diff --git a/test/passing/refs.ahrefs/attributes.ml.ref b/test/passing/refs.ahrefs/attributes.ml.ref
index 77573fd1cf..0529fd5486 100644
--- a/test/passing/refs.ahrefs/attributes.ml.ref
+++ b/test/passing/refs.ahrefs/attributes.ml.ref
@@ -584,3 +584,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]
diff --git a/test/passing/refs.default/attributes.ml.ref b/test/passing/refs.default/attributes.ml.ref
index 86eee43e3a..df28db0a04 100644
--- a/test/passing/refs.default/attributes.ml.ref
+++ b/test/passing/refs.default/attributes.ml.ref
@@ -463,3 +463,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]
diff --git a/test/passing/refs.janestreet/attributes.ml.ref b/test/passing/refs.janestreet/attributes.ml.ref
index 206cad90fa..d6b46bebe3 100644
--- a/test/passing/refs.janestreet/attributes.ml.ref
+++ b/test/passing/refs.janestreet/attributes.ml.ref
@@ -535,3 +535,5 @@ let _ =
 ;;
 
 let fold input ~init ~f = foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]
diff --git a/test/passing/refs.ocamlformat/attributes.ml.ref b/test/passing/refs.ocamlformat/attributes.ml.ref
index c920efff91..7ed152a669 100644
--- a/test/passing/refs.ocamlformat/attributes.ml.ref
+++ b/test/passing/refs.ocamlformat/attributes.ml.ref
@@ -513,3 +513,5 @@ let _ =
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]
diff --git a/test/passing/tests/attributes.ml b/test/passing/tests/attributes.ml
index c44870f805..541a743fd1 100644
--- a/test/passing/tests/attributes.ml
+++ b/test/passing/tests/attributes.ml
@@ -498,3 +498,5 @@ let _ = (begin[@a] x end [@b])
 
 let fold input ~init ~f =
   foldi input ~init ~f:(fun (_ : int) acc x -> f acc x) [@nontail]
+
+[@@@effect]
\ No newline at end of file
diff --git a/vendor/parser-extended/parser.mly b/vendor/parser-extended/parser.mly
index 93cc03212d..3d23c3e0f1 100644
--- a/vendor/parser-extended/parser.mly
+++ b/vendor/parser-extended/parser.mly
@@ -4018,6 +4018,7 @@ single_attr_id:
   | DO { "do" }
   | DONE { "done" }
   | DOWNTO { "downto" }
+  | EFFECT { "effect" }
   | ELSE { "else" }
   | END { "end" }
   | EXCEPTION { "exception" }
diff --git a/vendor/parser-standard/parser.mly b/vendor/parser-standard/parser.mly
index 1c8317c710..28afc9842c 100644
--- a/vendor/parser-standard/parser.mly
+++ b/vendor/parser-standard/parser.mly
@@ -4063,6 +4063,7 @@ single_attr_id:
   | DO { "do" }
   | DONE { "done" }
   | DOWNTO { "downto" }
+  | EFFECT { "effect" }
   | ELSE { "else" }
   | END { "end" }
   | EXCEPTION { "exception" }