diff --git a/src/dune_rules/format_rules.ml b/src/dune_rules/format_rules.ml
index 9a8ce5bd0a1..0695fe51ba0 100644
--- a/src/dune_rules/format_rules.ml
+++ b/src/dune_rules/format_rules.ml
@@ -1,15 +1,15 @@
 open Import
 
-let add_diff sctx loc alias ~dir ~input ~output =
+let add_diff sctx loc alias ~input ~output =
   let open Action_builder.O in
-  let action = Action.Chdir (Path.build dir, Promote.Diff_action.diff input output) in
-  Super_context.add_alias_action
-    sctx
-    alias
-    ~dir
-    ~loc
-    (Action_builder.paths [ input; Path.build output ]
-     >>> Action_builder.return (Action.Full.make action))
+  let dir = Alias.dir alias in
+  let action =
+    let dir = Path.Build.parent_exn dir in
+    Action.Chdir (Path.build dir, Promote.Diff_action.diff input output)
+  in
+  Action_builder.paths [ input; Path.build output ]
+  >>> Action_builder.return (Action.Full.make action)
+  |> Super_context.add_alias_action sctx alias ~dir ~loc
 ;;
 
 let rec subdirs_until_root dir =
@@ -178,7 +178,7 @@ let gen_rules_output
      in
      format_action format ~input ~output ~expander kind
      |> Memo.bind ~f:(Super_context.add_rule sctx ~mode:Standard ~loc ~dir)
-     >>> add_diff sctx loc alias_formatted ~dir ~input:(Path.build input) ~output)
+     >>> add_diff sctx loc alias_formatted ~input:(Path.build input) ~output)
     |> Memo.Option.iter ~f:Fun.id
   in
   let* source_dir = Source_tree.find_dir (Path.Build.drop_build_context_exn dir) in
@@ -205,7 +205,7 @@ let gen_rules_output
              Action.Full.make (action ~version input output))
             |> Action_builder.with_file_targets ~file_targets:[ output ]
             |> Super_context.add_rule sctx ~mode:Standard ~loc ~dir
-            >>> add_diff sctx loc alias_formatted ~dir ~input ~output)))
+            >>> add_diff sctx loc alias_formatted ~input ~output)))
   in
   Rules.Produce.Alias.add_deps alias_formatted (Action_builder.return ())
 ;;