diff --git a/fuzz/smart.ml b/fuzz/smart.ml
index 094505645..d0c1f3f43 100644
--- a/fuzz/smart.ml
+++ b/fuzz/smart.ml
@@ -74,11 +74,11 @@ let () =
 
   Crowbar.add_test ~name:"pkt-line" Crowbar.[ packet >>= bytes_fixed; range 4 ]
   @@ fun payload len ->
-  let str = Fmt.strf "%04x" (String.length payload + 4) ^ payload in
+  let str = Fmt.str "%04x" (String.length payload + 4) ^ payload in
   let res = of_string str in
   Crowbar.check_eq ~pp:(Fmt.fmt "%S") ~eq:String.equal ~cmp:String.compare res
     payload;
-  let str = Fmt.strf "%04x" len in
+  let str = Fmt.str "%04x" len in
   let res = of_string str in
   Crowbar.check_eq ~pp:(Fmt.fmt "%S") ~eq:String.equal ~cmp:String.compare res
     ""
diff --git a/src/carton-git/carton_git_unix.ml b/src/carton-git/carton_git_unix.ml
index e4ae04692..819473820 100644
--- a/src/carton-git/carton_git_unix.ml
+++ b/src/carton-git/carton_git_unix.ml
@@ -80,7 +80,7 @@ struct
   include Carton_git.Make (Carton_lwt.Scheduler) (Lwt) (Store) (Uid)
 
   let idx_major_uid_of_uid root uid =
-    Fpath.(root / Fmt.strf "pack-%s.idx" (Uid.to_hex uid))
+    Fpath.(root / Fmt.str "pack-%s.idx" (Uid.to_hex uid))
 
   let uid_of_major_uid path =
     let str = Fpath.basename (Fpath.rem_ext path) in
diff --git a/src/carton/dec.ml b/src/carton/dec.ml
index 7f27b84ce..9486809ec 100644
--- a/src/carton/dec.ml
+++ b/src/carton/dec.ml
@@ -93,7 +93,7 @@ module Fp (Uid : UID) = struct
   let is_inflate = function Inflate _ -> true | _ -> false
   let src_rem = i_rem
   let eoi d = { d with i = Bigstringaf.empty; i_pos = 0; i_len = min_int }
-  let malformedf fmt = Fmt.kstrf (fun err -> `Malformed err) fmt
+  let malformedf fmt = Fmt.kstr (fun err -> `Malformed err) fmt
 
   let src d s j l =
     if j < 0 || l < 0 || j + l > Bigstringaf.length s then
@@ -413,7 +413,7 @@ module Fp (Uid : UID) = struct
                   ctx = Uid.feed d.ctx d.i ~off:d.i_pos ~len;
                 }
           | `Flush z -> go (Zl.Inf.flush z)
-          | `Malformed err -> `Malformed (Fmt.strf "inflate: %s" err)
+          | `Malformed err -> `Malformed (Fmt.str "inflate: %s" err)
           | `End z ->
               let len = i_rem d - Zl.Inf.src_rem z in
               let crc = Checkseum.Crc32.digest_bigstring d.i d.i_pos len crc in
@@ -470,7 +470,7 @@ module Fp (Uid : UID) = struct
                 first := false );
 
               go (Zl.Inf.flush z)
-          | `Malformed err -> `Malformed (Fmt.strf "inflate: %s" err)
+          | `Malformed err -> `Malformed (Fmt.str "inflate: %s" err)
           | `End z ->
               if !first then (
                 let len = Bigstringaf.length d.o - Zl.Inf.dst_rem z in
diff --git a/src/carton/dec.mli b/src/carton/dec.mli
index 3d6afc205..98498ca73 100644
--- a/src/carton/dec.mli
+++ b/src/carton/dec.mli
@@ -405,7 +405,7 @@ val of_offset_with_path :
           | `B -> "tree"
           | `C -> "blob"
           | `D -> "tag" in
-        let hdr = Fmt.strf "%s %d\000" kind (len v) int
+        let hdr = Fmt.str "%s %d\000" kind (len v) int
         let ctx = Digest.empty in
         feed_string ctx hdr ;
         feed_bigstring ctx (Bigstringaf.sub (raw v) 0 (len v)) ;
diff --git a/src/carton/h.ml b/src/carton/h.ml
index 086edb526..02696c025 100644
--- a/src/carton/h.ml
+++ b/src/carton/h.ml
@@ -179,7 +179,7 @@ module M = struct
   let dst_rem d = bigstring_length d.dst - d.o_pos
   let src_len { src_len; _ } = src_len
   let dst_len { dst_len; _ } = dst_len
-  let malformedf fmt = Fmt.kstrf (fun s -> Malformed s) fmt
+  let malformedf fmt = Fmt.kstr (fun s -> Malformed s) fmt
 
