diff --git a/backend/src/routes/textbooks.js b/backend/src/routes/textbooks.js
index 3ca22ca..664b315 100644
--- a/backend/src/routes/textbooks.js
+++ b/backend/src/routes/textbooks.js
@@ -214,6 +214,8 @@ router.get('/:slug', (req, res) => {
 
 /* POST /api/textbooks/:slug/progress — update progress */
 router.post('/:slug/progress', (req, res) => {
+  // DEBUG: log incoming progress writes
+  console.log('[progress]', new Date().toISOString(), 'user', req.user && req.user.id, '| slug=', req.params.slug, '| body=', JSON.stringify(req.body || {}));
   const t = db.prepare('SELECT id FROM textbooks WHERE slug=? AND is_active=1').get(req.params.slug);
   if (!t) return res.status(404).json({ error: 'Учебник не найден' });
 
diff --git a/frontend/js/textbook-tracker.js b/frontend/js/textbook-tracker.js
index 9f3e21a..8d94a39 100644
--- a/frontend/js/textbook-tracker.js
+++ b/frontend/js/textbook-tracker.js
@@ -235,6 +235,30 @@
     refreshCheckUI(key);
     persist();
     syncToServer({ mark_read: key });
+    updateDebugBadge();
+  }
+
+  // DEBUG: визуальный бейдж прогресса в правом нижнем углу.
+  function ensureDebugBadge() {
+    if (document.getElementById('tb-debug-badge')) return;
+    const b = document.createElement('div');
+    b.id = 'tb-debug-badge';
+    b.style.cssText = 'position:fixed;bottom:12px;right:12px;z-index:99999;background:rgba(15,23,42,.92);color:#fff;padding:10px 14px;border-radius:10px;font-family:monospace;font-size:12px;line-height:1.5;box-shadow:0 4px 14px rgba(0,0,0,.3);max-width:260px';
+    document.body.appendChild(b);
+    updateDebugBadge();
+  }
+  function updateDebugBadge() {
+    const b = document.getElementById('tb-debug-badge');
+    if (!b) return;
+    const activeParaEl = document.querySelector('.para-pill.active[data-para]');
+    const active = activeParaEl ? activeParaEl.dataset.para : '?';
+    b.innerHTML =
+      '<div><b>tracker debug</b></div>' +
+      '<div>slug: ' + slug + '</div>' +
+      '<div>active pill: ' + active + '</div>' +
+      '<div>localState.last: ' + (localState.last || '?') + '</div>' +
+      '<div>read [' + localState.read.length + ']: ' + (localState.read.slice(-8).join(',') || '—') + '</div>' +
+      '<div style="opacity:.6;margin-top:4px">Кликай пилюли — должно меняться</div>';
   }
 
   function wirePillTracking() {
@@ -532,6 +556,8 @@
 
   function boot() {
     console.log('[tracker] boot, slug =', slug, '| LS:', typeof LS !== 'undefined', '| token:', typeof LS !== 'undefined' && LS.getToken && !!LS.getToken());
+    ensureDebugBadge();
+    setInterval(updateDebugBadge, 1000);
     injectStyles();
     installBackButton();
     installBookmarksBtn();