diff --git a/auth.php b/auth.php
index b404b1e..9439184 100644
--- a/auth.php
+++ b/auth.php
@@ -14,6 +14,44 @@ function getBrowserIdentifier() {
     return 'unknown';
 }
 
+function isGuestModeEnabled() {
+    return defined('GUEST_USER_ENABLED') && GUEST_USER_ENABLED === true;
+}
+
+function getGuestUsername() {
+    return defined('GUEST_USERNAME') ? (string) GUEST_USERNAME : 'guest';
+}
+
+function isGuestUsername($username) {
+    return isGuestModeEnabled() && strtolower(trim((string) $username)) === strtolower(getGuestUsername());
+}
+
+function isGuestSession() {
+    return isGuestModeEnabled() && !empty($_SESSION['auth_user']['username']) && isGuestUsername($_SESSION['auth_user']['username']);
+}
+
+function createGuestUser() {
+    return [
+        'username' => getGuestUsername(),
+        'role' => 'guest',
+        'guest' => true,
+        'email' => '',
+        'passwordHash' => ''
+    ];
+}
+
+function loginAsGuest() {
+    if (!isGuestModeEnabled()) {
+        return false;
+    }
+
+    $_SESSION['auth_user'] = [
+        'username' => getGuestUsername(),
+        'role' => 'guest'
+    ];
+    return true;
+}
+
 function getRememberMeCookieName($username) {
     return 'kai_remember_' . sanitizeUsername($username);
 }
@@ -136,6 +174,17 @@ function getUserChatPath($username) {
     return USER_CONFIG_DIR . '/' . sanitizeUsername($username) . '-chat.php';
 }
 
+function getConfiguredDefaultModel() {
+    $defaultModel = defined('DEFAULT_MODEL') ? trim((string) DEFAULT_MODEL) : '';
+    return $defaultModel !== '' ? $defaultModel : 'llama3';
+}
+
+function getDefaultModelForUser($username) {
+    $config = getUserConfig($username);
+    $storedModel = trim((string) ($config['defaultModel'] ?? ''));
+    return $storedModel !== '' ? $storedModel : getConfiguredDefaultModel();
+}
+
 function readUserConfig($username) {
     $path = getUserConfigPath($username);
     if (!file_exists($path)) {
@@ -159,7 +208,8 @@ function writeUserConfig($username, $config) {
     $data['role'] = (string) ($data['role'] ?? 'user');
     $data['email'] = (string) ($data['email'] ?? '');
     $data['passwordHash'] = (string) ($data['passwordHash'] ?? '');
-    $data['defaultModel'] = (string) ($data['defaultModel'] ?? '');
+    $storedDefaultModel = trim((string) ($data['defaultModel'] ?? ''));
+    $data['defaultModel'] = $storedDefaultModel !== '' ? $storedDefaultModel : getConfiguredDefaultModel();
 
     $php = "<?php\n\nreturn " . var_export($data, true) . ";\n";
     return file_put_contents(getUserConfigPath($username), $php) !== false;
@@ -268,7 +318,7 @@ function saveUsers($users, $previousUsers = null) {
         $entry['role'] = (string) ($entry['role'] ?? 'user');
         $entry['email'] = (string) ($entry['email'] ?? '');
         $entry['passwordHash'] = (string) ($entry['passwordHash'] ?? '');
-        $entry['defaultModel'] = (string) ($entry['defaultModel'] ?? '');
+        $entry['defaultModel'] = trim((string) ($entry['defaultModel'] ?? '')) !== '' ? trim((string) $entry['defaultModel']) : getConfiguredDefaultModel();
         $normalizedUsers[] = $entry;
     }
 
@@ -355,7 +405,7 @@ function createUser($username, $password, $role = 'user', $email = '') {
         'email' => $email,
         'passwordHash' => hashPassword($password),
         'role' => $role,
-        'defaultModel' => ''
+        'defaultModel' => getConfiguredDefaultModel()
     ];
 
     if (!writeUserChat($username, [])) {
@@ -449,6 +499,10 @@ function getAuthenticatedUser() {
         return null;
     }
 
