diff --git a/dune-project b/dune-project
index 57d8bfb..03dbd9e 100644
--- a/dune-project
+++ b/dune-project
@@ -29,7 +29,6 @@
   dune
   pprint
   crunch
-  containers
   ppx_deriving
   js_of_ocaml
   js_of_ocaml-compiler
diff --git a/lib/00_core/Pinc_Core.ml b/lib/00_core/Pinc_Core.ml
index fe3ca71..d5d2945 100644
--- a/lib/00_core/Pinc_Core.ml
+++ b/lib/00_core/Pinc_Core.ml
@@ -2,6 +2,7 @@ include StdlibExtension
 module StringMap = StringMap
 module StringSet = StringSet
 module Identifier = Identifier
+module Utf8String = Utf8String
 
 module Dedent = struct
   let indentation =
@@ -25,12 +26,12 @@ module Dedent = struct
 
   let lines string =
     let lines = String.split_on_char '\n' string in
-    let lines = List.map Containers.String.rtrim lines in
+    let lines = List.map String.trim_right lines in
     let lines =
       match lines with
       | [] -> []
       | "" :: rest -> drop_indentation rest
-      | first :: rest -> Containers.String.ltrim first :: drop_indentation rest
+      | first :: rest -> String.trim_left first :: drop_indentation rest
     in
     lines
   ;;
diff --git a/lib/00_core/StdlibExtension.ml b/lib/00_core/StdlibExtension.ml
index af910e1..38ff91c 100644
--- a/lib/00_core/StdlibExtension.ml
+++ b/lib/00_core/StdlibExtension.ml
@@ -119,4 +119,30 @@ module String = struct
     in
     take_first index s
   ;;
+
+  let trim_left s =
+    let len = length s in
+    let i = ref 0 in
+    while !i < len && Char.is_whitespace (unsafe_get s !i) do
+      incr i
+    done;
+    let j = len - 1 in
+    if j >= !i then
+      sub s !i (j - !i + 1)
+    else
+      empty
+  ;;
+
+  let trim_right s =
+    let len = length s in
+    let i = 0 in
+    let j = ref (len - 1) in
+    while !j >= i && Char.is_whitespace (unsafe_get s !j) do
+      decr j
+    done;
+    if !j >= i then
+      sub s i (!j - i + 1)
+    else
+      empty
+  ;;
 end
diff --git a/lib/00_core/Utf8String.ml b/lib/00_core/Utf8String.ml
new file mode 100644
index 0000000..9c7aa66
--- /dev/null
+++ b/lib/00_core/Utf8String.ml
@@ -0,0 +1,197 @@
+type t = String.t
+
+let to_string x = x
+
+(** State for decoding *)
+module Dec = struct
+  type t = {
+    s : string;
+    len : int;
+    mutable i : int;
+  }
+
+  let make ?(idx = 0) (s : string) : t = { s; i = idx; len = String.length s }
+end
+
+(** Malformed string at given offset *)
+exception Malformed of string * int
+
+(* decode next char. Mutate state, calls [yield c] if a char [c] is
+   read, [stop ()] otherwise.
+   @raise Malformed if an invalid substring is met *)
+let next_ (type a) (st : Dec.t) ~(yield : Uchar.t -> a) ~(stop : unit -> a) () : a =
+  let open Dec in
+  let malformed st = raise (Malformed (st.s, st.i)) in
+  (* read a multi-byte character.