   let t_need d n =
     d.t_len <- 0;
diff --git a/src/git-index/git_index.ml b/src/git-index/git_index.ml
index c03f85077..7486379af 100644
--- a/src/git-index/git_index.ml
+++ b/src/git-index/git_index.ml
@@ -150,7 +150,7 @@ module Entry = struct
           go ctx
       in
       let len = in_channel_length ic in
-      let ctx = Hash.feed_string ctx (Fmt.strf "blob %d\000" len) in
+      let ctx = Hash.feed_string ctx (Fmt.str "blob %d\000" len) in
       let hash = go ctx in
       close_in ic;
       Rresult.R.ok hash
@@ -168,7 +168,7 @@ module Entry = struct
     let ctx = Hash.empty in
     let ctx =
       Hash.feed_string ctx
-        (Fmt.strf "blob %d\000%s" (String.length contents) contents)
+        (Fmt.str "blob %d\000%s" (String.length contents) contents)
     in
     Rresult.R.ok (Hash.get ctx)
 
diff --git a/src/git-unix/git_unix.ml b/src/git-unix/git_unix.ml
index 13a52739e..c6c17cb6b 100644
--- a/src/git-unix/git_unix.ml
+++ b/src/git-unix/git_unix.ml
@@ -62,7 +62,7 @@ module Fold = struct
         | Unix.Unix_error (Unix.EINTR, _, _) -> contents ~dotfiles ~rel dir
         | Unix.Unix_error (err, _, _) ->
             let err =
-              Fmt.strf "directory contents %a: %s" Fpath.pp dir
+              Fmt.str "directory contents %a: %s" Fpath.pp dir
                 (Unix.error_message err)
             in
             Log.err (fun m -> m "%s" err);
@@ -616,10 +616,10 @@ module Make (Digestif : Digestif.S) = struct
     {
       Git.Store.pck_major_uid_of_uid =
         (fun root uid ->
-          Fpath.(root / Fmt.strf "pack-%s.pack" (Digestif.to_hex uid)));
+          Fpath.(root / Fmt.str "pack-%s.pack" (Digestif.to_hex uid)));
       Git.Store.idx_major_uid_of_uid =
         (fun root uid ->
-          Fpath.(root / Fmt.strf "pack-%s.idx" (Digestif.to_hex uid)));
+          Fpath.(root / Fmt.str "pack-%s.idx" (Digestif.to_hex uid)));
       Git.Store.uid_of_major_uid =
         (fun path ->
           let str = Fpath.basename (Fpath.rem_ext path) in
@@ -663,15 +663,15 @@ module Sync (Git_store : Git.S) (HTTP : Smart_git.HTTP) = struct
 
   let random_path pat =
     let rand = Random.State.bits (Lazy.force random_gen) land 0xFFFFFF in
-    Fpath.v (Fmt.strf pat (Fmt.strf "%06x" rand))
+    Fpath.v (Fmt.str pat (Fmt.str "%06x" rand))
 
-  let failwithf fmt = Fmt.kstrf (fun err -> Lwt.fail (Failure err)) fmt
+  let failwithf fmt = Fmt.kstr (fun err -> Lwt.fail (Failure err)) fmt
 
   let create_tmp_path mode dir pat =
     let rec loop count =
       if count < 0 then
         failwithf "Create a temporary file %s in %a: too many failing attempts"
-          (Fmt.strf pat "XXXXXX") Fpath.pp dir
+          (Fmt.str pat "XXXXXX") Fpath.pp dir
       else
         let file = random_path pat in
         let sfile = Fpath.to_string Fpath.(dir // file) in
diff --git a/src/git-unix/ogit-fetch/main.ml b/src/git-unix/ogit-fetch/main.ml
index b56df03fc..bd7072d7a 100644
--- a/src/git-unix/ogit-fetch/main.ml
+++ b/src/git-unix/ogit-fetch/main.ml
@@ -35,7 +35,7 @@ let reporter ppf =
       let dt = Mtime.Span.to_us (Mtime_clock.elapsed ()) in
       Fmt.kpf k ppf
         ("%s %a %a: @[" ^^ fmt ^^ "@]@.")
-        (pad 10 (Fmt.strf "%+04.0fus" dt))
+        (pad 10 (Fmt.str "%+04.0fus" dt))
         pp_header (level, h)
         Fmt.(styled `Magenta string)
         (pad 10 @@ Logs.Src.name src)
@@ -69,7 +69,7 @@ let ssh_cfg edn ssh_seed =
   let key = Awa.Keys.of_seed ssh_seed in
   match edn with
   | { Smart_git.scheme = `SSH user; path; _ } ->
-      let req = Awa.Ssh.Exec (Fmt.strf "git-upload-pack '%s'" path) in
+      let req = Awa.Ssh.Exec (Fmt.str "git-upload-pack '%s'" path) in
       Some { Awa_conduit.user; key; req; authenticator = None }
   | _ -> None
 
