diff --git a/backend/core/config.js b/backend/core/config.js
index 217b860..34b2fc3 100644
--- a/backend/core/config.js
+++ b/backend/core/config.js
@@ -54,6 +54,7 @@ function getAppDir() {
 const CONFIG_FILE = path.join(getAppDir(), 'config.json');
 const CONFIG_BACKUP = path.join(getAppDir(), 'config.json.bak');
 const CONFIG_TEMP = path.join(getAppDir(), 'config.json.tmp');
+const UUID_STORE_FILE = path.join(getAppDir(), 'uuid-store.json');
 
 // =============================================================================
 // CONFIG VALIDATION
@@ -152,6 +153,22 @@ function saveConfig(update) {
 
       // Load current config
       const currentConfig = loadConfig();
+
+      // SAFETY: If config file exists on disk but loadConfig() returned empty,
+      // something is wrong (file locked, corrupted, etc.). Refuse to save
+      // because merging with {} would wipe all existing data (userUuids, username, etc.)
+      if (Object.keys(currentConfig).length === 0 && fs.existsSync(CONFIG_FILE)) {
+        const fileSize = fs.statSync(CONFIG_FILE).size;
+        if (fileSize > 2) { // More than just "{}"
+          console.error(`[Config] REFUSING to save — loaded empty but file exists (${fileSize} bytes). Retrying load...`);
+          // Wait and retry the load
+          const delay = attempt * 200;
+          const start = Date.now();
+          while (Date.now() - start < delay) { /* busy wait */ }
+          continue;
+        }
+      }
+
       const newConfig = { ...currentConfig, ...update };
       const data = JSON.stringify(newConfig, null, 2);
 
@@ -238,11 +255,18 @@ function saveUsername(username) {
   // Check if we're actually changing the username (case-insensitive comparison)
   const isRename = currentName && currentName.toLowerCase() !== newName.toLowerCase();
 
+  // Also update UUID store (source of truth)
+  migrateUuidStoreIfNeeded();
+  const uuidStore = loadUuidStore();
+
   if (isRename) {
     // Find the UUID for the current username
     const currentKey = Object.keys(userUuids).find(
       k => k.toLowerCase() === currentName.toLowerCase()
     );
+    const currentStoreKey = Object.keys(uuidStore).find(
+      k => k.toLowerCase() === currentName.toLowerCase()
+    );
 
     if (currentKey && userUuids[currentKey]) {
       // Check if target username already exists (would be a different identity)
@@ -258,6 +282,9 @@ function saveUsername(username) {
         const uuid = userUuids[currentKey];
         delete userUuids[currentKey];
         userUuids[newName] = uuid;
+        // Same in UUID store
+        if (currentStoreKey) delete uuidStore[currentStoreKey];
+        uuidStore[newName] = uuid;
         console.log(`[Config] Renamed identity: "${currentKey}" → "${newName}" (UUID preserved: ${uuid})`);
       }
     }
@@ -270,11 +297,20 @@ function saveUsername(username) {
       const uuid = userUuids[currentKey];
       delete userUuids[currentKey];
       userUuids[newName] = uuid;
+      // Same in UUID store
+      const storeKey = Object.keys(uuidStore).find(k => k.toLowerCase() === currentName.toLowerCase());
+      if (storeKey) {
+        delete uuidStore[storeKey];
+        uuidStore[newName] = uuid;
+      }
       console.log(`[Config] Updated username case: "${currentKey}" → "${newName}"`);
     }
   }
 
-  // Save both username and updated userUuids
+  // Save UUID store
+  saveUuidStore(uuidStore);
+
+  // Save both username and updated userUuids to config
   saveConfig({ username: newName, userUuids });
   console.log(`[Config] Username saved: "${newName}"`);
   return newName;
@@ -310,6 +346,7 @@ function hasUsername() {
 
 // =============================================================================
 // UUID MANAGEMENT - Persistent and safe
+// Uses separate uuid-store.json as source of truth (survives config.json corruption)
 // =============================================================================
 
 /**
@@ -320,10 +357,55 @@ function normalizeUsername(username) {
   return username.trim().toLowerCase();
 }
 
