diff --git a/biothings/web/query/builder.py b/biothings/web/query/builder.py
index 436c3c6da..f3865d350 100644
--- a/biothings/web/query/builder.py
+++ b/biothings/web/query/builder.py
@@ -356,9 +356,15 @@ def apply_extras(self, search, options):
         # https://www.elastic.co/guide/en/elasticsearch/
         # reference/current/index-modules.html
 
+        # Feature: filter
+        # apply extra filter (as query_string query) to filter results
+        # Ref: https://www.elastic.co/guide/en/elasticsearch/reference/8.10/query-dsl-bool-query.html
+        if options.filter:
+            search = search.filter("query_string", query=options.filter)
+
         # Feature: post_filter
         # -- implementation using query string matching
-        # Ref: https://www.elastic.co/guide/en/elasticsearch/reference/8.1/filter-search-results.html#post-filter
+        # Ref: https://www.elastic.co/guide/en/elasticsearch/reference/8.10/filter-search-results.html#post-filter
         if options.post_filter:
             search = search.post_filter("query_string", query=options["post_filter"])
 
diff --git a/biothings/web/settings/default.py b/biothings/web/settings/default.py
index 398836080..6bc73335c 100644
--- a/biothings/web/settings/default.py
+++ b/biothings/web/settings/default.py
@@ -98,7 +98,7 @@
     "raw": {"type": bool, "default": False},
     "rawquery": {"type": bool, "default": False},
     # query builder stage
-    "_source": {"type": list, "max": 1000, "alias": ("fields", "field", "filter")},
+    "_source": {"type": list, "max": 1000, "alias": ("fields", "field")},
     "size": {"type": int, "max": 1000, "alias": "limit"},
     # formatter stage
     "dotfield": {"type": bool, "default": False},
@@ -124,6 +124,9 @@
         **{
             "from": {"type": int, "max": 10000, "alias": "skip"},
             "sort": {"type": list, "max": 10},
+            # use to set extra filter, as a filter clause in a boolean query
+            "filter": {"type": str, "default": None},
+            # use to set post_filter query, this one does not impact facets
             "post_filter": {"type": str, "default": None},
         },
     },  # for py3.9+, we can just use `|` operator like `COMMON_KWARGS.copy() | {...}`