diff --git a/src/dune b/src/dune
index d9a560ea..bd0df5e4 100644
--- a/src/dune
+++ b/src/dune
@@ -1,7 +1,7 @@
 (executable
  (public_name mte)
  (name mte)
- (modules assets assets_crunch headers mte config util)
+ (modules assets assets_crunch headers mte config util types)
  (libraries
   headers_lib
   syntax
diff --git a/src/mte.ml b/src/mte.ml
index cd406e4d..a9a60f15 100644
--- a/src/mte.ml
+++ b/src/mte.ml
@@ -13,13 +13,11 @@
     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <https://www.gnu.org/licenses/>. *)
 
-(* TODO ??: GANA error codes
-   https://git.gnunet.org/gana.git/tree/gnu-taler-error-codes/registry.rec *)
-
-(* https://docs.taler.net/core/api-common.html#tsref-type-ErrorDetail *)
-let error_detail ?hint status =
-  let _ = (hint, status) in
-  "{}"
+let error_detail ?hint _status =
+  let open Types.ErrorDetail in
+  let code = -1 in
+  let s = to_string (make code hint) in
+  s
 
 module Respond_with = struct
   open Vif.Response
diff --git a/src/types.ml b/src/types.ml
new file mode 100644
index 00000000..7ba4d474
--- /dev/null
+++ b/src/types.ml
@@ -0,0 +1,19 @@
+(* https://docs.taler.net/core/api-common.html#tsref-type-ErrorDetail *)
+module ErrorDetail = struct
+  (* TODO GANA error codes
+   https://git.gnunet.org/gana.git/tree/gnu-taler-error-codes/registry.rec *)
+  type t = { code: int; hint: string option }
+
+  let make code hint = { code; hint }
+  let enc_code v = v.code
+  let enc_hint v = v.hint
+
+  let jsont =
+    Jsont.Object.map ~kind:"ErrorDetail" make
+    |> Jsont.Object.mem "code" Jsont.int ~enc:enc_code
+    |> Jsont.Object.opt_mem "hint" Jsont.(string) ~enc:enc_hint
+    |> Jsont.Object.finish
+
+  let to_string v = Jsont_bytesrw.encode_string jsont v |> Result.get_ok
+  let of_string v = Jsont_bytesrw.decode_string jsont v
+end