diff --git a/src/cm_kind.ml b/src/cm_kind.ml
index 0b52c7af..f2959463 100644
--- a/src/cm_kind.ml
+++ b/src/cm_kind.ml
@@ -1,3 +1,5 @@
+open Stdune
+
 type t = Cmi | Cmo | Cmx
 
 let all = [Cmi; Cmo; Cmx]
@@ -35,3 +37,10 @@ module Dict = struct
     ; cmx = x
     }
 end
+
+let to_sexp =
+  let open Sexp.To_sexp in
+  function
+  | Cmi -> string "cmi"
+  | Cmo -> string "cmo"
+  | Cmx -> string "cmx"
diff --git a/src/cm_kind.mli b/src/cm_kind.mli
index 71e8d087..ec8e4501 100644
--- a/src/cm_kind.mli
+++ b/src/cm_kind.mli
@@ -1,3 +1,5 @@
+open Stdune
+
 type t = Cmi | Cmo | Cmx
 
 val all : t list
@@ -5,6 +7,8 @@ val all : t list
 val ext : t -> string
 val source : t -> Ml_kind.t
 
+val to_sexp : t Sexp.To_sexp.t
+
 module Dict : sig
   type cm_kind = t