+     @param acc the accumulator (containing the first byte of the char)
+     @param n_bytes number of bytes to read (i.e. [width char - 1])
+     @param overlong minimal bound on second byte (to detect overlong encoding)
+  *)
+  let read_multi ?(overlong = 0) n_bytes acc =
+    (* inner loop j = 1..jmax *)
+    let rec aux j acc =
+      let c = Char.code st.s.[st.i + j] in
+      (* check that c is in 0b10xxxxxx *)
+      if c lsr 6 <> 0b10 then
+        malformed st;
+      (* overlong encoding? *)
+      if j = 1 && overlong <> 0 && c land 0b111111 < overlong then
+        malformed st;
+      (* except for first, each char gives 6 bits *)
+      let next = (acc lsl 6) lor (c land 0b111111) in
+      if j = n_bytes then
+        if
+          (* done reading the codepoint *)
+          Uchar.is_valid next
+        then (
+          st.i <- st.i + j + 1;
+          (* +1 for first char *)
+          yield (Uchar.unsafe_of_int next))
+        else
+          malformed st
+      else
+        aux (j + 1) next
+    in
+    assert (n_bytes >= 1);
+    (* is the string long enough to contain the whole codepoint? *)
+    if st.i + n_bytes < st.len then
+      aux 1 acc
+    (* start with j=1, first char is already processed! *)
+    else
+      (* char is truncated *)
+      malformed st
+  in
+  if st.i >= st.len then
+    stop ()
+  else (
+    let c = st.s.[st.i] in
+    (* find leading byte, and detect some impossible cases
+       according to https://en.wikipedia.org/wiki/Utf8#Codepage_layout *)
+    match c with
+    | '\000' .. '\127' ->
+        st.i <- 1 + st.i;
+        yield (Uchar.of_int @@ Char.code c) (* 0xxxxxxx *)
+    | '\194' .. '\223' -> read_multi 1 (Char.code c land 0b11111) (* 110yyyyy *)
+    | '\225' .. '\239' -> read_multi 2 (Char.code c land 0b1111) (* 1110zzzz *)
+    | '\241' .. '\244' -> read_multi 3 (Char.code c land 0b111) (* 11110uuu *)
+    | '\224' ->
+        (* overlong: if next byte is < than [0b001000000] then the char
+         would fit in 1 byte *)
+        read_multi ~overlong:0b00100000 2 (Char.code c land 0b1111)
+        (* 1110zzzz *)
+    | '\240' ->
+        (* overlong: if next byte is < than [0b000100000] then the char
+         would fit in 2 bytes *)
+        read_multi ~overlong:0b00010000 3 (Char.code c land 0b111)
+        (* 11110uuu *)
+    | '\128' .. '\193' (* 192,193 are forbidden *) | '\245' .. '\255' -> malformed st)
+;;
+
+let fold ?idx f acc s =
+  let st = Dec.make ?idx s in
+  let rec aux acc =
+    next_
+      st
+      ~yield:(fun x ->
+        let acc = f acc x in
+        aux acc)
+      ~stop:(fun () -> acc)
+      ()
+  in
+  aux acc
+;;
+
+let n_chars = fold (fun x _ -> x + 1) 0
+
+let to_list ?(idx = 0) s : Uchar.t list =
+  fold ~idx (fun acc x -> x :: acc) [] s |> List.rev
+;;
+
+(* Convert a code point (int) into a string;
+   There are various equally trivial versions of this around.
+*)
+
+let[@inline] uchar_to_bytes (c : Uchar.t) (f : char -> unit) : unit =
+  let c = Uchar.to_int c in
+  let mask = 0b111111 in
+  assert (Uchar.is_valid c);
+  if c <= 0x7f then
+    f (Char.unsafe_chr c)
+  else if c <= 0x7ff then (
+    f (Char.unsafe_chr (0xc0 lor (c lsr 6)));
+    f (Char.unsafe_chr (0x80 lor (c land mask))))
+  else if c <= 0xffff then (
+    f (Char.unsafe_chr (0xe0 lor (c lsr 12)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 6) land mask)));
