diff --git a/bin/migrations/builder_migrations.ml b/bin/migrations/builder_migrations.ml
index c751981..edcb8bf 100644
--- a/bin/migrations/builder_migrations.ml
+++ b/bin/migrations/builder_migrations.ml
@@ -178,6 +178,7 @@ let () =
         actions (module M20210712c);
         [ f20210910 ];
         actions (module M20211105);
+        actions (module M20220509);
       ])
   |> Cmd.eval
   |> exit
diff --git a/bin/migrations/m20220509.ml b/bin/migrations/m20220509.ml
new file mode 100644
index 0000000..59f7983
--- /dev/null
+++ b/bin/migrations/m20220509.ml
@@ -0,0 +1,48 @@
+let new_version = 16L and old_version = 15L
+and identifier = "2022-05-09"
+and migrate_doc = "switch uuid encoding to hex"
+and rollback_doc = "switch uuid encoding back to binary"
+
+let old_uuid_rep =
+  let encode uuid = Ok (Uuidm.to_bytes uuid) in
+  let decode s =
+    Uuidm.of_bytes s
+    |> Option.to_result ~none:"failed to decode uuid"
+  in
+  Caqti_type.custom ~encode ~decode Caqti_type.string
+
+let uuids_byte_encoded_q =
+  Caqti_request.collect ~oneshot:true
+    Caqti_type.unit
+    (Caqti_type.tup2 (Builder_db.Rep.id (`build : [`build])) old_uuid_rep)
+    "SELECT id, uuid FROM build"
+
+let uuids_hex_encoded_q =
+  Caqti_request.collect ~oneshot:true
+    Caqti_type.unit
+    (Caqti_type.tup2 (Builder_db.Rep.id (`build : [`build])) Builder_db.Rep.uuid)
+    "SELECT id, uuid FROM build"
+
+let migrate_q =
+  Caqti_request.exec ~oneshot:true
+    (Caqti_type.tup2 (Builder_db.Rep.id (`build : [`build])) Builder_db.Rep.uuid)
+    "UPDATE build SET uuid = $2 WHERE id = $1"
+
+let rollback_q =
+  Caqti_request.exec ~oneshot:true
+    (Caqti_type.tup2 (Builder_db.Rep.id (`build : [`build])) old_uuid_rep)
+    "UPDATE build SET uuid = $2 WHERE id = $1"
+
+let migrate _datadir (module Db : Caqti_blocking.CONNECTION) =
+  let open Grej.Infix in
+  Grej.check_version ~user_version:old_version (module Db) >>= fun () ->
+  Db.collect_list uuids_byte_encoded_q () >>= fun old_uuids ->
+  Grej.list_iter_result (Db.exec migrate_q) old_uuids >>= fun () ->
+  Db.exec (Grej.set_version new_version) ()
+
+let rollback _datadir (module Db : Caqti_blocking.CONNECTION) =
+  let open Grej.Infix in
+  Grej.check_version ~user_version:new_version (module Db) >>= fun () ->
+  Db.collect_list uuids_hex_encoded_q () >>= fun new_uuids ->
+  Grej.list_iter_result (Db.exec rollback_q) new_uuids >>= fun () ->
+  Db.exec (Grej.set_version old_version) ()
diff --git a/db/representation.ml b/db/representation.ml
index c95e0cb..4d94b93 100644
--- a/db/representation.ml
+++ b/db/representation.ml
@@ -36,9 +36,9 @@ type file = {
 }
 
 let uuid =
-  let encode uuid = Ok (Uuidm.to_bytes uuid) in
+  let encode uuid = Ok (Uuidm.to_string uuid) in
   let decode s =
-    Uuidm.of_bytes s
+    Uuidm.of_string s
     |> Option.to_result ~none:"failed to decode uuid"
   in
   Caqti_type.custom ~encode ~decode Caqti_type.string