fix: add file mtime-based cache invalidation for memory data

Implement automatic cache invalidation based on file modification time to ensure memory data consistency across Gateway API and agent prompts. The cache now automatically reloads when the memory file is updated externally.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/agents/memory/updater.py

@@ -43,28 +43,54 @@ def _create_empty_memory() -> dict[str, Any]:
 
 # Global memory data cache
 _memory_data: dict[str, Any] | None = None
+# Track file modification time for cache invalidation
+_memory_file_mtime: float | None = None
 
 
 def get_memory_data() -> dict[str, Any]:
-    """Get the current memory data (cached singleton).
+    """Get the current memory data (cached with file modification time check).
+
+    The cache is automatically invalidated if the memory file has been modified
+    since the last load, ensuring fresh data is always returned.
 
     Returns:
         The memory data dictionary.
     """
-    global _memory_data
-    if _memory_data is None:
+    global _memory_data, _memory_file_mtime
+
+    file_path = _get_memory_file_path()
+
+    # Get current file modification time
+    try:
+        current_mtime = file_path.stat().st_mtime if file_path.exists() else None
+    except OSError:
+        current_mtime = None
+
+    # Invalidate cache if file has been modified or doesn't exist
+    if _memory_data is None or _memory_file_mtime != current_mtime:
         _memory_data = _load_memory_from_file()
+        _memory_file_mtime = current_mtime
+
     return _memory_data
 
 
 def reload_memory_data() -> dict[str, Any]:
-    """Reload memory data from file.
+    """Reload memory data from file, forcing cache invalidation.
 
     Returns:
         The reloaded memory data dictionary.
     """
-    global _memory_data
+    global _memory_data, _memory_file_mtime
+
+    file_path = _get_memory_file_path()
     _memory_data = _load_memory_from_file()
+
+    # Update file modification time after reload
+    try:
+        _memory_file_mtime = file_path.stat().st_mtime if file_path.exists() else None
+    except OSError:
+        _memory_file_mtime = None
+
     return _memory_data
 
 
@@ -89,7 +115,7 @@ def _load_memory_from_file() -> dict[str, Any]:
 
 
 def _save_memory_to_file(memory_data: dict[str, Any]) -> bool:
-    """Save memory data to file.
+    """Save memory data to file and update cache.
 
     Args:
         memory_data: The memory data to save.
@@ -97,7 +123,7 @@ def _save_memory_to_file(memory_data: dict[str, Any]) -> bool:
     Returns:
         True if successful, False otherwise.
     """
-    global _memory_data
+    global _memory_data, _memory_file_mtime
     file_path = _get_memory_file_path()
 
     try:
@@ -115,8 +141,12 @@ def _save_memory_to_file(memory_data: dict[str, Any]) -> bool:
         # Rename temp file to actual file (atomic on most systems)
         temp_path.replace(file_path)
 
-        # Update cache
+        # Update cache and file modification time
         _memory_data = memory_data
+        try:
+            _memory_file_mtime = file_path.stat().st_mtime
+        except OSError:
+            _memory_file_mtime = None
 
         print(f"Memory saved to {file_path}")
         return True