+    f (Char.unsafe_chr (0x80 lor (c land mask))))
+  else if c <= 0x1fffff then (
+    f (Char.unsafe_chr (0xf0 lor (c lsr 18)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 12) land mask)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 6) land mask)));
+    f (Char.unsafe_chr (0x80 lor (c land mask))))
+  else (
+    f (Char.unsafe_chr (0xf8 lor (c lsr 24)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 18) land mask)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 12) land mask)));
+    f (Char.unsafe_chr (0x80 lor ((c lsr 6) land mask)));
+    f (Char.unsafe_chr (0x80 lor (c land mask))))
+;;
+
+(* number of bytes required to encode this codepoint. A skeleton version
+   of {!uchar_to_bytes}. *)
+let[@inline] uchar_num_bytes (c : Uchar.t) : int =
+  let c = Uchar.to_int c in
+  if c <= 0x7f then
+    1
+  else if c <= 0x7ff then
+    2
+  else if c <= 0xffff then
+    3
+  else if c <= 0x1fffff then
+    4
+  else
+    5
+;;
+
+let of_list l : t =
+  let len = List.fold_left (fun n c -> n + uchar_num_bytes c) 0 l in
+  if len > Sys.max_string_length then
+    invalid_arg "CCUtf8_string.of_list: string size limit exceeded";
+  let buf = Bytes.make len '\000' in
+  let i = ref 0 in
+  List.iter
+    (fun c ->
+      uchar_to_bytes c (fun byte ->
+          Bytes.unsafe_set buf !i byte;
+          incr i))
+    l;
+  assert (!i = len);
+  Bytes.unsafe_to_string buf
+;;
+
+let is_valid (s : string) : bool =
+  let exception Stop in
+  try
+    let st = Dec.make s in
+    while true do
+      next_ st ~yield:(fun _ -> ()) ~stop:(fun () -> raise Stop) ()
+    done;
+    assert false
+  with
+  | Malformed _ -> false
+  | Stop -> true
+;;
+
+let of_string_exn s =
+  if is_valid s then
+    s
+  else
+    invalid_arg "CCUtf8_string.of_string_exn"
+;;
+
+let of_string s =
+  if is_valid s then
+    Some s
+  else
+    None
+;;
diff --git a/lib/00_core/Utf8String.mli b/lib/00_core/Utf8String.mli
new file mode 100644
index 0000000..19b1e4b
--- /dev/null
+++ b/lib/00_core/Utf8String.mli
@@ -0,0 +1,8 @@
+type t
+
+val n_chars : t -> int
+val to_string : t -> string
+val to_list : ?idx:int -> t -> Uchar.t list
+val of_list : Uchar.t list -> t
+val of_string_exn : string -> t
+val of_string : string -> t option
diff --git a/lib/00_core/dune b/lib/00_core/dune
index 92a269b..a007f84 100644
--- a/lib/00_core/dune
+++ b/lib/00_core/dune
@@ -1,4 +1,4 @@
 (library
  (name Pinc_Core)
  (public_name pinc-lang.core)
- (libraries containers))
+ (libraries))
diff --git a/lib/pinc_backend/Externals.ml b/lib/pinc_backend/Externals.ml
index ff69a01..e81e47a 100644
--- a/lib/pinc_backend/Externals.ml
+++ b/lib/pinc_backend/Externals.ml
@@ -43,8 +43,7 @@ module PincString = struct
             value_loc
             (Printf.sprintf
                "The argument given to the String.length function is not of type string")
-      | Ok v ->
-          v |> Containers.Utf8_string.of_string_exn |> Containers.Utf8_string.n_chars
+      | Ok v -> v |> Pinc_Core.Utf8String.of_string_exn |> Pinc_Core.Utf8String.n_chars
     in
 
     let output = Helpers.Value.int result in
@@ -100,12 +99,12 @@ module PincString = struct
 
     let result =
       str
-      |> Containers.Utf8_string.of_string_exn
-      |> Containers.Utf8_string.to_list
-      |> Containers.List.drop offset
