diff --git a/auth.php b/auth.php
index 30978c9..b404b1e 100644
--- a/auth.php
+++ b/auth.php
@@ -4,65 +4,208 @@ if (session_status() === PHP_SESSION_NONE) {
     session_start();
 }
 
-function ensureConfigDirectory() {
-    if (!is_dir(CONFIG_PATH)) {
-        if (!mkdir(CONFIG_PATH, 0755, true) && !is_dir(CONFIG_PATH)) {
-            throw new RuntimeException('Unable to create config directory.');
-        }
+require_once __DIR__ . '/default.php';
+
+function getBrowserIdentifier() {
+    $agent = $_SERVER['HTTP_USER_AGENT'] ?? 'unknown';
+    if (preg_match('/(Edg|Chrome|Chromium|Firefox|Safari|Opera|MSIE|Trident)/i', $agent, $matches)) {
+        return trim($matches[1]);
     }
+    return 'unknown';
 }
 
-function getConfigData() {
-    ensureConfigDirectory();
+function getRememberMeCookieName($username) {
+    return 'kai_remember_' . sanitizeUsername($username);
+}
 
-    if (!file_exists(CONFIG_FILE)) {
-        return ['defaultModel' => '', 'users' => []];
+function createRememberMeToken($username) {
+    $browser = getBrowserIdentifier();
+    $randomValue = bin2hex(random_bytes(16));
+    $token = hash_hmac('sha256', $username . '|' . $browser . '|' . $randomValue, APP_SECRET);
+
+    return [
+        'token' => $token,
+        'browser' => $browser,
+        'createdAt' => time(),
+        'lastUsedAt' => time()
+    ];
+}
+
+function setRememberMeCookie($username) {
+    $username = trim((string) $username);
+    if ($username === '') {
+        return false;
     }
 
-    $content = @file_get_contents(CONFIG_FILE);
-    if ($content === false) {
-        return ['defaultModel' => '', 'users' => []];
+    $tokenData = createRememberMeToken($username);
+    $config = getUserConfig($username);
+    $tokens = is_array($config['rememberMeTokens'] ?? null) ? $config['rememberMeTokens'] : [];
+    $tokens[] = $tokenData;
+
+    if (count($tokens) > 10) {
+        usort($tokens, function ($a, $b) {
+            return ($a['lastUsedAt'] ?? 0) <=> ($b['lastUsedAt'] ?? 0);
+        });
+        $tokens = array_slice($tokens, -10);
     }
 
-    $data = json_decode($content, true);
-    if (!is_array($data)) {
-        return ['defaultModel' => '', 'users' => []];
+    $config['rememberMeTokens'] = $tokens;
+    if (!saveUserConfigData($username, $config)) {
+        return false;
     }
 
-    if (!isset($data['users']) || !is_array($data['users'])) {
-        $data['users'] = [];
+    $cookieName = getRememberMeCookieName($username);
+    return setcookie($cookieName, $tokenData['token'], time() + 60 * 60 * 24 * 30, '/', '', false, true);
+}
+
+function clearRememberMeCookie($username) {
+    $username = trim((string) $username);
+    if ($username === '') {
+        return;
     }
 
-    return $data;
+    $cookieName = getRememberMeCookieName($username);
+    setcookie($cookieName, '', time() - 3600, '/', '', false, true);
 }
 