+/**
+ * Load UUID store from separate file (independent of config.json)
+ */
+function loadUuidStore() {
+  try {
+    if (fs.existsSync(UUID_STORE_FILE)) {
+      const data = fs.readFileSync(UUID_STORE_FILE, 'utf8');
+      if (data.trim()) {
+        return JSON.parse(data);
+      }
+    }
+  } catch (err) {
+    console.error('[UUID Store] Failed to load:', err.message);
+  }
+  return {};
+}
+
+/**
+ * Save UUID store to separate file (atomic write)
+ */
+function saveUuidStore(store) {
+  try {
+    const dir = path.dirname(UUID_STORE_FILE);
+    if (!fs.existsSync(dir)) fs.mkdirSync(dir, { recursive: true });
+    const tmpFile = UUID_STORE_FILE + '.tmp';
+    fs.writeFileSync(tmpFile, JSON.stringify(store, null, 2), 'utf8');
+    fs.renameSync(tmpFile, UUID_STORE_FILE);
+  } catch (err) {
+    console.error('[UUID Store] Failed to save:', err.message);
+  }
+}
+
+/**
+ * One-time migration: copy userUuids from config.json to uuid-store.json
+ */
+function migrateUuidStoreIfNeeded() {
+  if (fs.existsSync(UUID_STORE_FILE)) return; // Already migrated
+  const config = loadConfig();
+  if (config.userUuids && Object.keys(config.userUuids).length > 0) {
+    console.log('[UUID Store] Migrating', Object.keys(config.userUuids).length, 'UUIDs from config.json');
+    saveUuidStore(config.userUuids);
+  }
+}
+
 /**
  * Get UUID for a username
- * Creates new UUID only if user explicitly doesn't exist
- * Uses case-insensitive lookup to prevent duplicates, but preserves original case for display
+ * Source of truth: uuid-store.json (separate from config.json)
+ * Also writes to config.json for backward compatibility
+ * Creates new UUID only if user doesn't exist in EITHER store
  */
 function getUuidForUser(username) {
   const { v4: uuidv4 } = require('uuid');
@@ -335,32 +417,69 @@ function getUuidForUser(username) {
   const displayName = username.trim();
   const normalizedLookup = displayName.toLowerCase();
 
-  const config = loadConfig();
-  const userUuids = config.userUuids || {};
+  // Ensure UUID store exists (one-time migration from config.json)
+  migrateUuidStoreIfNeeded();
 
-  // Case-insensitive lookup - find existing key regardless of case
-  const existingKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
+  // 1. Check UUID store first (source of truth)
+  const uuidStore = loadUuidStore();
+  const storeKey = Object.keys(uuidStore).find(k => k.toLowerCase() === normalizedLookup);
 
-  if (existingKey) {
-    // Found existing - return UUID, update display name if case changed
-    const existingUuid = userUuids[existingKey];
+  if (storeKey) {
+    const existingUuid = uuidStore[storeKey];
 
-    // If user typed different case, update the key to new case (preserving UUID)
-    if (existingKey !== displayName) {
-      console.log(`[Config] Updating username case: "${existingKey}" → "${displayName}"`);
-      delete userUuids[existingKey];
-      userUuids[displayName] = existingUuid;
-      saveConfig({ userUuids });
+    // Update case if needed
+    if (storeKey !== displayName) {
+      console.log(`[UUID Store] Updating username case: "${storeKey}" → "${displayName}"`);
+      delete uuidStore[storeKey];
+      uuidStore[displayName] = existingUuid;
+      saveUuidStore(uuidStore);
     }
 
+    // Sync to config.json (backward compat, non-critical)
+    try {
+      const config = loadConfig();
+      const configUuids = config.userUuids || {};
+      const configKey = Object.keys(configUuids).find(k => k.toLowerCase() === normalizedLookup);
+      if (!configKey || configUuids[configKey] !== existingUuid) {
+        if (configKey) delete configUuids[configKey];
+        configUuids[displayName] = existingUuid;
+        saveConfig({ userUuids: configUuids });
+      }
+    } catch (e) {
+      // Non-critical — UUID store is the source of truth
+    }
+
+    console.log(`[UUID] ${displayName} → ${existingUuid} (from uuid-store)`);
     return existingUuid;
   }
 
-  // Create new UUID for new user - store with original case
+  // 2. Fallback: check config.json (recovery if uuid-store.json was lost)
+  const config = loadConfig();
+  const userUuids = config.userUuids || {};
+  const configKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
+
+  if (configKey) {
+    const recoveredUuid = userUuids[configKey];
+    console.warn(`[UUID] RECOVERED "${displayName}" → ${recoveredUuid} from config.json (uuid-store was missing)`);
+
+    // Save to UUID store
+    uuidStore[displayName] = recoveredUuid;
+    saveUuidStore(uuidStore);
+
+    return recoveredUuid;
+  }
+
+  // 3. New user — generate UUID, save to BOTH stores
   const newUuid = uuidv4();
+  console.log(`[UUID] NEW user "${displayName}" → ${newUuid}`);
+
+  // Save to UUID store (source of truth)
+  uuidStore[displayName] = newUuid;
+  saveUuidStore(uuidStore);
+
+  // Save to config.json (backward compat)
   userUuids[displayName] = newUuid;
   saveConfig({ userUuids });
-  console.log(`[Config] Created new UUID for "${displayName}": ${newUuid}`);
 
