diff --git a/src/config/__tests__/config-loader-facade.test.ts b/src/config/__tests__/config-loader-facade.test.ts
index 837ce030..1a9f9f8b 100644
--- a/src/config/__tests__/config-loader-facade.test.ts
+++ b/src/config/__tests__/config-loader-facade.test.ts
@@ -208,5 +208,27 @@ describe('config-loader-facade', () => {
       expect(config.profiles).toBeDefined();
       expect(config.cliproxy).toBeDefined();
     });
+
+    it('auto-invalidates when config file is modified externally', async () => {
+      const facade = await importFacade();
+
+      // Prime the cache
+      const first = facade.getCachedConfig();
+      expect(first.version).toBeDefined();
+
+      // Modify config file directly on disk (simulating external code
+      // bypassing the facade)
+      const configPath = path.join(tempHome, '.ccs', 'config.yaml');
+      // Touch the file to update mtime (wait briefly for mtime resolution)
+      const content = fs.readFileSync(configPath, 'utf8');
+      await new Promise((r) => setTimeout(r, 10));
+      fs.writeFileSync(configPath, content + '# touched\n', 'utf8');
+
+      // getCachedConfig should detect the mtime change and re-read
+      const second = facade.getCachedConfig();
+      expect(first).not.toBe(second);
+      // Content should still be valid (re-read from disk)
+      expect(second.version).toBeDefined();
+    });
   });
 });
diff --git a/src/config/config-loader-facade.ts b/src/config/config-loader-facade.ts
index 19188ba4..462efa37 100644
--- a/src/config/config-loader-facade.ts
+++ b/src/config/config-loader-facade.ts
@@ -52,34 +52,42 @@ export type { GeminiWebSearchInfo } from './unified-config-loader';
 export { loadSettings, loadConfigSafe, readConfig, getCcsDir } from '../utils/config-manager';
 
 // Internal imports for memoization wrappers
+import { statSync } from 'fs';
 import type { UnifiedConfig } from './unified-config-types';
 import {
   loadOrCreateUnifiedConfig as _loadOrCreateUnifiedConfig,
   saveUnifiedConfig as _saveUnifiedConfig,
   mutateUnifiedConfig as _mutateUnifiedConfig,
   updateUnifiedConfig as _updateUnifiedConfig,
+  getConfigYamlPath as _getConfigYamlPath,
 } from './unified-config-loader';
 
 // ---------------------------------------------------------------------------
-// Memoization cache
+// Memoization cache with mtime-based staleness detection
 // ---------------------------------------------------------------------------
 
 let _configCache: UnifiedConfig | null = null;
+let _cacheMtimeMs: number = 0;
+
+function getConfigFileMtime(): number {
+  try {
+    return statSync(_getConfigYamlPath()).mtimeMs;
+  } catch {
+    return 0;
+  }
+}
 
 /**
  * Get the unified config with in-memory caching.
- * First call reads from disk; subsequent calls return a deep copy.
- * Returns a copy to prevent callers from silently mutating the cache.
- *
- * NOTE: `loadOrCreateUnifiedConfig` (re-exported above) is NOT cached.
- * Use `getCachedConfig()` for cached reads, or the direct import for uncached.
- *
- * Call invalidateConfigCache() or use mutateConfig()/updateConfig()
- * to force a re-read from disk.
+ * Checks file mtime on each call — if the config file was modified
+ * externally (e.g. by code importing unified-config-loader directly),
+ * the cache is automatically invalidated and re-read from disk.
  */
 export function getCachedConfig(): UnifiedConfig {
-  if (!_configCache) {
+  const currentMtime = getConfigFileMtime();
+  if (!_configCache || currentMtime > _cacheMtimeMs) {
     _configCache = _loadOrCreateUnifiedConfig();
+    _cacheMtimeMs = getConfigFileMtime();
   }
   return structuredClone(_configCache);
 }
@@ -90,6 +98,7 @@ export function getCachedConfig(): UnifiedConfig {
  */
 export function invalidateConfigCache(): void {
   _configCache = null;
+  _cacheMtimeMs = 0;
 }
 
 /**