@@ -116,7 +116,7 @@ module Flag = struct
         match str with
         | "stdout" -> Ok Fmt.stdout
         | "stderr" -> Ok Fmt.stderr
-        | s -> Error (`Msg (Fmt.strf "%s is not an output." s))
+        | s -> Error (`Msg (Fmt.str "%s is not an output." s))
       in
       let print ppf v =
         Fmt.pf ppf "%s" (if v == Fmt.stdout then "stdout" else "stderr")
@@ -182,7 +182,7 @@ let setup_log =
 let main _ ssh_seed references directory repository _ =
   match Lwt_main.run (main ssh_seed references directory repository) with
   | Ok () -> `Ok ()
-  | Error (#error as err) -> `Error (false, Fmt.strf "%a" pp_error err)
+  | Error (#error as err) -> `Error (false, Fmt.str "%a" pp_error err)
 
 let command =
   let doc = "Fetch a Git repository by the HTTP protocol." in
diff --git a/src/git/blob.ml b/src/git/blob.ml
index 8be607031..bcc8c1178 100644
--- a/src/git/blob.ml
+++ b/src/git/blob.ml
@@ -59,7 +59,7 @@ module Make (Hash : S.HASH) = struct
 
   let digest cs =
     let ctx = Hash.init () in
-    let hdr = Fmt.strf "blob %Ld\000" (length cs) in
+    let hdr = Fmt.str "blob %Ld\000" (length cs) in
     let ctx = Hash.feed_string ctx hdr in
     let ctx = Hash.feed_cstruct ctx cs in
     Hash.get ctx
diff --git a/src/git/mem.ml b/src/git/mem.ml
index 4b9cfac50..1fc435c0b 100644
--- a/src/git/mem.ml
+++ b/src/git/mem.ml
@@ -83,7 +83,7 @@ let batch_write :
   in
   iter index ~f
 
-let failuref fmt = Fmt.kstrf (fun err -> Failure err) fmt
+let failuref fmt = Fmt.kstr (fun err -> Failure err) fmt
 
 (* XXX(dinosaure): a point about modules, functors and alias.
  * The choice was made to _defunctorize_ any types to avoid to
@@ -185,7 +185,7 @@ module Make (Digestif : Digestif.S) = struct
     let len = Cstruct.len raw in
     let ctx = Hash.init () in
     let hdr =
-      Fmt.strf "%s %d\000%!"
+      Fmt.str "%s %d\000%!"
         ( match kind with
         | `Commit -> "commit"
         | `Blob -> "blob"
@@ -206,7 +206,7 @@ module Make (Digestif : Digestif.S) = struct
         lazy
           ( match Value.of_raw ~kind inflated with
           | Error (`Msg err) ->
-              let str = Fmt.strf "Value.of_raw(%a): %s" Hash.pp hash err in
+              let str = Fmt.str "Value.of_raw(%a): %s" Hash.pp hash err in
               raise (Failure str)
           | Ok value -> value )
       in
@@ -242,7 +242,7 @@ module Make (Digestif : Digestif.S) = struct
             Hashtbl.add t.values h (lazy v);
             Ok v
         | Error (`Msg err) ->
-            let str = Fmt.strf "Value.of_raw(%a): %s" Hash.pp h err in
+            let str = Fmt.str "Value.of_raw(%a): %s" Hash.pp h err in
             raise (Failure str)
       with Not_found -> Error (`Not_found h) )
 
diff --git a/src/git/reference.ml b/src/git/reference.ml
index d11c1ccb1..e6d9b071f 100644
--- a/src/git/reference.ml
+++ b/src/git/reference.ml
@@ -10,7 +10,7 @@ type t = string (* non empty *)
 
 let dir_sep = "/"
 let dir_sep_char = '/'
-let error_msgf fmt = Fmt.kstrf (fun err -> Error (`Msg err)) fmt
+let error_msgf fmt = Fmt.kstr (fun err -> Error (`Msg err)) fmt
 
 let validate_and_collapse_seps p =
   let max_idx = String.length p - 1 in
@@ -233,8 +233,8 @@ let write { Carton.bind; Carton.return } t store reference contents =
 
   let str =
     match contents with
-    | Uid uid -> Fmt.strf "%s\n" (store.uid_to_hex uid)
-    | Ref t -> Fmt.strf "ref: %s\n" t
+    | Uid uid -> Fmt.str "%s\n" (store.uid_to_hex uid)
+    | Ref t -> Fmt.str "ref: %s\n" t
   in
   store.atomic_wr t reference str >>| reword_error (fun err -> `Store err)
 