   return newUuid;
 }
@@ -380,22 +499,26 @@ function getCurrentUuid() {
  * Get all UUID mappings (raw object)
  */
 function getAllUuidMappings() {
-  const config = loadConfig();
-  return config.userUuids || {};
+  migrateUuidStoreIfNeeded();
+  const uuidStore = loadUuidStore();
+  // Fallback to config if uuid-store is empty
+  if (Object.keys(uuidStore).length === 0) {
+    const config = loadConfig();
+    return config.userUuids || {};
+  }
+  return uuidStore;
 }
 
 /**
  * Get all UUID mappings as array with current user flag
  */
 function getAllUuidMappingsArray() {
-  const config = loadConfig();
-  const userUuids = config.userUuids || {};
+  const allMappings = getAllUuidMappings();
   const currentUsername = loadUsername();
-  // Case-insensitive comparison for isCurrent
   const normalizedCurrent = currentUsername ? currentUsername.toLowerCase() : null;
 
-  return Object.entries(userUuids).map(([username, uuid]) => ({
-    username,  // Original case preserved
+  return Object.entries(allMappings).map(([username, uuid]) => ({
+    username,
     uuid,
     isCurrent: username.toLowerCase() === normalizedCurrent
   }));
@@ -419,16 +542,20 @@ function setUuidForUser(username, uuid) {
 
   const displayName = username.trim();
   const normalizedLookup = displayName.toLowerCase();
+
+  // 1. Update UUID store (source of truth)
+  migrateUuidStoreIfNeeded();
+  const uuidStore = loadUuidStore();
+  const storeKey = Object.keys(uuidStore).find(k => k.toLowerCase() === normalizedLookup);
+  if (storeKey) delete uuidStore[storeKey];
+  uuidStore[displayName] = uuid;
+  saveUuidStore(uuidStore);
+
+  // 2. Update config.json (backward compat)
   const config = loadConfig();
   const userUuids = config.userUuids || {};
-
-  // Remove any existing entry with same name (case-insensitive)
   const existingKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
-  if (existingKey) {
-    delete userUuids[existingKey];
-  }
-
-  // Store with original case
+  if (existingKey) delete userUuids[existingKey];
   userUuids[displayName] = uuid;
   saveConfig({ userUuids });
 
@@ -454,20 +581,30 @@ function deleteUuidForUser(username) {
   }
 
   const normalizedLookup = username.trim().toLowerCase();
+  let deleted = false;
+
+  // 1. Delete from UUID store (source of truth)
+  migrateUuidStoreIfNeeded();
+  const uuidStore = loadUuidStore();
+  const storeKey = Object.keys(uuidStore).find(k => k.toLowerCase() === normalizedLookup);
+  if (storeKey) {
+    delete uuidStore[storeKey];
+    saveUuidStore(uuidStore);
+    deleted = true;
+  }
+
+  // 2. Delete from config.json (backward compat)
   const config = loadConfig();
   const userUuids = config.userUuids || {};
-
-  // Case-insensitive lookup
   const existingKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
-
   if (existingKey) {
     delete userUuids[existingKey];
     saveConfig({ userUuids });
-    console.log(`[Config] UUID deleted for "${username}"`);
-    return true;
+    deleted = true;
   }
 
-  return false;
+  if (deleted) console.log(`[Config] UUID deleted for "${username}"`);
+  return deleted;
 }
 
 /**
@@ -788,5 +925,6 @@ module.exports = {
   loadVersionBranch,
 
   // Constants
-  CONFIG_FILE
+  CONFIG_FILE,
+  UUID_STORE_FILE
 };
diff --git a/backend/managers/gameLauncher.js b/backend/managers/gameLauncher.js
index 4e66906..832357e 100644
--- a/backend/managers/gameLauncher.js
+++ b/backend/managers/gameLauncher.js
@@ -250,6 +250,7 @@ async function launchGame(playerNameOverride = null, progressCallback, javaPathO
   }
 
   const uuid = getUuidForUser(playerName);
+  console.log(`[Launcher] UUID for "${playerName}": ${uuid} (verify this stays constant across launches)`);
 
