diff --git a/backend/indexer.py b/backend/indexer.py
index 806d637..d2c4217 100644
--- a/backend/indexer.py
+++ b/backend/indexer.py
@@ -27,6 +27,19 @@ _async_index_lock: asyncio.Lock = None  # initialized lazily
 # (e.g. the inverted index in search.py) can detect staleness.
 _index_generation: int = 0
 
+# Hook for incremental inverted index updates: called as (action, vault, path, file_info)
+_on_index_change: callable = None
+
+
+def set_index_change_hook(hook):
+    """Register a callback for incremental inverted index updates.
+
+    The hook is called as ``hook(action, vault_name, path, file_info)``
+    where ``action`` is ``'add'`` or ``'remove'``.
+    """
+    global _on_index_change
+    _on_index_change = hook
+
 # O(1) lookup table for wikilink resolution: {filename_lower: [{vault, path}, ...]}
 _file_lookup: Dict[str, List[Dict[str, str]]] = {}