DOCS: Update README and system.xml with Varnish/Fastly configuration …

README.md

@@ -61,6 +61,75 @@ Note: bfcache availability may vary based on your Full Page Cache engine.
 * **Auto Close Menu** - Automatically close open menus when page is restored from back/forward cache (for compatible themes). (Default: Yes)
 * **Exclude URLs** - Optional configuration to exclude specific URL patterns from back/forward cache. Enter URL parts (substring), one per line. The extension automatically excludes non-cacheable URLs, so this is only needed for custom cached URLs that load private data via JavaScript.
 
+#### If you use Varnish FPC
+
+For the Back/Forward Cache feature to work with Varnish Full Page Cache, you must modify your VCL file's `vcl_deliver` subroutine by updating the existing Cache-Control header logic.
+
+```vcl
+sub vcl_deliver {
+  # Find the existing line that sets Cache-Control, like:
+  set resp.http.Cache-Control = "no-store, no-cache, must-revalidate, max-age=0";
+
+  # Replace it with:
+  if (resp.http.Cache-Control ~ "public") {
+      set resp.http.Cache-Control = "no-cache, must-revalidate, max-age=0";
+  } else {
+      set resp.http.Cache-Control = "no-store, no-cache, must-revalidate, max-age=0";
+  }
+}
+```
+
+- This modification requires manual VCL file editing and Varnish service restart
+- Test thoroughly in a staging environment before deploying to production
+- Consider using [elgentos/magento2-varnish-extended](https://github.com/elgentos/magento2-varnish-extended) for a more complete enhanced Varnish configuration
+
+#### If you use Fastly (including Adobe Commerce Cloud)
+
+For Fastly CDN, you must create two custom VCL snippets through the Magento admin panel, as follows:
+
+**Step 1: Access VCL Snippets**
+1. Navigate to **Stores** > **Settings** > **Configuration** > **Advanced** > **System**
+2. Expand **Full Page Cache** > **Fastly Configuration** > **Custom VCL Snippets**
+3. Click **Create Custom Snippet**
+
+**Step 2: Configure Snippet 1**
+- **Name**: `bfcache-preserve-public-private`
+- **Type**: `fetch`
+- **Priority**: `1`
+- **VCL Content**:
+
+```vcl
+if (beresp.http.Cache-Control) {
+    if (beresp.http.Cache-Control ~ "public") {
+        set beresp.http.X-MageOS-Bfcache = "public";
+    } else {
+        set beresp.http.X-MageOS-Bfcache  = "private";
+    }
+}
+```
+Save the snippet  
+Click **Create Custom Snippet** again
+
+**Step 3: Configure Snippet 2**
+- **Name**: `bfcache-remove-ccns`
+- **Type**: `deliver`
+- **Priority**: `100`
+- **VCL Content**:
+
+```vcl
+if (fastly.ff.visits_this_service == 0 && req.restarts == 0) {
+    if (resp.http.X-MageOS-Bfcache == "public") {
+       set resp.http.Cache-Control = "no-cache, must-revalidate, max-age=0";
+    }
+}
+
+unset resp.http.X-MageOS-Bfcache;
+```
+Save the snippet
+
+**Step 4: Deploy**
+
+Click **Upload VCL to Fastly**, and Activate the uploaded VCL
 ## Contributors
 
 Initial module, page transitions, and speculation rules contributed by [@rhoerr](https://github.com/rhoerr).

etc/adminhtml/system.xml

@@ -65,6 +65,7 @@
                 <label>Back/Forward Cache</label>
                 <comment><![CDATA[
                     <p>Go back and forwards in the browser history on most pages without reload, by allowing them to be cached in the browser temporarily.</p>
+                    <p><b>Important:</b> Support for bfcache depends on your Full Page Cache backend. While this module works out-of-the-box with Magento's built-in cache, using Varnish, Fastly, or other reverse proxies requires additional configuration. Please see the <a href="https://github.com/mage-os-lab/module-theme-optimization/blob/main/README.md#varnish--fastly-configuration" target="_blank">Varnish & Fastly Configuration</a> section in the README for details.</p>
                 ]]></comment>
                 <group id="general" translate="label comment" type="text" sortOrder="20" showInDefault="1" showInWebsite="1" showInStore="1">
                     <label>General Settings</label>