-function saveConfigData($config) {
+function tryRememberMeLogin() {
+    if (!empty($_SESSION['auth_user']['username'])) {
+        return true;
+    }
+
+    foreach ($_COOKIE as $cookieName => $cookieValue) {
+        if (strpos($cookieName, 'kai_remember_') !== 0) {
+            continue;
+        }
+
+        $username = sanitizeUsername(substr($cookieName, strlen('kai_remember_')));
+        if ($username === '') {
+            continue;
+        }
+
+        $user = findUserByUsername($username);
+        if ($user === null) {
+            continue;
+        }
+
+        $config = getUserConfig($username);
+        $tokens = is_array($config['rememberMeTokens'] ?? null) ? $config['rememberMeTokens'] : [];
+        $browser = getBrowserIdentifier();
+        foreach ($tokens as &$entry) {
+            if (($entry['token'] ?? '') !== $cookieValue) {
+                continue;
+            }
+            if (!empty($entry['browser']) && $entry['browser'] !== $browser) {
+                continue;
+            }
+
+            $entry['lastUsedAt'] = time();
+            $config['rememberMeTokens'] = $tokens;
+            saveUserConfigData($username, $config);
+
+            $_SESSION['auth_user'] = [
+                'username' => $user['username'],
+                'role' => $user['role'] ?? 'user'
+            ];
+            return true;
+        }
+    }
+
+    return false;
+}
+
+function ensureConfigDirectory() {
+    if (!is_dir(USER_CONFIG_DIR)) {
+        if (!mkdir(USER_CONFIG_DIR, 0755, true) && !is_dir(USER_CONFIG_DIR)) {
+            throw new RuntimeException('Unable to create config directory.');
+        }
+    }
+}
+
+function sanitizeUsername($username) {
+    $username = trim((string) $username);
+    $username = preg_replace('/[^A-Za-z0-9._-]/', '', $username);
+    return $username !== '' ? $username : 'user';
+}
+
+function getUserConfigPath($username) {
+    return USER_CONFIG_DIR . '/' . sanitizeUsername($username) . '-config.php';
+}
+
+function getUserChatPath($username) {
+    return USER_CONFIG_DIR . '/' . sanitizeUsername($username) . '-chat.php';
+}
+
+function readUserConfig($username) {
+    $path = getUserConfigPath($username);
+    if (!file_exists($path)) {
+        return [];
+    }
+
+    $data = include $path;
+    return is_array($data) ? $data : [];
+}
+
+function writeUserConfig($username, $config) {
     ensureConfigDirectory();
 
-    $data = getConfigData();
-    if (array_key_exists('defaultModel', $config)) {
-        $data['defaultModel'] = (string) $config['defaultModel'];
+    $username = trim((string) $username);
+    if ($username === '') {
+        return false;
     }
 
-    if (array_key_exists('users', $config)) {
-        $data['users'] = is_array($config['users']) ? $config['users'] : [];
+    $data = is_array($config) ? $config : [];
+    $data['username'] = $username;
+    $data['role'] = (string) ($data['role'] ?? 'user');
+    $data['email'] = (string) ($data['email'] ?? '');
+    $data['passwordHash'] = (string) ($data['passwordHash'] ?? '');
+    $data['defaultModel'] = (string) ($data['defaultModel'] ?? '');
+
+    $php = "<?php\n\nreturn " . var_export($data, true) . ";\n";
+    return file_put_contents(getUserConfigPath($username), $php) !== false;
+}
+
+function readUserChat($username) {
+    $path = getUserChatPath($username);
+    if (!file_exists($path)) {
+        return [];
     }
 
-    $json = json_encode($data, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE);
-    return file_put_contents(CONFIG_FILE, $json) !== false;
+    $data = include $path;
+    return is_array($data) ? $data : [];
 }
 
-function readConfig() {
-    $data = getConfigData();
+function writeUserChat($username, $chats) {
+    ensureConfigDirectory();
 
-    return [
-        'defaultModel' => isset($data['defaultModel']) ? (string) $data['defaultModel'] : '',
-        'users' => isset($data['users']) && is_array($data['users']) ? $data['users'] : []
-    ];
+    $username = trim((string) $username);
+    if ($username === '') {
+        return false;
+    }
+
+    $php = "<?php\n\nreturn " . var_export(is_array($chats) ? $chats : [], true) . ";\n";
+    return file_put_contents(getUserChatPath($username), $php) !== false;
 }
 
-function writeConfig($config) {
-    return saveConfigData($config);
+function renameUserFiles($oldUsername, $newUsername) {
+    $oldUsername = trim((string) $oldUsername);
+    $newUsername = trim((string) $newUsername);
+
+    if ($oldUsername === '' || $newUsername === '' || $oldUsername === $newUsername) {
+        return;
+    }
+
+    $oldConfigPath = getUserConfigPath($oldUsername);
+    $newConfigPath = getUserConfigPath($newUsername);
+    if (file_exists($oldConfigPath) && !file_exists($newConfigPath)) {
+        @rename($oldConfigPath, $newConfigPath);
+    }
+
+    $oldChatPath = getUserChatPath($oldUsername);
+    $newChatPath = getUserChatPath($newUsername);
+    if (file_exists($oldChatPath) && !file_exists($newChatPath)) {
+        @rename($oldChatPath, $newChatPath);
+    }
 }
 
 function hashPassword($password) {
@@ -73,13 +216,92 @@ function verifyPassword($password, $hash) {
     return password_verify($password, $hash);
 }
 
