diff --git a/openrag/components/indexer/vectordb/utils.py b/openrag/components/indexer/vectordb/utils.py
index 5bc582e0..0909e129 100644
--- a/openrag/components/indexer/vectordb/utils.py
+++ b/openrag/components/indexer/vectordb/utils.py
@@ -669,25 +669,30 @@ def get_existing_file_ids(self, partition: str, file_ids: list[str]) -> set[str]
             )
             return {r[0] for r in result.all()}
 
-    def add_files_to_workspace(self, workspace_id: str, file_ids: list[str]):
+    def add_files_to_workspace(self, workspace_id: str, file_ids: list[str]) -> list[str]:
+        """Add files to a workspace. Returns list of file_ids that could not be resolved."""
         with self.Session() as session:
             # Resolve the workspace's partition to scope the File lookup
             workspace = session.execute(
                 select(Workspace).where(Workspace.workspace_id == workspace_id)
             ).scalar_one_or_none()
             if workspace is None:
-                return
+                return file_ids
             partition = workspace.partition_name
-            for fid in file_ids:
-                file_row = session.execute(
-                    select(File.id).where(File.file_id == fid, File.partition_name == partition)
-                ).scalar_one_or_none()
-                if file_row is None:
-                    continue
-                stmt = pg_insert(WorkspaceFile).values(workspace_id=workspace_id, file_id=file_row)
+
+            # Bulk-resolve all file_ids → File.id in a single query
+            rows = session.execute(
+                select(File.file_id, File.id).where(File.file_id.in_(file_ids), File.partition_name == partition)
+            ).all()
+            id_map = {r[0]: r[1] for r in rows}
+            missing = [fid for fid in file_ids if fid not in id_map]
+
+            for fid, file_pk in id_map.items():
+                stmt = pg_insert(WorkspaceFile).values(workspace_id=workspace_id, file_id=file_pk)
                 stmt = stmt.on_conflict_do_nothing(constraint="uix_workspace_file")
                 session.execute(stmt)
             session.commit()
+            return missing
 
     def remove_file_from_workspace(self, workspace_id: str, file_id: str) -> bool:
         """Remove a file from a workspace. Returns True if the association existed, False otherwise."""
diff --git a/openrag/components/indexer/vectordb/vectordb.py b/openrag/components/indexer/vectordb/vectordb.py
index b060826a..17a3a13b 100644
--- a/openrag/components/indexer/vectordb/vectordb.py
+++ b/openrag/components/indexer/vectordb/vectordb.py
@@ -1127,8 +1127,8 @@ async def get_existing_file_ids(self, partition: str, file_ids: list[str]) -> li
         """Return the subset of file_ids that exist in the given partition."""
         return list(self.partition_file_manager.get_existing_file_ids(partition, file_ids))
 
-    async def add_files_to_workspace(self, workspace_id: str, file_ids: list[str]):
-        self.partition_file_manager.add_files_to_workspace(workspace_id, file_ids)
+    async def add_files_to_workspace(self, workspace_id: str, file_ids: list[str]) -> list[str]:
+        return self.partition_file_manager.add_files_to_workspace(workspace_id, file_ids)
 
     async def remove_file_from_workspace(self, workspace_id: str, file_id: str) -> bool:
         return self.partition_file_manager.remove_file_from_workspace(workspace_id, file_id)
diff --git a/openrag/routers/workspaces.py b/openrag/routers/workspaces.py
index fefc1533..3ed9cfe3 100644
--- a/openrag/routers/workspaces.py
+++ b/openrag/routers/workspaces.py
@@ -170,11 +170,17 @@ async def add_files_to_workspace(
             status_code=status.HTTP_404_NOT_FOUND,
             detail=f"File IDs not found in partition '{partition}': {unknown_ids}",
         )
-    await call_ray_actor_with_timeout(
+    missing = await call_ray_actor_with_timeout(
         vectordb.add_files_to_workspace.remote(workspace_id, body.file_ids),
         timeout=VECTORDB_TIMEOUT,
         task_description=f"add_files_to_workspace({workspace_id})",
     )
+    if missing:
+        # TOCTOU: files were deleted between the pre-check and the insert.
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail=f"File IDs not found in partition '{partition}': {sorted(missing)}",
+        )
     return {"status": "added", "file_ids": body.file_ids}