diff --git a/src/jbuild.ml b/src/jbuild.ml
index 4ab84962..ffab7cf7 100644
--- a/src/jbuild.ml
+++ b/src/jbuild.ml
@@ -942,7 +942,7 @@ end
 module Menhir = struct
   type t =
     { merge_into : string option
-    ; flags      : String_with_vars.t list
+    ; flags      : Ordered_set_lang.Unexpanded.t
     ; modules    : string list
     ; mode       : Rule.Mode.t
     ; loc        :  Loc.t
@@ -951,7 +951,7 @@ module Menhir = struct
   let v1 =
     record
       (field_o "merge_into" string >>= fun merge_into ->
-       field "flags" (list String_with_vars.t) ~default:[] >>= fun flags ->
+       field_oslu "flags" >>= fun flags ->
        field "modules" (list string) >>= fun modules ->
        Rule.Mode.field >>= fun mode ->
        return
diff --git a/src/jbuild.mli b/src/jbuild.mli
index e994d553..1ee4aba3 100644
--- a/src/jbuild.mli
+++ b/src/jbuild.mli
@@ -287,7 +287,7 @@ end
 module Menhir : sig
   type t =
     { merge_into : string option
-    ; flags      : String_with_vars.t list
+    ; flags      : Ordered_set_lang.Unexpanded.t
     ; modules    : string list
     ; mode       : Rule.Mode.t
     ; loc        : Loc.t
diff --git a/src/menhir.ml b/src/menhir.ml
index fced8e68..66d31eb1 100644
--- a/src/menhir.ml
+++ b/src/menhir.ml
@@ -1,17 +1,22 @@
 open Import
+open Build.O
+open! No_io
 
 let gen_rules sctx ~dir ~scope (t : Jbuild.Menhir.t) =
   let targets n = List.map ~f:(Path.relative dir) [n ^ ".ml"; n ^ ".mli"] in
   let flags =
-    List.map ~f:(Super_context.expand_vars sctx ~scope ~dir) t.flags in
+    Super_context.expand_and_eval_set sctx ~scope ~dir t.flags
+      ~standard:[] in
   let menhir =
     let menhir =
       Super_context.resolve_program sctx ~hint:"opam install menhir" "menhir" in
-    fun ~extra_targets ->
-      Build.run ~extra_targets
-        menhir
-        ~dir
-        ~context:(Super_context.context sctx) in
+    fun ~extra_targets args->
+      flags
+      >>> (Build.run ~extra_targets
+             menhir
+             ~dir
+             ~context:(Super_context.context sctx)
+             args) in
   let add_rule_get_targets =
     Super_context.add_rule_get_targets sctx ~mode:t.mode ~loc:t.loc in
   let mly name = Path.relative dir (name ^ ".mly") in
@@ -21,7 +26,7 @@ let gen_rules sctx ~dir ~scope (t : Jbuild.Menhir.t) =
       add_rule_get_targets (
         menhir
           ~extra_targets:(targets name)
-          [ As flags
+          [ Dyn (fun x -> As x)
           ; Dep (mly name)]
       )) t.modules
   | Some merge_into ->
@@ -29,7 +34,7 @@ let gen_rules sctx ~dir ~scope (t : Jbuild.Menhir.t) =
       menhir
         ~extra_targets:(targets merge_into)
         [ A "--base" ; A merge_into
-        ; As flags
+        ; Dyn (fun x -> As x)
         ; Deps (List.map ~f:mly t.modules)
         ]
     )