   // Fetch tokens from auth server
   if (progressCallback) {
diff --git a/package.json b/package.json
index a833176..606cdaa 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "hytale-f2p-launcher",
-  "version": "2.3.5",
+  "version": "2.3.6",
   "description": "A modern, cross-platform launcher for Hytale with automatic updates and multi-client support",
   "homepage": "https://git.sanhost.net/sanasol/hytale-f2p",
   "main": "main.js",
diff --git a/test-uuid-persistence.js b/test-uuid-persistence.js
new file mode 100644
index 0000000..33fa7f0
--- /dev/null
+++ b/test-uuid-persistence.js
@@ -0,0 +1,523 @@
+#!/usr/bin/env node
+/**
+ * UUID Persistence Tests
+ *
+ * Simulates the exact conditions that caused character data loss:
+ * - Config file corruption during updates
+ * - File locks making config temporarily unreadable
+ * - Username re-entry after config wipe
+ *
+ * Run: node test-uuid-persistence.js
+ */
+
+const fs = require('fs');
+const path = require('path');
+const os = require('os');
+
+// Use a temp directory so we don't mess with real config
+const TEST_DIR = path.join(os.tmpdir(), 'hytale-uuid-test-' + Date.now());
+const CONFIG_FILE = path.join(TEST_DIR, 'config.json');
+const CONFIG_BACKUP = path.join(TEST_DIR, 'config.json.bak');
+const CONFIG_TEMP = path.join(TEST_DIR, 'config.json.tmp');
+const UUID_STORE_FILE = path.join(TEST_DIR, 'uuid-store.json');
+
+// Track test results
+let passed = 0;
+let failed = 0;
+const failures = [];
+
+function assert(condition, message) {
+  if (condition) {
+    passed++;
+    console.log(`  ✓ ${message}`);
+  } else {
+    failed++;
+    failures.push(message);
+    console.log(`  ✗ FAIL: ${message}`);
+  }
+}
+
+function assertEqual(actual, expected, message) {
+  if (actual === expected) {
+    passed++;
+    console.log(`  ✓ ${message}`);
+  } else {
+    failed++;
+    failures.push(`${message} (expected: ${expected}, got: ${actual})`);
+    console.log(`  ✗ FAIL: ${message} (expected: "${expected}", got: "${actual}")`);
+  }
+}
+
+function cleanup() {
+  try {
+    if (fs.existsSync(TEST_DIR)) {
+      fs.rmSync(TEST_DIR, { recursive: true });
+    }
+  } catch (e) {}
+}
+
+function setup() {
+  cleanup();
+  fs.mkdirSync(TEST_DIR, { recursive: true });
+}
+
+// ============================================================================
+// Inline the config functions so we can override paths
+// (We can't require config.js directly because it uses hardcoded getAppDir())
+// ============================================================================
+
+function validateConfig(config) {
+  if (!config || typeof config !== 'object') return false;
+  if (config.userUuids !== undefined && typeof config.userUuids !== 'object') return false;
+  if (config.username !== undefined && (typeof config.username !== 'string')) return false;
+  return true;
+}
+
+function loadConfig() {
+  try {
+    if (fs.existsSync(CONFIG_FILE)) {
+      const data = fs.readFileSync(CONFIG_FILE, 'utf8');
+      if (data.trim()) {
+        const config = JSON.parse(data);
+        if (validateConfig(config)) return config;
+        console.warn('[Config] Primary config invalid structure, trying backup...');
+      }
+    }
+  } catch (err) {
+    console.error('[Config] Failed to load primary config:', err.message);
+  }
+
+  try {
+    if (fs.existsSync(CONFIG_BACKUP)) {
+      const data = fs.readFileSync(CONFIG_BACKUP, 'utf8');
+      if (data.trim()) {
+        const config = JSON.parse(data);
+        if (validateConfig(config)) {
+          console.log('[Config] Recovered from backup successfully');
+          try { fs.writeFileSync(CONFIG_FILE, data, 'utf8'); } catch (e) {}
+          return config;
+        }
+      }
+    }
+  } catch (err) {}
+
+  return {};
+}
+
+function saveConfig(update) {
+  const maxRetries = 3;
+  for (let attempt = 1; attempt <= maxRetries; attempt++) {
+    try {
+      if (!fs.existsSync(TEST_DIR)) fs.mkdirSync(TEST_DIR, { recursive: true });
+
+      const currentConfig = loadConfig();
+
+      // SAFETY CHECK: refuse to save if file exists but loaded empty
+      if (Object.keys(currentConfig).length === 0 && fs.existsSync(CONFIG_FILE)) {
+        const fileSize = fs.statSync(CONFIG_FILE).size;
+        if (fileSize > 2) {
+          console.error(`[Config] REFUSING to save — loaded empty but file exists (${fileSize} bytes). Retrying...`);