+function getUserConfig($username) {
+    $username = trim((string) $username);
+    if ($username === '') {
+        return [];
+    }
+
+    return readUserConfig($username);
+}
+
+function saveUserConfigData($username, $config) {
+    return writeUserConfig($username, $config);
+}
+
 function loadUsers() {
-    $config = readConfig();
-    return $config['users'];
+    ensureConfigDirectory();
+
+    $files = glob(USER_CONFIG_DIR . '/*-config.php');
+    if ($files === false) {
+        return [];
+    }
+
+    $users = [];
+    foreach ($files as $file) {
+        $username = basename($file, '-config.php');
+        $user = readUserConfig($username);
+        if (!empty($user) && !empty($user['username'])) {
+            $users[] = $user;
+        }
+    }
+
+    usort($users, function ($a, $b) {
+        return strcmp(($a['username'] ?? ''), ($b['username'] ?? ''));
+    });
+
+    return $users;
 }
 
-function saveUsers($users) {
-    return writeConfig(['users' => $users]);
+function saveUsers($users, $previousUsers = null) {
+    ensureConfigDirectory();
+
+    $normalizedUsers = [];
+    foreach ($users as $index => $user) {
+        $entry = is_array($user) ? $user : [];
+        $username = trim((string) ($entry['username'] ?? ''));
+        if ($username === '') {
+            continue;
+        }
+
+        $entry['username'] = $username;
+        $entry['role'] = (string) ($entry['role'] ?? 'user');
+        $entry['email'] = (string) ($entry['email'] ?? '');
+        $entry['passwordHash'] = (string) ($entry['passwordHash'] ?? '');
+        $entry['defaultModel'] = (string) ($entry['defaultModel'] ?? '');
+        $normalizedUsers[] = $entry;
+    }
+
+    $previousUsers = is_array($previousUsers) ? $previousUsers : loadUsers();
+    foreach ($normalizedUsers as $index => $entry) {
+        $oldUsername = isset($previousUsers[$index]) ? trim((string) ($previousUsers[$index]['username'] ?? '')) : '';
+        $newUsername = trim((string) ($entry['username'] ?? ''));
+        if ($oldUsername !== '' && $oldUsername !== $newUsername) {
+            renameUserFiles($oldUsername, $newUsername);
+        }
+
+        if (!writeUserConfig($newUsername, $entry)) {
+            return false;
+        }
+    }
+
+    $currentUsernames = [];
+    foreach ($normalizedUsers as $entry) {
+        $currentUsernames[] = sanitizeUsername($entry['username']);
+    }
+
+    $files = glob(USER_CONFIG_DIR . '/*-config.php');
+    if ($files !== false) {
+        foreach ($files as $file) {
+            $name = basename($file, '-config.php');
+            if (!in_array($name, $currentUsernames, true)) {
+                @unlink(USER_CONFIG_DIR . '/' . $name . '-config.php');
+                @unlink(USER_CONFIG_DIR . '/' . $name . '-chat.php');
+            }
+        }
+    }
+
+    return true;
 }
 
 function findUserByUsername($username) {
@@ -132,9 +354,14 @@ function createUser($username, $password, $role = 'user', $email = '') {
         'username' => $username,
         'email' => $email,
         'passwordHash' => hashPassword($password),
-        'role' => $role
+        'role' => $role,
+        'defaultModel' => ''
     ];
 
+    if (!writeUserChat($username, [])) {
+        return false;
+    }
+
     return saveUsers($users);
 }
 
@@ -222,7 +449,16 @@ function getAuthenticatedUser() {
         return null;
     }
 