diff --git a/src/git/store.ml b/src/git/store.ml
index ae562c987..c97808c84 100644
--- a/src/git/store.ml
+++ b/src/git/store.ml
@@ -237,7 +237,7 @@ struct
     read t hash >>= function
     | Ok v -> Lwt.return v
     | Error _ ->
-        let err = Fmt.strf "Git.Store.read_exn: %a not found" Hash.pp hash in
+        let err = Fmt.str "Git.Store.read_exn: %a not found" Hash.pp hash in
         Lwt.fail_invalid_arg err
 
   let stream_of_raw ?(chunk = De.io_buffer_size) raw =
@@ -308,7 +308,7 @@ struct
     Loose.atomic_add t.minor buffers v >>= function
     | Ok (hash, _) -> Lwt.return hash
     | Error (`Store err) ->
-        Lwt.fail (Failure (Fmt.strf "%a" pp_error (`Minor err)))
+        Lwt.fail (Failure (Fmt.str "%a" pp_error (`Minor err)))
     | Error `Non_atomic -> (
         let consumed = Stdlib.ref false in
         let stream () =
@@ -320,7 +320,7 @@ struct
         Loose.add t.minor buffers (kind, Int64.of_int len) stream >>= function
         | Ok (hash, _) -> Lwt.return hash
         | Error (`Store err) ->
-            Lwt.fail (Failure (Fmt.strf "%a" pp_error (`Minor err))) )
+            Lwt.fail (Failure (Fmt.str "%a" pp_error (`Minor err))) )
 
   let mem t hash =
     if not (Pack.exists t.major t.packs hash) then Loose.exists t.minor hash
diff --git a/src/git/stream.ml b/src/git/stream.ml
index 17cd4ab20..7d25ca0d6 100644
--- a/src/git/stream.ml
+++ b/src/git/stream.ml
@@ -4,10 +4,10 @@ let src =
 module Log = (val Logs.src_log src : Logs.LOG)
 
 let hdr = function
-  | `Blob -> Fmt.strf "blob %Ld\000"
-  | `Tree -> Fmt.strf "tree %Ld\000"
-  | `Tag -> Fmt.strf "tag %Ld\000"
-  | `Commit -> Fmt.strf "commit %Ld\000"
+  | `Blob -> Fmt.str "blob %Ld\000"
+  | `Tree -> Fmt.str "tree %Ld\000"
+  | `Tag -> Fmt.str "tag %Ld\000"
+  | `Commit -> Fmt.str "commit %Ld\000"
 
 type ('uid, 't) digest = {
   empty : 't;
diff --git a/src/loose/loose_git_unix.ml b/src/loose/loose_git_unix.ml
index e3da76333..4c62ea429 100644
--- a/src/loose/loose_git_unix.ml
+++ b/src/loose/loose_git_unix.ml
@@ -95,7 +95,7 @@ module Make (Uid : UID) = struct
   (* fold *)
 
   let always x _ = x
-  let failwithf fmt = Fmt.kstrf Lwt.fail_with fmt
+  let failwithf fmt = Fmt.kstr Lwt.fail_with fmt
 
   let contents ?(dotfiles = false) ?(rel = false) dir =
     let rec readdir dh acc =
diff --git a/src/not-so-smart/capability.ml b/src/not-so-smart/capability.ml
index 7b51268d2..562261375 100644
--- a/src/not-so-smart/capability.ml
+++ b/src/not-so-smart/capability.ml
@@ -34,7 +34,7 @@ let to_string = function
   | `Side_band -> "side-band"
   | `Side_band_64k -> "side-band-64k"
   | `Ofs_delta -> "ofs-delta"
-  | `Agent agent -> Fmt.strf "agent=%s" agent
+  | `Agent agent -> Fmt.str "agent=%s" agent
   | `Shallow -> "shallow"
   | `Deepen_since -> "deepen-since"
   | `Deepen_not -> "deepen-not"
@@ -47,10 +47,10 @@ let to_string = function
   | `Push_options -> "push-options"
   | `Allow_tip_sha1_in_want -> "allow-tip-sha1-in-want"
   | `Allow_reachable_sha1_in_want -> "allow-reachable-sha1-in-want"
-  | `Push_cert cert -> Fmt.strf "push-cert=%s" cert
-  | `Symref (ref0, ref1) -> Fmt.strf "symref=%s:%s" ref0 ref1
+  | `Push_cert cert -> Fmt.str "push-cert=%s" cert
+  | `Symref (ref0, ref1) -> Fmt.str "symref=%s:%s" ref0 ref1
   | `Other capability -> capability
-  | `Parameter (key, value) -> Fmt.strf "%s=%s" key value
+  | `Parameter (key, value) -> Fmt.str "%s=%s" key value
 
