diff --git a/app/login/api/engine/back.php b/app/login/api/engine/back.php
index a812d54..9b5daac 100644
--- a/app/login/api/engine/back.php
+++ b/app/login/api/engine/back.php
@@ -27,7 +27,7 @@ require_once '../../../assets/utils/db_auth.php';
 
 // Clear session token so the account is free to log in elsewhere immediately
 if (!empty($_SESSION['login_user_id'])) {
-    $pdo1->prepare("UPDATE user SET session_token = NULL, session_token_at = NULL WHERE user_id = :uid")
+    $pdo1->prepare("UPDATE user SET session_token = NULL, session_token_at = NULL, session_last_seen = NULL WHERE user_id = :uid")
          ->execute([':uid' => (int)$_SESSION['login_user_id']]);
 }
 
diff --git a/app/login/api/engine/login_confirm.php b/app/login/api/engine/login_confirm.php
index 6cc9341..073e43d 100644
--- a/app/login/api/engine/login_confirm.php
+++ b/app/login/api/engine/login_confirm.php
@@ -95,29 +95,7 @@ $otp_diff_minutes = $otp_diff_seconds / 60.0;
 $_SESSION["now"]  = $now;
 $_SESSION["diff"] = $otp_diff_minutes;
 
-// ── Step 4: Block login if another session is already active ─────────────────
-// If session_token is non-NULL AND was set within the last 8 hours, another
-// session is active — reject. Tokens older than 8 hours are treated as
-// abandoned (browser crash, PHP GC expiry, etc.) and cleared automatically.
-$sth_token = $pdo1->prepare("SELECT session_token, session_last_seen FROM user WHERE user_id = :uid LIMIT 1");
-$sth_token->execute([':uid' => $user_id]);
-$token_row      = $sth_token->fetch(PDO::FETCH_ASSOC);
-$existing_token = $token_row['session_token'] ?? null;
-if (!empty($existing_token)) {
-    $idle_seconds = PHP_INT_MAX;
-    if (!empty($token_row['session_last_seen'])) {
-        $idle_seconds = time() - strtotime($token_row['session_last_seen']);
-    }
-    if ($idle_seconds < (int)ini_get('session.gc_maxlifetime')) {
-        $answer["message"] = "This account is currently logged in on another device. Please wait for the other session to end.";
-        exit(json_encode($answer));
-    }
-    // PHP GC has expired this session — clear token and allow login
-    $pdo1->prepare("UPDATE user SET session_token = NULL, session_token_at = NULL, session_last_seen = NULL WHERE user_id = :uid")
-         ->execute([':uid' => $user_id]);
-}
-
-// ── Step 4b: Validate OTP value and expiry ────────────────────────────────────
+// ── Step 4: Validate OTP value and expiry ─────────────────────────────────────
 // Skipped for staff/viewer roles — login_otp.php sets skip_otp=true in session
 // so they never receive or enter an OTP. Admin/owner always go through this check.
 if (empty($_SESSION['skip_otp'])) {
@@ -127,10 +105,33 @@ if (empty($_SESSION['skip_otp'])) {
     }
 }
 
-// ── Step 4c: Claim session — write token so no one else can log in ────────────
+// ── Step 4b: Claim session atomically ─────────────────────────────────────────
+// The WHERE clause is the concurrency gate: only a free or stale token can be
+// replaced. session_last_seen is set immediately so a fresh login is live before
+// the first authenticated heartbeat in db_auth.php.
 $session_token = bin2hex(random_bytes(32));
-$pdo1->prepare("UPDATE user SET session_token = :token, session_token_at = NOW() WHERE user_id = :uid")
-     ->execute([':token' => $session_token, ':uid' => $user_id]);
+$sth_claim = $pdo1->prepare(
+    "UPDATE user
+     SET session_token = :token,
+         session_token_at = NOW(),
+         session_last_seen = NOW()
+     WHERE user_id = :uid
+       AND (
+           session_token IS NULL
+           OR session_last_seen IS NULL
+           OR TIMESTAMPDIFF(SECOND, session_last_seen, NOW()) >= :gc_maxlifetime
+       )"
+);
+$sth_claim->execute([
+    ':token'          => $session_token,
+    ':uid'            => $user_id,
+    ':gc_maxlifetime' => (int)ini_get('session.gc_maxlifetime'),
+]);
+
+if ($sth_claim->rowCount() !== 1) {
+    $answer["message"] = "This account is currently logged in on another device. Please wait for the other session to end.";
+    exit(json_encode($answer));
+}
 
 // ── Step 5a: Regenerate session ID ────────────────────────────────────────────
 // session_regenerate_id(true) issues a brand-new session ID and deletes the old