-    return $_SESSION['auth_user'];
+    return findUserByUsername($_SESSION['auth_user']['username'] ?? '');
+}
+
+function getAuthenticatedUsername() {
+    $user = getAuthenticatedUser();
+    if ($user !== null) {
+        return (string) ($user['username'] ?? '');
+    }
+
+    return (string) ($_SESSION['auth_user']['username'] ?? '');
 }
 
 function requireAuthentication() {
@@ -243,5 +479,10 @@ function requireAdmin() {
 }
 
 function logoutUser() {
+    $username = trim((string) ($_SESSION['auth_user']['username'] ?? ''));
     unset($_SESSION['auth_user']);
+
+    if ($username !== '') {
+        clearRememberMeCookie($username);
+    }
 }
diff --git a/chat.php b/chat.php
index 663f9e9..334ffd0 100644
--- a/chat.php
+++ b/chat.php
@@ -2,6 +2,35 @@
 
 header('Content-Type: application/json');
 require_once __DIR__ . '/default.php';
+require_once __DIR__ . '/auth.php';
+
+function appendChatAuditEntry($username, $chatMessages, $resultPayload, $ipAddress = null) {
+    ensureConfigDirectory();
+
+    $auditPath = USER_CONFIG_DIR . '/chat-requests.json';
+    $entries = [];
+
+    if (file_exists($auditPath)) {
+        $raw = file_get_contents($auditPath);
+        if ($raw !== false) {
+            $decoded = json_decode($raw, true);
+            if (is_array($decoded)) {
+                $entries = $decoded;
+            }
+        }
+    }
+
+    $entries[] = [
+        'timestamp' => date('c'),
+        'username' => (string) $username,
+        'ipAddress' => (string) ($ipAddress ?? ($_SERVER['REMOTE_ADDR'] ?? '')),
+        'chat' => is_array($chatMessages) ? $chatMessages : [$chatMessages],
+        'result' => $resultPayload
+    ];
+
+    $payload = json_encode($entries, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
+    return $payload !== false && file_put_contents($auditPath, $payload . PHP_EOL) !== false;
+}
 
 // 1. Handle fetching available models
 if (isset($_GET['action']) && $_GET['action'] === 'models') {
@@ -24,6 +53,10 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     
     $model = $input['model'] ?? 'llama3';
     $messages = $input['messages'] ?? [];
+    $username = '';
+    if (!empty($_SESSION['auth_user']['username'])) {
+        $username = sanitizeUsername($_SESSION['auth_user']['username']);
+    }
 
     $payload = json_encode([
         'model' => $model,
@@ -38,13 +71,19 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
 
     $response = curl_exec($ch);
-    
+    $resultPayload = null;
+
     if (curl_errno($ch)) {
-        echo json_encode(['error' => 'Error communicating with remote Ollama: ' . curl_error($ch)]);
+        $resultPayload = ['error' => 'Error communicating with remote Ollama: ' . curl_error($ch)];
+        echo json_encode($resultPayload);
     } else {
+        $decodedResponse = json_decode($response, true);
+        $resultPayload = $decodedResponse !== null ? $decodedResponse : $response;
         echo $response;
     }
     curl_close($ch);
+
+    appendChatAuditEntry($username, $messages, $resultPayload, $_SERVER['REMOTE_ADDR'] ?? '');
     exit;
 }
 
diff --git a/config.php b/config.php
index ca4e7d1..2d319ca 100644
--- a/config.php
+++ b/config.php
@@ -1,40 +1,4 @@
 <?php
 
-header('Content-Type: application/json');
-require_once __DIR__ . '/default.php';
-require_once __DIR__ . '/auth.php';
-
-$action = $_GET['action'] ?? '';
-
-switch ($action) {
-    case 'load':
-        echo json_encode(readConfig());
-        exit;
-
-    case 'save':
-        if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
-            echo json_encode(['error' => 'POST required.']);
-            exit;
-        }
-
-        $input = json_decode(file_get_contents('php://input'), true);
-        if (!is_array($input)) {
-            echo json_encode(['error' => 'Invalid JSON.']);
-            exit;
-        }
-
-        $config = readConfig();
-        $config['defaultModel'] = isset($input['defaultModel']) ? (string) $input['defaultModel'] : '';
-
-        if (!writeConfig($config)) {
-            echo json_encode(['error' => 'Unable to save config.']);
-            exit;
-        }
-
-        echo json_encode(['success' => true, 'defaultModel' => $config['defaultModel']]);
-        exit;
-
-    default:
-        echo json_encode(['error' => 'Unknown action.']);
-        exit;
-}
+define('USER_CONFIG_DIR', '/var/lib/KaI/users');
+define('OLLAMA_URL', 'http://127.0.0.1:11434');
diff --git a/default.php b/default.php
index 3584794..74943cc 100644
--- a/default.php
+++ b/default.php
@@ -1,10 +1,9 @@
 <?php
 
