy-paste</h3>
+        <p class="section-desc" style="margin-bottom:8px">
+          Pick the same folder in each browser once. Changes are written to
+          <code>silent-send-sync.json</code> automatically whenever settings change,
+          and read back whenever this page is opened or regains focus.
+          Use a shared folder (Dropbox, OneDrive, iCloud Drive, or any network share)
+          for cross-computer sync too.
+        </p>
+        <div class="bulk-actions" style="align-items:center">
+          <button class="btn btn-primary" id="btnPickSyncFolder">Choose Sync Folder</button>
+          <span id="syncFolderName" style="font-size:12px;color:#6b7280;font-family:monospace"></span>
+          <button class="btn btn-danger btn-sm" id="btnClearSyncFolder" style="display:none">Clear</button>
+        </div>
+        <div id="fileSyncStatus" style="margin-top:6px;font-size:12px;min-height:16px"></div>
+      </div>
     </section>
 
     <section class="section">
diff --git a/src/options/options.js b/src/options/options.js
index 2bad103..4e29c99 100644
--- a/src/options/options.js
+++ b/src/options/options.js
@@ -95,6 +95,29 @@ document.addEventListener('DOMContentLoaded', async () => {
     setSyncStatus('', 'neutral');
   });
 
+  // --- File-based auto-sync ---
+  await initFileSync();
+
+  $('#btnPickSyncFolder').addEventListener('click', pickSyncFolder);
+  $('#btnClearSyncFolder').addEventListener('click', async () => {
+    await SilentSendSync.clearSyncDirHandle();
+    syncDirHandle = null;
+    updateFileSyncUI();
+    setFileSyncStatus('Sync folder cleared.', 'neutral');
+  });
+
+  // Auto-write when local storage changes (catches popup edits, page-world adds, etc.)
+  api.storage.onChanged.addListener(async (changes, area) => {
+    if (area === 'local' && (changes.ss_settings || changes.ss_mappings || changes.ss_identity)) {
+      await writeToSyncFile();
+    }
+  });
+
+  // Re-check sync file whenever the options page regains focus
+  window.addEventListener('focus', async () => {
+    await checkFileSyncUpdate();
+  });
+
   // Transfer data
   $('#btnExportAll').addEventListener('click', exportAllPlain);
   $('#btnExportEncrypted').addEventListener('click', exportAllEncrypted);
@@ -465,6 +488,119 @@ async function importAll(e) {
   e.target.value = '';
 }
 
+// ----------------------------------------------------------------
+// File-based auto-sync (File System Access API)
+// ----------------------------------------------------------------
+
+let syncDirHandle = null;
+const SYNC_FILE_NAME = 'silent-send-sync.json';
+
+async function initFileSync() {
+  syncDirHandle = await SilentSendSync.loadSyncDirHandle();
+  updateFileSyncUI();
+  if (syncDirHandle) {
+    await checkFileSyncUpdate();
+  }
+}
+
+async function pickSyncFolder() {
+  if (!window.showDirectoryPicker) {
+    setFileSyncStatus('Your browser does not support the File System Access API.', 'error');
+    return;
+  }
+  try {
+    const handle = await window.showDirectoryPicker({ mode: 'readwrite', id: 'ss-sync' });
+    syncDirHandle = handle;
+    await SilentSendSync.saveSyncDirHandle(handle);
+    updateFileSyncUI();
+    // Write current settings immediately so the file exists for the other browser
+    await writeToSyncFile();
+    setFileSyncStatus('Sync folder set. Settings will sync automatically.', 'ok');
+  } catch (e) {
+    if (e.name !== 'AbortError') {
+      setFileSyncStatus('Could not set sync folder: ' + e.message, 'error');
+    }
+  }
+}
+
+async function writeToSyncFile() {
+  if (!syncDirHandle) return;
+  try {
+    // Re-verify permission is still granted (required after browser restart)
+    const perm = await syncDirHandle.requestPermission({ mode: 'readwrite' });
+    if (perm !== 'granted') return;
+
+    const data = await SilentSendSync._getAllData();
+    const fileHandle = await syncDirHandle.getFileHandle(SYNC_FILE_NAME, { create: true });
+    const writable = await fileHandle.createWritable();
+    await writable.write(JSON.stringify(data, null, 2));
+    await writable.close();
+  } catch (e) {
+    // Permission denied or folder removed — don't spam errors
+    console.warn('[Silent Send] writeToSyncFile failed:', e.message);
+  }
+}
+
+async function checkFileSyncUpdate() {
+  if (!syncDirHandle) return;
+  try {
+    const perm = await syncDirHandle.queryPermission({ mode: 'readwrite' });
+    if (perm === 'prompt') {
+      // Need a user gesture to re-request — skip silently
+      return;
+    }
+    if (perm !== 'granted') return;
+
+    const fileHandle = await syncDirHandle.getFileHandle(SYNC_FILE_NAME);
+    const file = await fileHandle.getFile();
+    const data = JSON.parse(await file.text());
+
+    if (!data.version || !data.lastModified) return;
+
+    const local = await SilentSendSync._getAllData();
+    if (data.lastModified > (local.lastModified || 0)) {
+      await SilentSendSync._applyData(data);
+      mappings = await Storage.getMappings();
+      settings = await Storage.getSettings();
+      $('#browserSync').checked = settings.browserSync === true;
+      renderMappings();
+      renderDomains();
+      renderLog();
+      setFileSyncStatus(
+        'Auto-synced from folder (' + new Date(data.lastModified).toLocaleString() + ').',
+        'ok'
+      );
+    }
+  } catch (e) {
+    if (e.name !== 'NotFoundError') {
+      console.warn('[Silent Send] checkFileSyncUpdate failed:', e.message);
+    }
+  }
+}
+
+function updateFileSyncUI() {
+  const nameEl = $('#syncFolderName');
+  const clearBtn = $('#btnClearSyncFolder');
+  const pickBtn = $('#btnPickSyncFolder');
+  if (!nameEl) return;
+  if (syncDirHandle) {
+    nameEl.textContent = syncDirHandle.name + '/';
+    clearBtn.style.display = '';
+    pickBtn.textContent = 'Change Folder';
+  } else {
+    nameEl.textContent = '';
+    clearBtn.style.display = 'none';
+    pickBtn.textContent = 'Choose Sync Folder';
+  }
+}
+
+function setFileSyncStatus(msg, type) {
+  const el = $('#fileSyncStatus');
+  if (!el) return;
+  el.textContent = msg;
+  el.style.color = type === 'ok' ? '#10b981' : type === 'warn' ? '#f59e0b' : type === 'error' ? '#dc2626' : '#6b7280';
+}
+
 function setSyncStatus(msg, type) {
   const el = $('#syncStatus');
   if (!el) return;