diff --git a/promisor-remote.h b/promisor-remote.h
index 3d4d2de018..301f5ac5cb 100644
--- a/promisor-remote.h
+++ b/promisor-remote.h
@@ -29,6 +29,17 @@ int repo_has_promisor_remote(struct repository *r);
  * Fetches all requested objects from all promisor remotes, trying them one at
  * a time until all objects are fetched.
  *
+ * Callers are responsible for filtering out OIDs that are already present
+ * locally before calling this function: every supplied OID is sent in the
+ * fetch request, even if the object already exists in the local object
+ * store. (Only after a fetch failure does this function fall back to
+ * stripping already-present OIDs from the list before trying the next
+ * configured promisor remote.) Callers should also deduplicate the OIDs.
+ *
+ * To test for local presence without triggering a lazy fetch (which would
+ * defeat the purpose of batching), use odb_has_object(..., 0) or
+ * odb_read_object_info_extended() with OBJECT_INFO_FOR_PREFETCH.
+ *
  * If oid_nr is 0, this function returns immediately.
  */
 void promisor_remote_get_direct(struct repository *repo,