-define('OLLAMA_URL', 'http://127.0.0.1:11434');
-define('CONFIG_PATH', '/var/lib/KaI');
-define('CONFIG_FILE', CONFIG_PATH . '/config.json');
-define('CHAT_HISTORY_FILE', CONFIG_PATH . '/chat.history.data');
+require_once __DIR__ . '/config.php';
+
 define('MAIL_FROM', getenv('MAIL_FROM') ?: 'noreply@kai.local');
 define('MAIL_FROM_NAME', getenv('MAIL_FROM_NAME') ?: 'KaI');
 define('APP_BASE_URL', getenv('APP_BASE_URL') ?: 'http://127.0.0.1/KaI-OllamaChat');
+define('APP_SECRET', getenv('APP_SECRET') ?: 'change-this-secret');
 
diff --git a/index.php b/index.php
index 8727831..4b8d5ad 100644
--- a/index.php
+++ b/index.php
@@ -82,7 +82,7 @@ function appendMessage(role, text) {
 
 async function loadConfig() {
     try {
-        const response = await fetch('config.php?action=load');
+        const response = await fetch('savechat.php?action=load-config');
         const data = await response.json();
         return data.defaultModel || '';
     } catch (err) {
@@ -93,7 +93,7 @@ async function loadConfig() {
 
 async function saveConfig(defaultModel) {
     try {
-        await fetch('config.php?action=save', {
+        await fetch('savechat.php?action=save-config', {
             method: 'POST',
             headers: { 'Content-Type': 'application/json' },
             body: JSON.stringify({ defaultModel })
diff --git a/login.php b/login.php
index a39a3ae..6c1d4a9 100644
--- a/login.php
+++ b/login.php
@@ -23,6 +23,13 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                 'username' => $user['username'],
                 'role' => $user['role'] ?? 'user'
             ];
+
+            if (!empty($_POST['remember'])) {
+                setRememberMeCookie($user['username']);
+            } else {
+                clearRememberMeCookie($user['username']);
+            }
+
             header('Location: index.php');
             exit;
         }
@@ -36,9 +43,7 @@ if (isLoggedIn()) {
     exit;
 }
 
-if (!file_exists(CONFIG_FILE)) {
-    ensureConfigDirectory();
-}
+ensureConfigDirectory();
 
 if (empty(loadUsers())) {
     $created = createUser('admin', 'changeme', 'admin', 'admin@example.com');
@@ -75,6 +80,11 @@ if (empty(loadUsers())) {
             <label for="password">Password</label>
             <input id="password" name="password" type="password" required>
 
+            <label style="font-weight:normal; margin-top:12px;">
+                <input type="checkbox" name="remember" value="1">
+                Remember me on this browser for 30 days
+            </label>
+
             <button type="submit">Sign in</button>
         </form>
         <?php if (!empty($errors)): ?>
diff --git a/savechat.php b/savechat.php
index d1b8b56..1db7a79 100644
--- a/savechat.php
+++ b/savechat.php
@@ -2,169 +2,118 @@
 
 header('Content-Type: application/json');
 require_once __DIR__ . '/default.php';
+require_once __DIR__ . '/auth.php';
 
-/*----------------------------------------------------------
-  -  Create storage file if necessary
-  ----------------------------------------------------------*/
-if (!file_exists(CHAT_HISTORY_FILE)) {
-    if (@file_put_contents(CHAT_HISTORY_FILE, '') === false) {
-        http_response_code(500);
-        echo json_encode([
-            'error' => 'Cannot create history file.'
-        ]);
-        exit;
-    }
-}
-
-/*----------------------------------------------------------
-  - Helper
-  ----------------------------------------------------------*/
-function readChats() {
-    $result = [];
-
-    $fp = fopen(CHAT_HISTORY_FILE, 'c+');
-    if (!$fp) {
-        return [];
-    }
-
-    flock($fp, LOCK_SH);
-    while (($line = fgets($fp)) !== false) {
-
-        $line = trim($line);
-        if ($line === '') {
-            continue;
-        }
-
-        $obj = json_decode($line, true);
-        if (is_array($obj)) {
-            $result[] = $obj;
-        }
-    }
+requireAuthentication();
 
