diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 8388309570..27886bc7d3 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -60,6 +60,38 @@ export default async function RootLayout({ children }) {
   return (
     <html lang={locale} dir={isRtl ? "rtl" : "ltr"} suppressHydrationWarning>
       <head>
+        {/* Pre-hydration cleanup: browser extensions (Bitdefender's
+            bis_skin_checked, Grammarly's data-gr-ext-installed, LanguageTool's
+            data-lt-installed, etc.) inject attributes into the DOM after SSR
+            but before React hydrates, causing hydration mismatch warnings in
+            dev. Strip them synchronously and observe for late injections; the
+            observer auto-disconnects after 5s (well past typical hydration). */}
+        <script
+          dangerouslySetInnerHTML={{
+            __html: `
+              (function () {
+                var ATTRS = ["bis_skin_checked", "data-google-query-id", "data-new-gr-c-s-check-loaded", "data-gr-ext-installed", "data-lt-installed", "data-lt-tmp-id"];
+                function strip(el) {
+                  for (var i = 0; i < ATTRS.length; i++) {
+                    if (el.hasAttribute(ATTRS[i])) el.removeAttribute(ATTRS[i]);
+                  }
+                }
+                strip(document.documentElement);
+                if (typeof MutationObserver === "undefined") return;
+                var obs = new MutationObserver(function (muts) {
+                  for (var i = 0; i < muts.length; i++) {
+                    var m = muts[i];
+                    if (m.type === "attributes" && ATTRS.indexOf(m.attributeName) !== -1) {
+                      m.target.removeAttribute(m.attributeName);
+                    }
+                  }
+                });
+                obs.observe(document.documentElement, { attributes: true, subtree: true, attributeFilter: ATTRS });
+                setTimeout(function () { obs.disconnect(); }, 5000);
+              })();
+            `,
+          }}
+        />
         {/* Material Symbols icon font is self-hosted via globals.css
             (@import "material-symbols/outlined.css") so icons render even when
             the Google Fonts CDN is unreachable (#3695). */}
diff --git a/tests/unit/dashboard/hydration-extension-attrs.test.ts b/tests/unit/dashboard/hydration-extension-attrs.test.ts
new file mode 100644
index 0000000000..4840761d95
--- /dev/null
+++ b/tests/unit/dashboard/hydration-extension-attrs.test.ts
@@ -0,0 +1,72 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { resolve, join } from "node:path";
+
+// Regression guard for the browser-extension hydration-mismatch fix: extensions
+// like Bitdefender (bis_skin_checked), Google Search (data-google-query-id),
+// Grammarly (data-new-gr-c-s-check-loaded / data-gr-ext-installed) and
+// LanguageTool (data-lt-installed / data-lt-tmp-id) inject attributes into the
+// DOM after SSR but before React hydrates. React's suppressHydrationWarning on
+// <html>/<body> only applies one level deep, so the mismatch still surfaces on
+// Next.js's internal elements. The fix adds a synchronous pre-hydration <script>
+// in src/app/layout.tsx that strips the known attributes from
+// document.documentElement and keeps stripping late injections via a
+// MutationObserver that auto-disconnects after 5s. These assertions fail on the
+// pre-fix tree (no such script present) and stay green afterwards, preventing a
+// future layout.tsx refactor from silently dropping this script.
+
+const cwd = process.cwd();
+const layoutPath = resolve(join(cwd, "src/app/layout.tsx"));
+
+const EXPECTED_ATTRS = [
+  "bis_skin_checked",
+  "data-google-query-id",
+  "data-new-gr-c-s-check-loaded",
+  "data-gr-ext-installed",
+  "data-lt-installed",
+  "data-lt-tmp-id",
+];
+
+test("layout.tsx strips the full known browser-extension attribute list before hydration", () => {
+  const layout = readFileSync(layoutPath, "utf8");
+  for (const attr of EXPECTED_ATTRS) {
+    assert.ok(
+      layout.includes(`"${attr}"`),
+      `layout.tsx must list "${attr}" among the pre-hydration extension attributes to strip`
+    );
+  }
+  assert.match(
+    layout,
+    /removeAttribute\s*\(\s*ATTRS\s*\[\s*i\s*\]\s*\)/,
+    "layout.tsx must call removeAttribute for each known extension attribute"
+  );
+});
+
+test("layout.tsx observes late attribute injections via MutationObserver and auto-disconnects", () => {
+  const layout = readFileSync(layoutPath, "utf8");
+  assert.match(
+    layout,
+    /new MutationObserver\s*\(/,
+    "layout.tsx must set up a MutationObserver to catch extension attributes injected after the initial strip"
+  );
+  assert.match(
+    layout,
+    /attributeFilter\s*:\s*ATTRS/,
+    "the MutationObserver must filter on the same known extension ATTRS list"
+  );
+  assert.match(
+    layout,
+    /setTimeout\s*\(\s*function\s*\(\s*\)\s*\{\s*obs\.disconnect\(\)\s*;?\s*\}\s*,\s*5000\s*\)/,
+    "the MutationObserver must auto-disconnect after 5s (well past typical hydration) to avoid a long-lived observer"
+  );
+});
+
+test("layout.tsx strips extension attributes from document.documentElement synchronously (before React hydrates)", () => {
+  const layout = readFileSync(layoutPath, "utf8");
+  assert.match(
+    layout,
+    /strip\s*\(\s*document\.documentElement\s*\)/,
+    "layout.tsx must run the strip synchronously against document.documentElement on initial script execution"
+  );
+});