-      |> Containers.List.take length
-      |> Containers.Utf8_string.of_list
-      |> Containers.Utf8_string.to_string
+      |> Pinc_Core.Utf8String.of_string_exn
+      |> Pinc_Core.Utf8String.to_list
+      |> List.drop offset
+      |> List.take length
+      |> Pinc_Core.Utf8String.of_list
+      |> Pinc_Core.Utf8String.to_string
     in
 
     let output = Helpers.Value.string result in
diff --git a/lib/pinc_backend/Interpreter.ml b/lib/pinc_backend/Interpreter.ml
index bcfa786..631f00a 100644
--- a/lib/pinc_backend/Interpreter.ml
+++ b/lib/pinc_backend/Interpreter.ml
@@ -839,8 +839,8 @@ and eval_binary_bracket_access ~state left right =
       let output =
         try
           a
-          |> CCUtf8_string.of_string_exn
-          |> CCUtf8_string.to_list
+          |> Pinc_Core.Utf8String.of_string_exn
+          |> Pinc_Core.Utf8String.to_list
           |> Fun.flip List.nth b
           |> Helpers.Value.char
                ~loc:(Location.merge ~s:left.expression_loc ~e:right.expression_loc ())
@@ -1116,16 +1116,17 @@ and eval_for_in ~state ~index_ident ~ident ~reverse ~iterable body =
       let map s =
         if reverse then
           s
-          |> CCUtf8_string.to_seq
-          |> CCSeq.to_rev_list
-          |> List.to_seq
-          |> Seq.map (fun c -> c |> Helpers.Value.char ~loc:iterable_value.value_loc)
+          |> Pinc_Core.Utf8String.to_list
+          |> List.rev
+          |> List.map (fun c -> c |> Helpers.Value.char ~loc:iterable_value.value_loc)
         else
           s
-          |> CCUtf8_string.to_seq
-          |> Seq.map (fun c -> c |> Helpers.Value.char ~loc:iterable_value.value_loc)
+          |> Pinc_Core.Utf8String.to_list
+          |> List.map (fun c -> c |> Helpers.Value.char ~loc:iterable_value.value_loc)
+      in
+      let state, res =
+        s |> Pinc_Core.Utf8String.of_string_exn |> map |> List.to_seq |> loop ~state []
       in
-      let state, res = s |> CCUtf8_string.of_string_exn |> map |> loop ~state [] in
       state
       |> State.add_output ~output:(res |> Helpers.Value.list ~loc:body.expression_loc)
   | Portal l ->
diff --git a/lib/pinc_backend/dune b/lib/pinc_backend/dune
index 0dee9d1..2fc2b20 100644
--- a/lib/pinc_backend/dune
+++ b/lib/pinc_backend/dune
@@ -2,13 +2,7 @@
  (name Pinc_Backend)
  (public_name pinc-lang.backend)
  (flags :standard -open Pinc_Core)
- (libraries
-  unix
-  Pinc_Core
-  Pinc_Source
-  Pinc_Diagnostics
-  Pinc_Parser
-  containers)
+ (libraries Pinc_Core Pinc_Source Pinc_Diagnostics Pinc_Parser)
  (preprocess
   (pps ppx_deriving.show))
  (instrumentation
diff --git a/pinc-lang.opam b/pinc-lang.opam
index 177bc38..7ed46f6 100644
--- a/pinc-lang.opam
+++ b/pinc-lang.opam
@@ -12,7 +12,6 @@ depends: [
   "dune" {>= "3.17"}
   "pprint"
   "crunch"
-  "containers"
   "ppx_deriving"
   "js_of_ocaml"
   "js_of_ocaml-compiler"
diff --git a/pinc-lang.opam.locked b/pinc-lang.opam.locked
index d093ff1..79536c0 100644
--- a/pinc-lang.opam.locked
+++ b/pinc-lang.opam.locked
@@ -9,77 +9,81 @@ homepage: "https://github.com/pinc-official/pinc-lang"
 bug-reports: "https://github.com/pinc-official/pinc-lang/issues"
 depends: [
   "astring" {= "0.8.5" & with-dev-setup}
-  "base" {= "v0.17.1" & with-dev-setup}
+  "base" {= "v0.17.3" & with-dev-setup}
   "base-bigarray" {= "base"}
