diff --git a/src/headers_lib.ml b/src/headers_lib.ml
index 079c176d..289e8018 100644
--- a/src/headers_lib.ml
+++ b/src/headers_lib.ml
@@ -1,11 +1,11 @@
-(* independent module for headers fields value *)
+(* independent library for headers fields value *)
 
 (* TODO
    - test
    - can still bypass this module and directly set Etag header, but its fine *)
 module Etag : sig
-  (* module to parse < "*" / #entity-tag >,
-     used for If-Match and If-None-Match headers
+  (* module to parse etags header fields
+     used by If-Match and If-None-Match headers
 
      https://httpwg.org/specs/rfc9110.html#field.etag *)
   type t
diff --git a/src/mte.ml b/src/mte.ml
index 79ecd9c5..cd406e4d 100644
--- a/src/mte.ml
+++ b/src/mte.ml
@@ -13,7 +13,9 @@
     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 *)
+(* 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
@@ -23,6 +25,11 @@ module Respond_with = struct
   open Vif.Response
   open Syntax
 
+  let bad_request ?hint req =
+    let body = error_detail ?hint `Bad_request in
+    let* () = with_string ?compression:None req body in
+    respond `Bad_request
+
   let not_modified () =
     let* () = empty in
     respond `Not_modified
@@ -48,21 +55,19 @@ end
      include a "Avail-Languages" header: a comma-separated list of the languages available *)
 module Static = struct
   let f kind req _server _env =
-    let get_ok = function
-      | Error e -> Fmt.failwith "TODO handle me, %s" e
-      | Ok v -> v
-    in
     let etag = Assets.etag kind in
     let headers = Vif.Request.headers req in
     let has_matching_etag =
       match Vif.Headers.get headers "if-none-match" with
-      | None -> false
+      | None -> Ok false
       | Some s ->
-          Headers_lib.Etag.parse s |> get_ok |> Headers_lib.Etag.evaluate etag
+          Headers_lib.Etag.parse s
+          |> Result.map (Headers_lib.Etag.evaluate etag)
     in
     match has_matching_etag with
-    | true -> Respond_with.not_modified ()
-    | false -> (
+    | Error e -> Respond_with.bad_request ~hint:e req
+    | Ok true -> Respond_with.not_modified ()
+    | Ok false -> (
         match Headers.select_mimetype headers with
         | None -> Respond_with.unsupported_media_type req
         | Some mime ->