+          const delay = attempt * 50; // shorter delay for tests
+          const start = Date.now();
+          while (Date.now() - start < delay) {}
+          continue;
+        }
+      }
+
+      const newConfig = { ...currentConfig, ...update };
+      const data = JSON.stringify(newConfig, null, 2);
+
+      fs.writeFileSync(CONFIG_TEMP, data, 'utf8');
+      const verification = JSON.parse(fs.readFileSync(CONFIG_TEMP, 'utf8'));
+      if (!validateConfig(verification)) throw new Error('Validation failed');
+
+      if (fs.existsSync(CONFIG_FILE)) {
+        try {
+          const currentData = fs.readFileSync(CONFIG_FILE, 'utf8');
+          if (currentData.trim()) fs.writeFileSync(CONFIG_BACKUP, currentData, 'utf8');
+        } catch (e) {}
+      }
+
+      fs.renameSync(CONFIG_TEMP, CONFIG_FILE);
+      return true;
+    } catch (err) {
+      try { if (fs.existsSync(CONFIG_TEMP)) fs.unlinkSync(CONFIG_TEMP); } catch (e) {}
+      if (attempt >= maxRetries) throw err;
+    }
+  }
+}
+
+function loadUuidStore() {
+  try {
+    if (fs.existsSync(UUID_STORE_FILE)) {
+      const data = fs.readFileSync(UUID_STORE_FILE, 'utf8');
+      if (data.trim()) return JSON.parse(data);
+    }
+  } catch (err) {}
+  return {};
+}
+
+function saveUuidStore(store) {
+  const tmpFile = UUID_STORE_FILE + '.tmp';
+  fs.writeFileSync(tmpFile, JSON.stringify(store, null, 2), 'utf8');
+  fs.renameSync(tmpFile, UUID_STORE_FILE);
+}
+
+function migrateUuidStoreIfNeeded() {
+  if (fs.existsSync(UUID_STORE_FILE)) return;
+  const config = loadConfig();
+  if (config.userUuids && Object.keys(config.userUuids).length > 0) {
+    console.log('[UUID Store] Migrating', Object.keys(config.userUuids).length, 'UUIDs');
+    saveUuidStore(config.userUuids);
+  }
+}
+
+function getUuidForUser(username) {
+  const { v4: uuidv4 } = require('uuid');
+  if (!username || !username.trim()) throw new Error('Username required');
+
+  const displayName = username.trim();
+  const normalizedLookup = displayName.toLowerCase();
+
+  migrateUuidStoreIfNeeded();
+
+  // 1. Check UUID store (source of truth)
+  const uuidStore = loadUuidStore();
+  const storeKey = Object.keys(uuidStore).find(k => k.toLowerCase() === normalizedLookup);
+  if (storeKey) {
+    const existingUuid = uuidStore[storeKey];
+    if (storeKey !== displayName) {
+      delete uuidStore[storeKey];
+      uuidStore[displayName] = existingUuid;
+      saveUuidStore(uuidStore);
+    }
+    // Sync to config (non-critical)
+    try {
+      const config = loadConfig();
+      const configUuids = config.userUuids || {};
+      const configKey = Object.keys(configUuids).find(k => k.toLowerCase() === normalizedLookup);
+      if (!configKey || configUuids[configKey] !== existingUuid) {
+        if (configKey) delete configUuids[configKey];
+        configUuids[displayName] = existingUuid;
+        saveConfig({ userUuids: configUuids });
+      }
+    } catch (e) {}
+    return existingUuid;
+  }
+
+  // 2. Fallback: check config.json
+  const config = loadConfig();
+  const userUuids = config.userUuids || {};
+  const configKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
+  if (configKey) {
+    const recoveredUuid = userUuids[configKey];
+    uuidStore[displayName] = recoveredUuid;
+    saveUuidStore(uuidStore);
+    return recoveredUuid;
+  }
+
+  // 3. New user — generate UUID
+  const newUuid = uuidv4();
+  uuidStore[displayName] = newUuid;
+  saveUuidStore(uuidStore);
+  userUuids[displayName] = newUuid;
+  saveConfig({ userUuids });
+  return newUuid;
+}
+
+// ============================================================================
+// OLD CODE (before fix) — for comparison testing
+// ============================================================================
+
+function getUuidForUser_OLD(username) {
+  const { v4: uuidv4 } = require('uuid');
+  if (!username || !username.trim()) throw new Error('Username required');
+  const displayName = username.trim();
+  const normalizedLookup = displayName.toLowerCase();
+
+  const config = loadConfig();
+  const userUuids = config.userUuids || {};
+  const existingKey = Object.keys(userUuids).find(k => k.toLowerCase() === normalizedLookup);
+
+  if (existingKey) {
+    return userUuids[existingKey];
+  }
+
+  // New user
+  const newUuid = uuidv4();
+  userUuids[displayName] = newUuid;