 exception Capability_expect_value of string
 
diff --git a/src/not-so-smart/decoder.ml b/src/not-so-smart/decoder.ml
index 14990a9f9..b9ef45312 100644
--- a/src/not-so-smart/decoder.ml
+++ b/src/not-so-smart/decoder.ml
@@ -192,7 +192,7 @@ let error_end_of_input decoder () = fail decoder `End_of_input
 let reliable_pkt k decoder () =
   match get_pkt_len decoder with
   | Some _len ->
-      let hdr = Fmt.strf "%04X" (decoder.max - decoder.pos) in
+      let hdr = Fmt.str "%04X" (decoder.max - decoder.pos) in
       Bytes.blit_string hdr 0 decoder.buffer decoder.pos 4;
       (* unsafe! *)
       k decoder
diff --git a/src/not-so-smart/find_common.ml b/src/not-so-smart/find_common.ml
index 68f1269f9..c91717fc9 100644
--- a/src/not-so-smart/find_common.ml
+++ b/src/not-so-smart/find_common.ml
@@ -65,7 +65,7 @@ let run :
    translated as is [fetch-pack.c:find_common] in OCaml. *)
 
 let unsafe_write_have ctx hex =
-  let packet = Fmt.strf "have %s\n" hex in
+  let packet = Fmt.str "have %s\n" hex in
   Smart.Unsafe.write ctx packet
 
 let next_flush stateless count =
diff --git a/src/not-so-smart/protocol.ml b/src/not-so-smart/protocol.ml
index 345378967..0b9cb168d 100644
--- a/src/not-so-smart/protocol.ml
+++ b/src/not-so-smart/protocol.ml
@@ -722,7 +722,7 @@ module Encoder = struct
     k0 encoder;
     (* XXX(dinosaure): or [encoder.pos <- encoder.pos + 4]? *)
     let len = encoder.pos - pos in
-    Bytes.blit_string (Fmt.strf "%04X" len) 0 encoder.payload pos 4;
+    Bytes.blit_string (Fmt.str "%04X" len) 0 encoder.payload pos 4;
     flush k1 encoder
 
   let kdone _encoder = Done
@@ -741,17 +741,17 @@ module Encoder = struct
       | `Upload_archive -> write encoder "git-upload-archive"
     in
     let write_version encoder version =
-      let version = Fmt.strf "version=%d" version in
+      let version = Fmt.str "version=%d" version in
       write encoder version
     in
     let write_host encoder = function
       | host, Some port ->
           let host =
-            Fmt.strf "host=%s:%d" (Conduit.Endpoint.to_string host) port
+            Fmt.str "host=%s:%d" (Conduit.Endpoint.to_string host) port
           in
           write encoder host
       | host, None ->
-          let host = Fmt.strf "host=%s" (Conduit.Endpoint.to_string host) in
+          let host = Fmt.str "host=%s" (Conduit.Endpoint.to_string host) in
           write encoder host
     in
     let k encoder =
@@ -860,7 +860,7 @@ module Encoder = struct
     encoder.pos <- encoder.pos + 4;
     Encoder.write encoder packet;
     let len = encoder.pos - pos in
-    Bytes.blit_string (Fmt.strf "%04X" len) 0 encoder.payload pos 4
+    Bytes.blit_string (Fmt.str "%04X" len) 0 encoder.payload pos 4
 
   let write_command encoder = function
     | Commands.Create (uid, r) ->
diff --git a/src/not-so-smart/smart_git.ml b/src/not-so-smart/smart_git.ml
index 29dcf0d01..a301f3b74 100644
--- a/src/not-so-smart/smart_git.ml
+++ b/src/not-so-smart/smart_git.ml
@@ -91,7 +91,7 @@ let endpoint_of_string str =
         let user =
           String.concat "."
             (List.map
-               (function `Atom x -> x | `String x -> Fmt.strf "%S" x)
+               (function `Atom x -> x | `String x -> Fmt.str "%S" x)
                m.Emile.local)
         in
         ( match fst m.Emile.domain with
@@ -186,10 +186,10 @@ struct
 
