diff --git a/src/documents/tests/search/test_acceptance.py b/src/documents/tests/search/test_acceptance.py
index efbc890a2..b19e2524a 100644
--- a/src/documents/tests/search/test_acceptance.py
+++ b/src/documents/tests/search/test_acceptance.py
@@ -245,3 +245,19 @@ class TestMultitokenInNestedOr:
         backend.add_or_update(doc_b)
         matched = _matched_ids(backend, 'tag:"multi word tag" OR title:B')
         assert matched == {doc_a.pk, doc_b.pk}
+
+
+class TestUnregisteredIdFieldFoldsToLiteralText:
+    """tag_id, owner_id, etc. are intentionally excluded from the
+    FieldRegistry - whoosh-compat parity leniency folds them into a literal
+    text search rather than raising a diagnostic/400 (see docs/usage.md's
+    advanced-search section). Prove the fold is inert against real data, not
+    just that parsing doesn't raise."""
+
+    def test_tag_id_query_matches_nothing(
+        self,
+        backend: TantivyBackend,
+        indexed_documents: dict[str, int],
+    ) -> None:
+        matched = _matched_ids(backend, "tag_id:5")
+        assert matched == set()
diff --git a/src/documents/tests/search/test_query.py b/src/documents/tests/search/test_query.py
index a1c63bd6e..fcb9c9286 100644
--- a/src/documents/tests/search/test_query.py
+++ b/src/documents/tests/search/test_query.py
@@ -152,6 +152,9 @@ class TestParseUserQuery:
     ) -> None:
         # tag_id is intentionally excluded from the FieldRegistry — whoosh-compat
         # parity leniency folds it into literal text, not a diagnostic/400.
+        # A result-level assertion that this fold actually matches nothing
+        # against real documents lives in
+        # test_acceptance.py::TestUnregisteredIdFieldFoldsToLiteralText.
         q = parse_user_query(query_index, "tag_id:5", UTC)
         assert isinstance(q, tantivy.Query)