diff --git a/src/backfill/sparse.rs b/src/backfill/sparse.rs
index 8189133..a71bd4e 100644
--- a/src/backfill/sparse.rs
+++ b/src/backfill/sparse.rs
@@ -439,6 +439,12 @@ async fn persist_sparse_backfill(
         let mut collection_counts: HashMap<SmolStr, u64> = HashMap::new();
 
         let mut txn = DbTxn::new(db);
+        // permanent mode locks before the existing-head snapshot so a live
+        // write or operator redaction cannot invalidate the diff. ephemeral
+        // mode has no heads and therefore no shared record state to lock.
+        if !app_state.ephemeral {
+            txn.hold_repo_write_lock(&did);
+        }
 
         let prefix = keys::record_prefix_did(&did);
         let mut existing_cids: HashMap<(SmolStr, DbRkey), SmolStr> = HashMap::new();