diff --git a/src/MainWindow.cpp b/src/MainWindow.cpp
index 5350c93..4ce2fea 100644
--- a/src/MainWindow.cpp
+++ b/src/MainWindow.cpp
@@ -115,11 +115,29 @@ MainWindow::MainWindow(QWidget *parent)
   connect(m_authManager, &AuthManager::signedOut, m_authDialog, &AuthDialog::reset);
   connect(m_authManager, &AuthManager::signedOut, m_authDialog, &AuthDialog::show);
   connect(m_authManager, &AuthManager::signedIn, this, [this]() {
+    m_store->setCurrentUserEmail(m_authManager->sessionData()->email);
     m_store->loadStartupData();
   });
   connect(m_store, &Store::startupDataLoaded, this, [this]() {
     ui->topLevelParticlesView->expandAll();
   });
+  connect(m_store, &Store::particleUpdated, this, [this](const QString &, const QString &particleId) {
+    if (particleId == m_selectedParticleId)
+    {
+      const Particle *particle = m_store->particleById(particleId);
+      if (particle && !particle->ackedByEmails.empty())
+      {
+        QStringList acked;
+        for (const auto &email : particle->ackedByEmails)
+          acked.append(email);
+        ui->label_3->setText("Acked by: " + acked.join(", "));
+      }
+      else
+      {
+        ui->label_3->setText("");
+      }
+    }
+  });
 
   // DocumentSend toolbar button opens text input dialog
   connect(ui->toolButton_3, &QToolButton::clicked, this, &MainWindow::showTextInputDialog);
@@ -329,6 +347,7 @@ void MainWindow::onParticleSelectionChanged(const QItemSelection &selected, cons
   {
     ui->label->setText("");
     ui->label_2->setText("");
+    ui->label_3->setText("");
     m_selectedParticleId.clear();
     m_selectedParticleRow = -1;
     updateStatusBar();
@@ -352,6 +371,19 @@ void MainWindow::onParticleSelectionChanged(const QItemSelection &selected, cons
       ui->label->setText("");
     }
 
+    // Show ack info
+    if (!particle->ackedByEmails.empty())
+    {
+      QStringList acked;
+      for (const auto &email : particle->ackedByEmails)
+        acked.append(email);
+      ui->label_3->setText("Acked by: " + acked.join(", "));
+    }
+    else
+    {
+      ui->label_3->setText("");
+    }
+
     // Mark seen on selection
     if (!particle->seen)
     {
diff --git a/src/store.cpp b/src/store.cpp
index 1150597..ca50496 100644
--- a/src/store.cpp
+++ b/src/store.cpp
@@ -791,8 +791,29 @@ Operation *Store::markParticlesSeen(const QStringList &particleIds)
   return new Operation(reply, this);
 }
 
+void Store::setCurrentUserEmail(const QString &email) { m_currentUserEmail = email; }
+
 Operation *Store::ackParticle(const QString &particleId)
 {
+  // Optimistic local update
+  for (auto it = m_particles.begin(); it != m_particles.end(); ++it)
+  {
+    for (int i = 0; i < it.value().size(); ++i)
+    {
+      if (it.value()[i].id == particleId)
+      {
+        auto &emails = it.value()[i].ackedByEmails;
+        if (!m_currentUserEmail.isEmpty() &&
+            std::find(emails.begin(), emails.end(), m_currentUserEmail) == emails.end())
+        {
+          emails.push_back(m_currentUserEmail);
+          emit particleUpdated(it.key(), particleId);
+        }
+        break;
+      }
+    }
+  }
+
   QNetworkReply *reply = NetworkManager::instance().post(
       QString("/particles/%1/ack").arg(particleId), QJsonDocument());
   return new Operation(reply, this);
diff --git a/src/store.h b/src/store.h
index 7c74128..e3ab598 100644
--- a/src/store.h
+++ b/src/store.h
@@ -85,6 +85,8 @@ public:
   Operation *markParticlesSeen(const QStringList &particleIds);
   Operation *ackParticle(const QString &particleId);
 
+  void setCurrentUserEmail(const QString &email);
+
 signals:
   // ── Data-change signals ──
 
@@ -106,6 +108,7 @@ private:
   QMap<QString, QList<Particle>> m_particles; // streamId -> particles
   QMap<QString, int> m_networkIndex;          // networkId -> index in m_networks
   QMap<QString, QString> m_streamToNetwork;   // streamId -> networkId
+  QString m_currentUserEmail;
 
   // ── JSON parsing ──