+  saveConfig({ userUuids });
+  return newUuid;
+}
+
+function saveConfig_OLD(update) {
+  // OLD saveConfig without safety check
+  if (!fs.existsSync(TEST_DIR)) fs.mkdirSync(TEST_DIR, { recursive: true });
+  const currentConfig = loadConfig();
+  // NO SAFETY CHECK — this is the bug
+  const newConfig = { ...currentConfig, ...update };
+  fs.writeFileSync(CONFIG_FILE, JSON.stringify(newConfig, null, 2), 'utf8');
+  return true;
+}
+
+// ============================================================================
+// TESTS
+// ============================================================================
+
+console.log('\n' + '='.repeat(70));
+console.log('UUID PERSISTENCE TESTS — Simulating update corruption scenarios');
+console.log('='.repeat(70));
+
+// --------------------------------------------------------------------------
+// TEST 1: Normal flow — UUID stays consistent
+// --------------------------------------------------------------------------
+console.log('\n--- Test 1: Normal flow — UUID stays consistent ---');
+setup();
+
+const uuid1 = getUuidForUser('SpecialK');
+const uuid2 = getUuidForUser('SpecialK');
+const uuid3 = getUuidForUser('specialk'); // case insensitive
+
+assertEqual(uuid1, uuid2, 'Same username returns same UUID');
+assertEqual(uuid1, uuid3, 'Case-insensitive lookup returns same UUID');
+assert(uuid1.match(/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i), 'UUID is valid v4 format');
+
+// --------------------------------------------------------------------------
+// TEST 2: Simulate update corruption (THE BUG) — old code
+// --------------------------------------------------------------------------
+console.log('\n--- Test 2: OLD CODE — Config wipe during update loses UUID ---');
+setup();
+
+// Setup: player has UUID
+const oldConfig = { username: 'SpecialK', userUuids: { 'SpecialK': 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee' }, hasLaunchedBefore: true };
+fs.writeFileSync(CONFIG_FILE, JSON.stringify(oldConfig, null, 2), 'utf8');
+
+const uuidBefore = getUuidForUser_OLD('SpecialK');
+assertEqual(uuidBefore, 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee', 'UUID correct before corruption');
+
+// Simulate: config.json gets corrupted (loadConfig returns {} because file locked)
+// This simulates what happens when saveConfig reads an empty/locked file
+fs.writeFileSync(CONFIG_FILE, '', 'utf8'); // Simulate corruption: empty file
+
+// Old saveConfig behavior: reads empty, merges with update, saves
+// This wipes userUuids
+saveConfig_OLD({ hasLaunchedBefore: true });
+
+const configAfterCorruption = JSON.parse(fs.readFileSync(CONFIG_FILE, 'utf8'));
+assert(!configAfterCorruption.userUuids, 'OLD CODE: userUuids wiped after corruption');
+assert(!configAfterCorruption.username, 'OLD CODE: username wiped after corruption');
+
+// Player re-enters name, gets NEW UUID (character data lost!)
+const uuidAfterOld = getUuidForUser_OLD('SpecialK');
+assert(uuidAfterOld !== uuidBefore, 'OLD CODE: UUID changed after corruption (BUG!)');
+
+// --------------------------------------------------------------------------
+// TEST 3: NEW CODE — Config wipe during update, UUID survives via uuid-store
+// --------------------------------------------------------------------------
+console.log('\n--- Test 3: NEW CODE — Config wipe + UUID survives via uuid-store ---');
+setup();
+
+// Setup: player has UUID (stored in both config.json AND uuid-store.json)
+const initialConfig = { username: 'SpecialK', userUuids: { 'SpecialK': 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee' }, hasLaunchedBefore: true };
+fs.writeFileSync(CONFIG_FILE, JSON.stringify(initialConfig, null, 2), 'utf8');
+
+// First call migrates to uuid-store
+const uuidFirst = getUuidForUser('SpecialK');
+assertEqual(uuidFirst, 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee', 'UUID correct before corruption');
+assert(fs.existsSync(UUID_STORE_FILE), 'uuid-store.json created');
+
+// Simulate: config.json gets wiped (same as the update bug)
+fs.writeFileSync(CONFIG_FILE, '{}', 'utf8');
+
+// Verify config is empty
+const wipedConfig = loadConfig();
+assert(!wipedConfig.userUuids || Object.keys(wipedConfig.userUuids).length === 0, 'Config wiped — no userUuids');
+assert(!wipedConfig.username, 'Config wiped — no username');
+
+// Player re-enters same name → UUID recovered from uuid-store!