     let ctx =
       match kind with
-      | `A -> feed_string ctx (Fmt.strf "commit %d\000" len)
-      | `B -> feed_string ctx (Fmt.strf "tree %d\000" len)
-      | `C -> feed_string ctx (Fmt.strf "blob %d\000" len)
-      | `D -> feed_string ctx (Fmt.strf "tag %d\000" len)
+      | `A -> feed_string ctx (Fmt.str "commit %d\000" len)
+      | `B -> feed_string ctx (Fmt.str "tree %d\000" len)
+      | `C -> feed_string ctx (Fmt.str "blob %d\000" len)
+      | `D -> feed_string ctx (Fmt.str "tag %d\000" len)
     in
     let ctx = Uid.feed ctx ~off ~len buf in
     Uid.get ctx
@@ -394,12 +394,12 @@ struct
       endpoint path ~resolvers ?deepen ?want store access fetch_cfg pack =
     let open Rresult in
     let open Lwt.Infix in
-    let uri0 = Fmt.strf "%a/info/refs?service=git-upload-pack" Uri.pp uri in
+    let uri0 = Fmt.str "%a/info/refs?service=git-upload-pack" Uri.pp uri in
     let uri0 = Uri.of_string uri0 in
     HTTP.get ~resolvers ~headers uri0
     >|= R.reword_error (R.msgf "%a" HTTP.pp_error)
     >>? fun (_resp, contents) ->
-    let uri1 = Fmt.strf "%a/git-upload-pack" Uri.pp uri in
+    let uri1 = Fmt.str "%a/git-upload-pack" Uri.pp uri in
     let uri1 = Uri.of_string uri1 in
     let flow =
       {
@@ -472,12 +472,12 @@ struct
             match scheme with
             | `HTTP headers ->
                 ( Uri.of_string
-                    (Fmt.strf "http://%a%s.git" Conduit.Endpoint.pp endpoint
+                    (Fmt.str "http://%a%s.git" Conduit.Endpoint.pp endpoint
                        path),
                   headers )
             | `HTTPS headers ->
                 ( Uri.of_string
-                    (Fmt.strf "https://%a%s.git" Conduit.Endpoint.pp endpoint
+                    (Fmt.str "https://%a%s.git" Conduit.Endpoint.pp endpoint
                        path),
                   headers )
           in
diff --git a/test/carton/test.ml b/test/carton/test.ml
index b1594bd6c..0102c544d 100644
--- a/test/carton/test.ml
+++ b/test/carton/test.ml
@@ -33,7 +33,7 @@ let () = Random.full_init seed
 
 open Prelude
 
-let failf fmt = Fmt.kstrf Alcotest.fail fmt
+let failf fmt = Fmt.kstr Alcotest.fail fmt
 
 let bigstringaf =
   Alcotest.testable
@@ -236,10 +236,10 @@ let digest_like_git ~kind ?(off = 0) ?len buf =
 
   let ctx =
     match kind with
-    | `A -> Digestif.SHA1.feed_string ctx (Fmt.strf "commit %d\000" len)
-    | `B -> Digestif.SHA1.feed_string ctx (Fmt.strf "tree %d\000" len)
-    | `C -> Digestif.SHA1.feed_string ctx (Fmt.strf "blob %d\000" len)
-    | `D -> Digestif.SHA1.feed_string ctx (Fmt.strf "tag %d\000" len)
+    | `A -> Digestif.SHA1.feed_string ctx (Fmt.str "commit %d\000" len)
+    | `B -> Digestif.SHA1.feed_string ctx (Fmt.str "tree %d\000" len)
+    | `C -> Digestif.SHA1.feed_string ctx (Fmt.str "blob %d\000" len)
+    | `D -> Digestif.SHA1.feed_string ctx (Fmt.str "tag %d\000" len)
   in
   let ctx = Digestif.SHA1.feed_bigstring ctx ~off ~len buf in
   Digestif.SHA1.get ctx
@@ -986,7 +986,7 @@ let decode_index_stream () =
         ~f:(fun ~uid ~offset ~crc:_ ->
           match Carton.Dec.Idx.find index0 uid with
           | Some (_, offset') ->
-              Alcotest.(check int64) (Fmt.strf "%a" Uid.pp uid) offset offset'
+              Alcotest.(check int64) (Fmt.str "%a" Uid.pp uid) offset offset'
           | None -> Alcotest.failf "%a not found" Uid.pp uid)
         index1
   | Error err ->
diff --git a/test/carton/test_lwt.ml b/test/carton/test_lwt.ml
index 178025ddf..a30b70761 100644
--- a/test/carton/test_lwt.ml
+++ b/test/carton/test_lwt.ml
@@ -20,8 +20,7 @@ let create root path =
   and error = function
     | Unix.Unix_error (Unix.ENOENT, _, _) | Unix.Unix_error (Unix.EACCES, _, _)
       ->
-        Lwt.return_error
-          (`Msg (Fmt.strf "Impossible to open %a." Fpath.pp path))
+        Lwt.return_error (`Msg (Fmt.str "Impossible to open %a." Fpath.pp path))
     | Unix.Unix_error (Unix.EINTR, _, _) -> Lwt.catch process error
     | exn -> Lwt.fail exn
   in
@@ -97,10 +96,10 @@ let digest ~kind ?(off = 0) ?len buf =
 