-    flock($fp, LOCK_UN);
-    fclose($fp);
-
-    return $result;
-}
-
-function writeChats($chats) {
-    $fp = fopen(CHAT_HISTORY_FILE, 'w');
-
-    if (!$fp) {
-        return false;
-    }
-
-    flock($fp, LOCK_EX);
-    foreach ($chats as $chat) {
-        fwrite($fp, json_encode($chat, JSON_UNESCAPED_UNICODE) . PHP_EOL);
-    }
-
-    fflush($fp);
-
-    flock($fp, LOCK_UN);
-    fclose($fp);
-
-    return true;
-}
+$username = getAuthenticatedUsername();
 
 function createTitle($messages) {
     foreach ($messages as $msg) {
         if (($msg['role'] ?? '') === 'user') {
             $title = trim($msg['content']);
             if (mb_strlen($title) > 60) {
-                $title = mb_substr($title, 0, 60) . "...";
+                $title = mb_substr($title, 0, 60) . '...';
             }
             return $title;
         }
     }
-    return "Untitled Chat";
+    return 'Untitled Chat';
 }
 
-/*----------------------------------------------------------
-  - Action
-  ----------------------------------------------------------*/
-
 $action = $_GET['action'] ?? '';
 switch ($action) {
-    /*------------------------------------------------------
-      - LIST
-      ------------------------------------------------------*/
+    case 'load-config':
+        $config = getUserConfig($username);
+        echo json_encode([
+            'defaultModel' => (string) ($config['defaultModel'] ?? '')
+        ]);
+        exit;
+
+    case 'save-config':
+        if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
+            echo json_encode(['error' => 'POST required.']);
+            exit;
+        }
+
+        $input = json_decode(file_get_contents('php://input'), true);
+        if (!is_array($input)) {
+            echo json_encode(['error' => 'Invalid JSON.']);
+            exit;
+        }
+
+        $config = getUserConfig($username);
+        $config['defaultModel'] = isset($input['defaultModel']) ? (string) $input['defaultModel'] : '';
+
+        if (!saveUserConfigData($username, $config)) {
+            echo json_encode(['error' => 'Unable to save config.']);
+            exit;
+        }
+
+        echo json_encode(['success' => true, 'defaultModel' => $config['defaultModel']]);
+        exit;
+
     case 'list':
-        $chats = readChats();
+        $chats = readUserChat($username);
         $list = [];
         foreach ($chats as $chat) {
             $list[] = [
-                'id' => $chat['id'],
-                'title' => $chat['title'],
-                'created' => $chat['created'],
+                'id' => $chat['id'] ?? '',
+                'title' => $chat['title'] ?? 'Untitled Chat',
+                'created' => $chat['created'] ?? '',
                 'model' => $chat['model'] ?? ''
             ];
         }
 
-        usort($list, function($a, $b) {
+        usort($list, function ($a, $b) {
             return strcmp($b['created'], $a['created']);
         });
 
         echo json_encode($list);
         exit;
 
-    /*------------------------------------------------------
-      - LOAD
-      ------------------------------------------------------*/
     case 'load':
         $id = $_GET['id'] ?? '';
-        if ($id == '') {
-            echo json_encode([
-                'error' => 'Missing id.'
-            ]);
+        if ($id === '') {
+            echo json_encode(['error' => 'Missing id.']);
             exit;
         }
 
-        $chats = readChats();
+        $chats = readUserChat($username);
         foreach ($chats as $chat) {
-            if ($chat['id'] === $id) {
+            if (($chat['id'] ?? '') === $id) {
                 echo json_encode($chat);
                 exit;
             }
         }
-        echo json_encode([
-            'error' => 'Chat not found.'
-        ]);
+        echo json_encode(['error' => 'Chat not found.']);
         exit;
-    /*------------------------------------------------------
-      -  SAVE
-      ------------------------------------------------------*/
+
     case 'save':
         if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
-            echo json_encode([
-                'error' => 'POST required.'
-            ]);
+            echo json_encode(['error' => 'POST required.']);
             exit;
         }
 