+const uuidAfterNew = getUuidForUser('SpecialK');
+assertEqual(uuidAfterNew, 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee', 'NEW CODE: UUID preserved after config wipe!');
+
+// --------------------------------------------------------------------------
+// TEST 4: saveConfig safety check — refuses to overwrite good data with empty
+// --------------------------------------------------------------------------
+console.log('\n--- Test 4: saveConfig safety check — blocks destructive writes ---');
+setup();
+
+// Setup: valid config file with data
+const goodConfig = { username: 'SpecialK', userUuids: { 'SpecialK': 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee' }, hasLaunchedBefore: true, installPath: 'C:\\Games\\Hytale' };
+fs.writeFileSync(CONFIG_FILE, JSON.stringify(goodConfig, null, 2), 'utf8');
+
+// Make the file temporarily unreadable by writing garbage (simulates file lock/corruption)
+const originalContent = fs.readFileSync(CONFIG_FILE, 'utf8');
+fs.writeFileSync(CONFIG_FILE, 'NOT VALID JSON!!!', 'utf8');
+
+// Try to save — should refuse because file exists but can't be parsed
+let saveThrew = false;
+try {
+  saveConfig({ someNewField: true });
+} catch (e) {
+  saveThrew = true;
+}
+
+// The file should still have the garbage (not overwritten with { someNewField: true })
+const afterContent = fs.readFileSync(CONFIG_FILE, 'utf8');
+
+// Restore original for backup recovery test
+fs.writeFileSync(CONFIG_FILE, JSON.stringify(goodConfig, null, 2), 'utf8');
+
+// Note: with invalid JSON, loadConfig returns {} and safety check triggers
+// The save may eventually succeed on retry if the file becomes readable
+// What matters is that it doesn't blindly overwrite
+assert(afterContent !== '{\n  "someNewField": true\n}', 'Safety check prevented blind overwrite of corrupted file');
+
+// --------------------------------------------------------------------------
+// TEST 5: Backup recovery — config.json corrupted, recovered from .bak
+// --------------------------------------------------------------------------
+console.log('\n--- Test 5: Backup recovery — auto-recover from .bak ---');
+setup();
+
+// Create config and backup
+const validConfig = { username: 'SpecialK', userUuids: { 'SpecialK': 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee' } };
+fs.writeFileSync(CONFIG_BACKUP, JSON.stringify(validConfig, null, 2), 'utf8');
+fs.writeFileSync(CONFIG_FILE, 'CORRUPTED', 'utf8');
+
+const recovered = loadConfig();
+assertEqual(recovered.username, 'SpecialK', 'Username recovered from backup');
+assert(recovered.userUuids && recovered.userUuids['SpecialK'] === 'aaaaaaaa-bbbb-4ccc-9ddd-eeeeeeeeeeee', 'UUID recovered from backup');
+
+// --------------------------------------------------------------------------
+// TEST 6: Full update simulation — the exact scenario from player report
+// --------------------------------------------------------------------------
+console.log('\n--- Test 6: Full update simulation (player report scenario) ---');
+setup();
+
+// Step 1: Player installs v2.3.4, sets username, plays game
+console.log('  Step 1: Player sets up profile...');
+saveConfig({ username: 'Special K', hasLaunchedBefore: true });
+const originalUuid = getUuidForUser('Special K');
+console.log(`  Original UUID: ${originalUuid}`);
+
+// Step 2: v2.3.5 auto-update — new app launches
+console.log('  Step 2: Simulating v2.3.5 update...');
+
+// Simulate the 3 saveConfig calls that happen during startup
+// But first, simulate config being temporarily locked (returns empty)
+const preUpdateContent = fs.readFileSync(CONFIG_FILE, 'utf8');
+fs.writeFileSync(CONFIG_FILE, '', 'utf8'); // Simulate: file empty during write (race condition)
+
+// These are the 3 calls from: profileManager.init, migrateUserDataToCentralized, handleFirstLaunchCheck
+// With our safety check, they should NOT wipe the data
+try { saveConfig({ hasLaunchedBefore: true }); } catch (e) { /* expected — safety check blocks it */ }
+
+// Simulate file becomes readable again (antivirus releases lock)
+fs.writeFileSync(CONFIG_FILE, preUpdateContent, 'utf8');
+
+// Step 3: Player re-enters username (because UI might show empty)
+console.log('  Step 3: Player re-enters username...');
+const postUpdateUuid = getUuidForUser('Special K');
+console.log(`  Post-update UUID: ${postUpdateUuid}`);
+
+assertEqual(postUpdateUuid, originalUuid, 'UUID survived the full update cycle!');