+    if (isGuestSession()) {
+        return createGuestUser();
+    }
+
     return findUserByUsername($_SESSION['auth_user']['username'] ?? '');
 }
 
diff --git a/chat.php b/chat.php
index 334ffd0..85449eb 100644
--- a/chat.php
+++ b/chat.php
@@ -51,7 +51,9 @@ if (isset($_GET['action']) && $_GET['action'] === 'models') {
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     $input = json_decode(file_get_contents('php://input'), true);
     
-    $model = $input['model'] ?? 'llama3';
+    $model = isset($input['model']) && trim((string) $input['model']) !== ''
+        ? (string) $input['model']
+        : getConfiguredDefaultModel();
     $messages = $input['messages'] ?? [];
     $username = '';
     if (!empty($_SESSION['auth_user']['username'])) {
diff --git a/config.php b/config.php
index 2d319ca..16a6cde 100644
--- a/config.php
+++ b/config.php
@@ -2,3 +2,7 @@
 
 define('USER_CONFIG_DIR', '/var/lib/KaI/users');
 define('OLLAMA_URL', 'http://127.0.0.1:11434');
+define('GUEST_USER_ENABLED', true);
+define('GUEST_USERNAME', 'guest');
+define('DEFAULT_MODEL', 'llama3.2:latest');
+
diff --git a/index.php b/index.php
index 4b8d5ad..a415e63 100644
--- a/index.php
+++ b/index.php
@@ -7,12 +7,17 @@ if (session_status() === PHP_SESSION_NONE) {
 require_once __DIR__ . '/default.php';
 require_once __DIR__ . '/auth.php';
 
+if (!isLoggedIn() && isGuestModeEnabled()) {
+    loginAsGuest();
+}
+
 if (!isLoggedIn()) {
     header('Location: login.php');
     exit;
 }
 
 $authUser = getAuthenticatedUser();
+$isGuestSession = isGuestSession();
 ?>
 <!DOCTYPE html>
 <html lang="en">
@@ -35,10 +40,13 @@ $authUser = getAuthenticatedUser();
         <div class="chat-header">
             <h2>I'm KaI your - Ollama Chat</h2>
             <div class="controls">
-                <span style="font-size:12px; color:#aaa;">Signed in as <?php echo htmlspecialchars($authUser['username'] ?? ''); ?></span>
+                <span style="font-size:12px; color:#aaa;">Signed in as <?php echo htmlspecialchars(($authUser['username'] ?? 'guest')); ?></span>
                 <?php if (($authUser['role'] ?? 'user') === 'admin'): ?>
                     <a href="useradmin.php" style="color:#89b4fa; text-decoration:none;">User Admin</a>
                 <?php endif; ?>
+                <?php if (isGuestModeEnabled()): ?>
+                    <a href="login.php" style="color:#89b4fa; text-decoration:none;">Login</a>
+                <?php endif; ?>
                 <a href="logout.php" style="color:#f38ba8; text-decoration:none;">Logout</a>
                 <select id="model-select">
                     <option>Loading models...</option>
diff --git a/login.php b/login.php
index 6c1d4a9..dd053e4 100644
--- a/login.php
+++ b/login.php
@@ -11,6 +11,12 @@ $errors = [];
 $message = '';
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
+    if (!empty($_POST['guest']) && isGuestModeEnabled()) {
+        loginAsGuest();
+        header('Location: index.php');
+        exit;
+    }
+
     $username = trim($_POST['username'] ?? '');
     $password = (string) ($_POST['password'] ?? '');
 