         $input = json_decode(file_get_contents('php://input'), true);
         if (!is_array($input)) {
-            echo json_encode([
-                'error' => 'Invalid JSON.'
-            ]);
+            echo json_encode(['error' => 'Invalid JSON.']);
             exit;
         }
 
         $messages = $input['messages'] ?? [];
-        if (!is_array($messages) || count($messages) == 0) {
-            echo json_encode([
-                'error' => 'No messages.'
-            ]);
+        if (!is_array($messages) || count($messages) === 0) {
+            echo json_encode(['error' => 'No messages.']);
             exit;
         }
 
         $id = $input['id'] ?? '';
-        if ($id == '') {
+        if ($id === '') {
             $id = uniqid('', true);
         }
 
         $title = createTitle($messages);
-        $model = $input['model'] ?? '';
+        $model = isset($input['model']) ? (string) $input['model'] : '';
         $chat = [
             'id' => $id,
             'title' => $title,
@@ -173,10 +122,10 @@ switch ($action) {
             'messages' => $messages
         ];
 
-        $chats = readChats();
+        $chats = readUserChat($username);
         $found = false;
         foreach ($chats as &$existing) {
-            if ($existing['id'] === $id) {
+            if (($existing['id'] ?? '') === $id) {
                 $existing = $chat;
                 $found = true;
                 break;
@@ -187,10 +136,10 @@ switch ($action) {
             $chats[] = $chat;
         }
 
-        if (!writeChats($chats)) {
-            echo json_encode([
-                'error' => 'Unable to save.'
-            ]);
+        $config = getUserConfig($username);
+        $config['defaultModel'] = $model;
+        if (!saveUserConfigData($username, $config) || !writeUserChat($username, $chats)) {
+            echo json_encode(['error' => 'Unable to save.']);
             exit;
         }
 
@@ -201,40 +150,26 @@ switch ($action) {
         ]);
         exit;
 
-    /*------------------------------------------------------
-      -  DELETE
-      ------------------------------------------------------*/
     case 'delete':
         $id = $_GET['id'] ?? '';
-
-        if ($id == '') {
-            echo json_encode([
-                'error' => 'Missing id.'
-            ]);
+        if ($id === '') {
+            echo json_encode(['error' => 'Missing id.']);
             exit;
         }
 
-        $chats = readChats();
+        $chats = readUserChat($username);
         $newChats = [];
-
         foreach ($chats as $chat) {
-            if ($chat['id'] !== $id) {
+            if (($chat['id'] ?? '') !== $id) {
                 $newChats[] = $chat;
             }
         }
 
-        writeChats($newChats);
-        echo json_encode([
-            'success' => true
-        ]);
+        writeUserChat($username, $newChats);
+        echo json_encode(['success' => true]);
         exit;
-    /*------------------------------------------------------
-      -  DEFAULT
-      ------------------------------------------------------*/
 
     default:
-        echo json_encode([
-            'error' => 'Unknown action.'
-        ]);
+        echo json_encode(['error' => 'Unknown action.']);
         exit;
 }
diff --git a/useradmin.php b/useradmin.php
index 55ff182..7c9912f 100644
--- a/useradmin.php
+++ b/useradmin.php
@@ -11,6 +11,7 @@ requireAuthentication();
 requireAdmin();
 
 $users = loadUsers();
+$previousUsers = $users;
 $errors = [];
 $success = '';
 
@@ -49,7 +50,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                 $users[$index]['passwordHash'] = hashPassword($password);
             }
             $users[$index]['role'] = $role;
-            if (saveUsers($users)) {
+            if (saveUsers($users, $previousUsers)) {
                 $success = 'User updated successfully.';
             } else {
                 $errors[] = 'Unable to update user.';
@@ -62,7 +63,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         if ($index >= 0 && isset($users[$index])) {
             unset($users[$index]);
             $users = array_values($users);
-            if (saveUsers($users)) {
+            if (saveUsers($users, $previousUsers)) {
                 $success = 'User deleted successfully.';
             } else {
                 $errors[] = 'Unable to delete user.';