+
+// --------------------------------------------------------------------------
+// TEST 7: Multiple users — UUIDs stay independent
+// --------------------------------------------------------------------------
+console.log('\n--- Test 7: Multiple users — UUIDs stay independent ---');
+setup();
+
+const uuidAlice = getUuidForUser('Alice');
+const uuidBob = getUuidForUser('Bob');
+const uuidCharlie = getUuidForUser('Charlie');
+
+assert(uuidAlice !== uuidBob, 'Alice and Bob have different UUIDs');
+assert(uuidBob !== uuidCharlie, 'Bob and Charlie have different UUIDs');
+
+// Wipe config, all should survive
+fs.writeFileSync(CONFIG_FILE, '{}', 'utf8');
+
+assertEqual(getUuidForUser('Alice'), uuidAlice, 'Alice UUID survived config wipe');
+assertEqual(getUuidForUser('Bob'), uuidBob, 'Bob UUID survived config wipe');
+assertEqual(getUuidForUser('Charlie'), uuidCharlie, 'Charlie UUID survived config wipe');
+
+// --------------------------------------------------------------------------
+// TEST 8: UUID store deleted — recovery from config.json
+// --------------------------------------------------------------------------
+console.log('\n--- Test 8: UUID store deleted — recovery from config.json ---');
+setup();
+
+// Create UUID via normal flow (saves to both stores)
+const uuidOriginal = getUuidForUser('TestPlayer');
+
+// Delete uuid-store.json (simulates user manually deleting it or disk issue)
+fs.unlinkSync(UUID_STORE_FILE);
+assert(!fs.existsSync(UUID_STORE_FILE), 'uuid-store.json deleted');
+
+// UUID should be recovered from config.json
+const uuidRecovered = getUuidForUser('TestPlayer');
+assertEqual(uuidRecovered, uuidOriginal, 'UUID recovered from config.json after uuid-store deletion');
+assert(fs.existsSync(UUID_STORE_FILE), 'uuid-store.json recreated after recovery');
+
+// --------------------------------------------------------------------------
+// TEST 9: Both stores deleted — new UUID generated (fresh install)
+// --------------------------------------------------------------------------
+console.log('\n--- Test 9: Both stores deleted — new UUID (fresh install) ---');
+setup();
+
+const uuidFresh = getUuidForUser('NewPlayer');
+
+// Delete both
+fs.unlinkSync(UUID_STORE_FILE);
+fs.unlinkSync(CONFIG_FILE);
+
+const uuidAfterWipe = getUuidForUser('NewPlayer');
+assert(uuidAfterWipe !== uuidFresh, 'New UUID generated when both stores are gone (expected for true fresh install)');
+
+// --------------------------------------------------------------------------
+// TEST 10: Worst case — config.json wiped AND uuid-store.json exists
+// Simulates the EXACT player-reported scenario with new code
+// --------------------------------------------------------------------------
+console.log('\n--- Test 10: Exact player scenario with new code ---');
+setup();
+
+// Player has been playing for a while
+saveConfig({
+  username: 'Special K',
+  hasLaunchedBefore: true,
+  installPath: 'C:\\Games\\Hytale',
+  version_client: '2026.02.19-1a311a592',
+  version_branch: 'release',
+  userUuids: { 'Special K': '11111111-2222-4333-9444-555555555555' }
+});
+
+// First call creates uuid-store.json
+const originalUuid10 = getUuidForUser('Special K');
+assertEqual(originalUuid10, '11111111-2222-4333-9444-555555555555', 'Original UUID loaded');
+
+// BOOM: Update happens, config.json completely wiped
+fs.writeFileSync(CONFIG_FILE, '{}', 'utf8');
+
+// Username lost — player has to re-enter
+const loadedUsername = loadConfig().username;
+assert(!loadedUsername, 'Username is gone from config (simulating what player saw)');
+
+// Player types "Special K" again in settings
+saveConfig({ username: 'Special K' });
+
+// Player clicks Play — getUuidForUser called
+const recoveredUuid10 = getUuidForUser('Special K');
+assertEqual(recoveredUuid10, '11111111-2222-4333-9444-555555555555', 'UUID recovered — character data preserved!');
+
+// ============================================================================
+// RESULTS
+// ============================================================================
+console.log('\n' + '='.repeat(70));
+console.log(`RESULTS: ${passed} passed, ${failed} failed`);
+if (failed > 0) {
+  console.log('\nFailures:');
+  failures.forEach(f => console.log(`  ✗ ${f}`));
+}
+console.log('='.repeat(70));
+
+cleanup();
+process.exit(failed > 0 ? 1 : 0);