@@ -38,7 +44,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     }
 }
 
-if (isLoggedIn()) {
+if (isLoggedIn() && !isGuestSession()) {
     header('Location: index.php');
     exit;
 }
@@ -65,6 +71,7 @@ if (empty(loadUsers())) {
         label { display: block; margin-top: 12px; font-weight: bold; }
         input { width: 100%; padding: 10px; margin-top: 6px; border-radius: 6px; border: 1px solid #45475a; background: #313244; color: #fff; }
         button { margin-top: 16px; width: 100%; padding: 10px; border: none; border-radius: 6px; background: #89b4fa; color: #11111b; font-weight: bold; cursor: pointer; }
+        .guest-btn { background: #a6e3a1; color: #11111b; }
         .error { color: #f38ba8; margin-top: 12px; }
         .message { color: #a6e3a1; margin-top: 12px; }
         .small { font-size: 0.9rem; color: #999; margin-top: 12px; }
@@ -73,6 +80,12 @@ if (empty(loadUsers())) {
 <body>
     <div class="card">
         <h1>KaI Login</h1>
+        <?php if (isGuestModeEnabled()): ?>
+            <form method="post">
+                <input type="hidden" name="guest" value="1">
+                <button type="submit" class="guest-btn">Continue as Guest</button>
+            </form>
+        <?php endif; ?>
         <form method="post">
             <label for="username">Username</label>
             <input id="username" name="username" required>
diff --git a/savechat.php b/savechat.php
index 1db7a79..2c67c96 100644
--- a/savechat.php
+++ b/savechat.php
@@ -7,6 +7,34 @@ require_once __DIR__ . '/auth.php';
 requireAuthentication();
 
 $username = getAuthenticatedUsername();
+$action = $_GET['action'] ?? '';
+$defaultModel = getConfiguredDefaultModel();
+
+if (isGuestSession()) {
+    switch ($action) {
+        case 'load-config':
+            echo json_encode(['defaultModel' => $defaultModel]);
+            exit;
+        case 'save-config':
+            echo json_encode(['success' => true, 'defaultModel' => $defaultModel]);
+            exit;
+        case 'list':
+            echo json_encode([]);
+            exit;
+        case 'load':
+            echo json_encode(['error' => 'Guest users do not have saved chats.']);
+            exit;
+        case 'save':
+            echo json_encode(['success' => true, 'id' => '', 'title' => '']);
+            exit;
+        case 'delete':
+            echo json_encode(['success' => true]);
+            exit;
+        default:
+            echo json_encode(['error' => 'Unknown action.']);
+            exit;
+    }
+}
 
 function createTitle($messages) {
     foreach ($messages as $msg) {
@@ -21,12 +49,10 @@ function createTitle($messages) {
     return 'Untitled Chat';
 }
 
-$action = $_GET['action'] ?? '';
 switch ($action) {
     case 'load-config':
-        $config = getUserConfig($username);
         echo json_encode([
-            'defaultModel' => (string) ($config['defaultModel'] ?? '')
+            'defaultModel' => getDefaultModelForUser($username)
         ]);
         exit;
 
@@ -43,7 +69,9 @@ switch ($action) {
         }
 
         $config = getUserConfig($username);
-        $config['defaultModel'] = isset($input['defaultModel']) ? (string) $input['defaultModel'] : '';
+        $config['defaultModel'] = isset($input['defaultModel']) && trim((string) $input['defaultModel']) !== ''
+            ? (string) $input['defaultModel']
+            : getConfiguredDefaultModel();
 
         if (!saveUserConfigData($username, $config)) {
             echo json_encode(['error' => 'Unable to save config.']);
@@ -113,7 +141,9 @@ switch ($action) {
         }
 
         $title = createTitle($messages);
-        $model = isset($input['model']) ? (string) $input['model'] : '';
+        $model = isset($input['model']) && trim((string) $input['model']) !== ''
+            ? (string) $input['model']
+            : getDefaultModelForUser($username);
         $chat = [
             'id' => $id,
             'title' => $title,