   let ctx =
     match kind with
-    | `A -> Digestif.SHA1.feed_string ctx (Fmt.strf "commit %d\000" len)
-    | `B -> Digestif.SHA1.feed_string ctx (Fmt.strf "tree %d\000" len)
-    | `C -> Digestif.SHA1.feed_string ctx (Fmt.strf "blob %d\000" len)
-    | `D -> Digestif.SHA1.feed_string ctx (Fmt.strf "tag %d\000" len)
+    | `A -> Digestif.SHA1.feed_string ctx (Fmt.str "commit %d\000" len)
+    | `B -> Digestif.SHA1.feed_string ctx (Fmt.str "tree %d\000" len)
+    | `C -> Digestif.SHA1.feed_string ctx (Fmt.str "blob %d\000" len)
+    | `D -> Digestif.SHA1.feed_string ctx (Fmt.str "tag %d\000" len)
   in
   let ctx = Digestif.SHA1.feed_bigstring ctx ~off ~len buf in
   Digestif.SHA1.get ctx
diff --git a/test/mem/test.ml b/test/mem/test.ml
index 3f49d5ba3..74351b48c 100644
--- a/test/mem/test.ml
+++ b/test/mem/test.ml
@@ -55,7 +55,7 @@ let store =
   Arg.conv (parser, pp)
 
 let random =
-  let v = Fpath.v (Fmt.strf "git-%06x" (Random.bits () land 0xffffff)) in
+  let v = Fpath.v (Fmt.str "git-%06x" (Random.bits () land 0xffffff)) in
   match Lwt_main.run (Store.v v) with
   | Ok v -> v
   | Error err -> Fmt.failwith "empty store: %a" Store.pp_error err
diff --git a/test/smart/store_backend.ml b/test/smart/store_backend.ml
index b82cbaf74..68d9bb16e 100644
--- a/test/smart/store_backend.ml
+++ b/test/smart/store_backend.ml
@@ -13,7 +13,7 @@ type git = Store.t
 
 let store_prj = Store.prj
 let store_inj = Store.inj
-let failwithf fmt = Fmt.kstrf (fun err -> raise (Failure err)) fmt
+let failwithf fmt = Fmt.kstr (fun err -> raise (Failure err)) fmt
 
 let kind_of_object path uid =
   let open Bos in
@@ -160,7 +160,7 @@ let commit_of_tag path uid =
       fun () ->
         out_string ~trim:true
           (run_out ~err:err_null
-             Cmd.(v "git" % "rev-parse" % Fmt.strf "%s^{commit}" uid)))
+             Cmd.(v "git" % "rev-parse" % Fmt.str "%s^{commit}" uid)))
     ()
   |> R.join
   |> function
@@ -304,7 +304,7 @@ let parents :
             | Ok None ->
                 assert false
                 (* XXX(dinosaure): impossible, [git] can not give to us unknown object. *)
-            | Error err -> Stdlib.raise (Failure (Fmt.strf "%a" R.pp_msg err)) )
+            | Error err -> Stdlib.raise (Failure (Fmt.str "%a" R.pp_msg err)) )
       in
       try
         let objs = List.map map uids in
@@ -400,7 +400,7 @@ let parse_shallow ic () =
 
