diff --git a/www/firebaseConfig.js b/www/firebaseConfig.js
index e60f98d..a0e1861 100644
--- a/www/firebaseConfig.js
+++ b/www/firebaseConfig.js
@@ -95,12 +95,27 @@ if (APP_ENV !== 'production') {
 }
 
 // Initialize Firebase
-let app;
+//
+// IMPORTANT: variable is intentionally NOT named `app`. A `let app`
+// declared at the top level of a classic <script> creates a binding in
+// the global lexical environment that SHADOWS `window.app` for inline
+// event handlers (`onclick="app.x()"`) — even though it is not visible
+// as a `window.app` property in DevTools. That broke every
+// `onclick="app.chat.X()"` button in www/templates/chat.html: the AI
+// Assistant Send button silently threw `Cannot read properties of
+// undefined (reading 'sendFromInput')` because inline handlers
+// resolved `app` to *this* Firebase App instance instead of the
+// application's `window.app` object.
+//
+// Found via Playwright interaction-driver in the staging clone repo
+// (DDancingDeath/AadhatManagementApp-staging). Full RCA in that repo's
+// docs/REVIEW_ISSUES.md → STG-WALK-5.
+let firebaseApp;
 try {
-  app = firebase.initializeApp(firebaseConfig);
+  firebaseApp = firebase.initializeApp(firebaseConfig);
 } catch (error) {
   if (error.code === 'app/duplicate-app') {
-    app = firebase.app();
+    firebaseApp = firebase.app();
   } else {
     console.error('Firebase initialization error:', error);
     throw error;
diff --git a/www/js/__tests__/firebase-app-shadow.test.js b/www/js/__tests__/firebase-app-shadow.test.js
new file mode 100644
index 0000000..bba6d26
--- /dev/null
+++ b/www/js/__tests__/firebase-app-shadow.test.js
@@ -0,0 +1,68 @@
+/**
+ * Regression test for STG-WALK-5 — pinned 2026-01-08.
+ *
+ * `firebaseConfig.js` is loaded as a *classic* (non-module) `<script>`. In
+ * that context, `let app` or `const app` at the top level creates a binding
+ * in the **global lexical environment** that lives "above" `window` in the
+ * scope chain seen by inline event handlers (`onclick="app.x()"`).
+ *
+ * That binding then SHADOWS `window.app` — even though it is *not* exposed
+ * as `window.app` to JavaScript code. So `typeof window.app === 'object'`
+ * and `window.app.chat.sendFromInput` is a function, but inline handlers
+ * resolve `app` to the Firebase App instance instead. Result: every
+ * `onclick="app.chat.X()"` button silently throws "Cannot read properties
+ * of undefined (reading 'X')" because the Firebase App has no `.chat`.
+ *
+ * This bug took out the entire AI Assistant feature without anyone
+ * noticing because `chat-render.test.js` only string-matches the HTML —
+ * it doesn't actually click anything.
+ *
+ * To prevent recurrence:
+ *   - Don't name any top-level binding in any `<script>` tag `app`.
+ *   - chat.html (and any future inline handlers) must use `window.app.*`.
+ */
+const fs = require('fs');
+const path = require('path');
+
+const STAGING_ROOT = path.resolve(__dirname, '..', '..', '..');
+
+describe('firebaseConfig.js — STG-WALK-5 regression', () => {
+    let src;
+    beforeAll(() => {
+        src = fs.readFileSync(
+            path.join(STAGING_ROOT, 'www/firebaseConfig.js'),
+            'utf8'
+        );
+    });
+
+    test('does NOT use `let app` or `const app` at top level (would shadow window.app for inline handlers)', () => {
+        // Match `let app` / `const app` / `var app` *only when followed by
+        // = / ; / EOL* — i.e. the declaration form. Excludes accidental
+        // matches inside strings or comments via simple heuristics.
+        const lines = src.split(/\r?\n/);
+        const offenders = [];
+        lines.forEach((line, i) => {
+            // Strip line comments.
+            const code = line.replace(/\/\/.*$/, '');
+            if (/^\s*(?:let|const|var)\s+app\s*[;=]/.test(code)) {
+                offenders.push((i + 1) + ': ' + line.trim());
+            }
+        });
+        if (offenders.length) {
+            throw new Error(
+                'firebaseConfig.js declares a top-level `app` binding. This shadows ' +
+                '`window.app` for inline event handlers (`onclick="app.x()"`) and ' +
+                'breaks the AI Assistant Send button. Rename to `firebaseApp` or ' +
+                'similar. Offenders:\n' + offenders.join('\n')
+            );
+        }
+    });
+
+    test('Firebase init result is bound to a name OTHER than `app`', () => {
+        // Sanity: the file does still call initializeApp(), and assigns its
+        // result to *some* variable.
+        expect(src).toMatch(/firebase\.initializeApp\s*\(/);
+        // A common safe name; this test just pins our convention.
+        expect(src).toMatch(/(?:let|const|var)\s+(?:firebaseApp|fbApp|_app)\b/);
+    });
+});