 let save_shallow oc lst =
   let ppf = Format.formatter_of_out_channel oc in
-  let str = Fmt.strf "%a" Fmt.(list ~sep:(always "\n") Uid.pp) lst in
+  let str = Fmt.str "%a" Fmt.(list ~sep:(always "\n") Uid.pp) lst in
   Log.debug (fun m -> m "Want to save: %S." str);
   Fmt.pf ppf "%a%!" Fmt.(list ~sep:(always "\n") Uid.pp) lst;
   Rresult.R.ok ()
diff --git a/test/smart/test.ml b/test/smart/test.ml
index 9adf7c1cc..1d3c36310 100644
--- a/test/smart/test.ml
+++ b/test/smart/test.ml
@@ -84,14 +84,14 @@ let create_tmp_dir ?(mode = 0o700) ?prefix_path pat =
   let failed_too_many_times () =
     Rresult.R.error_msgf
       "create temporary directory %s in %a: too many failing attempts"
-      (Fmt.strf pat "XXXXXX") Fpath.pp dir
+      (Fmt.str pat "XXXXXX") Fpath.pp dir
   in
   let rec loop count =
     if count < 0 then failed_too_many_times ()
     else
       let dir =
         let rand = Random.bits () land 0xffffff in
-        Fpath.(dir / Fmt.strf pat (Fmt.strf "%06x" rand))
+        Fpath.(dir / Fmt.str pat (Fmt.str "%06x" rand))
       in
       try
         Ok
@@ -102,7 +102,7 @@ let create_tmp_dir ?(mode = 0o700) ?prefix_path pat =
       | Unix.Unix_error (Unix.EINTR, _, _) -> loop count
       | Unix.Unix_error (e, _, _) ->
           Rresult.R.error_msgf "create temporary directory %s in %a: %s"
-            (Fmt.strf pat "XXXXXX") Fpath.pp dir (Unix.error_message e)
+            (Fmt.str pat "XXXXXX") Fpath.pp dir (Unix.error_message e)
   in
   match loop 10000 with
   | Ok dir as r ->
@@ -526,7 +526,7 @@ let test_fetch_empty () =
             / ".git"
             / "objects"
             / "pack"
-            / Fmt.strf "pack-%a.pack" Uid.pp uid)
+            / Fmt.str "pack-%a.pack" Uid.pp uid)
         in
         Bos.OS.Path.move tmp1 dst |> Lwt.return >>? fun () ->
         let dst =
@@ -535,7 +535,7 @@ let test_fetch_empty () =
             / ".git"
             / "objects"
             / "pack"
-            / Fmt.strf "pack-%a.idx" Uid.pp uid)
+            / Fmt.str "pack-%a.idx" Uid.pp uid)
         in
         Bos.OS.Path.move tmp2 dst |> Lwt.return >>? fun () ->
         let update (refname, uid) =
@@ -1076,14 +1076,14 @@ let create_fifo_path mode dir pat =
   let err () =
     Rresult.R.error_msgf
       "create temporary fifo %s in %a: too many failing attempts"
-      (Fmt.strf pat "XXXXXX") Fpath.pp dir
+      (Fmt.str pat "XXXXXX") Fpath.pp dir
   in
   let rec loop count =
     if count < 0 then err ()
     else
       let file =
         let rand = Random.bits () land 0xffffff in
-        Fpath.(dir / Fmt.strf pat (Fmt.strf "%06x" rand))
+        Fpath.(dir / Fmt.str pat (Fmt.str "%06x" rand))
       in
       let sfile = Fpath.to_string file in
       try Ok (file, Unix.mkfifo sfile mode) with
@@ -1560,7 +1560,7 @@ let test_partial_fetch_ssh () =
             / ".git"
             / "objects"
             / "pack"
-            / Fmt.strf "pack-%a.pack" Uid.pp uid)
+            / Fmt.str "pack-%a.pack" Uid.pp uid)
         in
         Bos.OS.Path.move tmp1 dst |> Lwt.return >>? fun () ->
         let dst =
@@ -1569,7 +1569,7 @@ let test_partial_fetch_ssh () =
             / ".git"
             / "objects"
             / "pack"
-            / Fmt.strf "pack-%a.idx" Uid.pp uid)
+            / Fmt.str "pack-%a.idx" Uid.pp uid)
         in
         Bos.OS.Path.move tmp2 dst |> Lwt.return >>? fun () ->
         let update (refname, uid) =
diff --git a/test/test_store.ml b/test/test_store.ml
index 8ce0ac10f..8195fbc75 100644
--- a/test/test_store.ml
+++ b/test/test_store.ml
@@ -136,16 +136,16 @@ struct
     x >>= function
     | Ok x -> f x
     | Error err ->
-        Fmt.kstrf (fun err -> Lwt.fail (Failure err)) "%a" Store.pp_error err
+        Fmt.kstr (fun err -> Lwt.fail (Failure err)) "%a" Store.pp_error err
 
   let check_write store name k v =
     let open Lwt.Infix in
     Store.write store v >>? fun (k', _) ->
-    Alcotest.(check hash) (Fmt.strf "set %s" name) k k';
+    Alcotest.(check hash) (Fmt.str "set %s" name) k k';
     Store.read_exn store k' >>= fun v' ->
-    Alcotest.(check value) (Fmt.strf "get %s" name) v v';
+    Alcotest.(check value) (Fmt.str "get %s" name) v v';
     Store.write store v >>? fun (k'', _) ->
-    Alcotest.(check hash) (Fmt.strf "set %s" name) k' k'';
+    Alcotest.(check hash) (Fmt.str "set %s" name) k' k'';
     Lwt.return_unit
 
   module Search = Git.Search.Make (Digestif) (Store)
@@ -333,7 +333,7 @@ struct
     Store.Ref.resolve store refa >>= function
     | Ok _ -> Alcotest.failf "Unexpected ok value"
     | Error err ->
-        let err = Fmt.strf "%a" Store.pp_error err in
+        let err = Fmt.str "%a" Store.pp_error err in
         Alcotest.(check string) "cycle" err _err_cycle;
         Lwt.return_unit
 
diff --git a/test/unix/test.ml b/test/unix/test.ml
index 69481b0f9..19760760f 100644
--- a/test/unix/test.ml
+++ b/test/unix/test.ml
@@ -48,7 +48,7 @@ let git_object_exist path hash =
       Bos.OS.Cmd.run
         Bos.Cmd.(v "git" % "cat-file" % "-e" % Digestif.SHA1.to_hex hash)
       >>= fun () ->
-      Alcotest.(check pass) (Fmt.strf "%a" Digestif.SHA1.pp hash) () ();
+      Alcotest.(check pass) (Fmt.str "%a" Digestif.SHA1.pp hash) () ();
       R.ok ()
 
 let check_blobs_with_git =