-  "base-bytes" {= "base" & with-dev-setup}
   "base-domains" {= "base"}
   "base-effects" {= "base"}
   "base-nnp" {= "base"}
   "base-threads" {= "base"}
   "base-unix" {= "base"}
   "camlp-streams" {= "5.0.1" & with-dev-setup}
-  "chrome-trace" {= "3.17.2" & with-dev-setup}
-  "cmdliner" {= "1.3.0"}
-  "containers" {= "3.17"}
+  "chrome-trace" {= "3.23.1" & with-dev-setup}
+  "cmdliner" {= "2.1.1"}
   "cppo" {= "1.8.0"}
   "crunch" {= "4.0.0"}
-  "csexp" {= "1.5.2"}
-  "dune" {= "3.21.0"}
-  "dune-build-info" {= "3.17.2" & with-dev-setup}
-  "dune-configurator" {= "3.17.2"}
-  "dune-rpc" {= "3.21.0" & with-dev-setup}
-  "dyn" {= "3.21.0" & with-dev-setup}
-  "either" {= "1.0.0"}
+  "csexp" {= "1.5.2" & with-dev-setup}
+  "dune" {= "3.23.1"}
+  "dune-build-info" {= "3.23.1" & with-dev-setup}
+  "dune-configurator" {= "3.23.1" & with-dev-setup}
+  "dune-rpc" {= "3.23.1" & with-dev-setup}
+  "dyn" {= "3.23.1" & with-dev-setup}
+  "either" {= "1.0.0" & with-dev-setup}
   "fiber" {= "3.7.0" & with-dev-setup}
-  "fix" {= "20230505" & with-dev-setup}
-  "fmt" {= "0.9.0"}
+  "fix" {= "20250919" & with-dev-setup}
+  "fmt" {= "0.11.0"}
   "fpath" {= "0.7.3" & with-dev-setup}
-  "fs-io" {= "3.21.0" & with-dev-setup}
-  "jsonrpc" {= "1.25.0" & with-dev-setup}
-  "lsp" {= "1.25.0" & with-dev-setup}
-  "menhir" {= "20240715" & with-dev-setup}
-  "menhirCST" {= "20240715" & with-dev-setup}
-  "menhirLib" {= "20240715" & with-dev-setup}
-  "menhirSdk" {= "20240715" & with-dev-setup}
-  "merlin-lib" {= "5.6.1-504" & with-dev-setup}
-  "ocaml" {= "5.4.0"}
-  "ocaml-compiler" {= "5.4.0"}
+  "fs-io" {= "3.23.1" & with-dev-setup}
+  "gen" {= "1.1"}
+  "js_of_ocaml" {= "6.3.2"}
+  "js_of_ocaml-compiler" {= "6.3.2"}
+  "jsonrpc" {= "1.26.0" & with-dev-setup}
+  "lsp" {= "1.26.0" & with-dev-setup}
+  "menhir" {= "20260209"}
+  "menhirCST" {= "20260209"}
+  "menhirGLR" {= "20260209"}
+  "menhirLib" {= "20260209"}
+  "menhirSdk" {= "20260209"}
+  "merlin-lib" {= "5.7.1-504" & with-dev-setup}
+  "ocaml" {= "5.4.1"}
+  "ocaml-base-compiler" {= "5.4.1"}
+  "ocaml-compiler" {= "5.4.1"}
   "ocaml-compiler-libs" {= "v0.17.0"}
   "ocaml-config" {= "3"}
-  "ocaml-index" {= "5.6.1-504" & with-dev-setup}
-  "ocaml-lsp-server" {= "1.25.0" & with-dev-setup}
-  "ocaml-variants" {= "5.4.0+options"}
-  "ocaml-version" {= "4.0.3" & with-dev-setup}
-  "ocaml_intrinsics_kernel" {= "v0.17.1" & with-dev-setup}
-  "ocamlbuild" {= "0.15.0"}
-  "ocamlc-loc" {= "3.21.0" & with-dev-setup}
+  "ocaml-index" {= "5.7.1-504" & with-dev-setup}
+  "ocaml-lsp-server" {= "1.26.0" & with-dev-setup}
+  "ocaml-options-vanilla" {= "1"}
+  "ocaml-version" {= "4.1.1" & with-dev-setup}
+  "ocaml_intrinsics_kernel" {= "v0.17.2" & with-dev-setup}
+  "ocamlbuild" {= "0.16.1"}
+  "ocamlc-loc" {= "3.23.1" & with-dev-setup}
   "ocamlfind" {= "1.9.8"}
-  "ocamlformat" {= "0.28.1" & with-dev-setup}
-  "ocamlformat-lib" {= "0.28.1" & with-dev-setup}
-  "ocamlformat-rpc-lib" {= "0.27.0" & with-dev-setup}
-  "ocp-indent" {= "1.8.1" & with-dev-setup}
-  "ordering" {= "3.21.0" & with-dev-setup}
+  "ocamlformat" {= "0.29.0" & with-dev-setup}
+  "ocamlformat-lib" {= "0.29.0" & with-dev-setup}
+  "ocamlformat-rpc-lib" {= "0.29.0" & with-dev-setup}
+  "ocp-indent" {= "1.9.0" & with-dev-setup}
+  "ordering" {= "3.23.1" & with-dev-setup}
   "pp" {= "2.0.0" & with-dev-setup}
   "pprint" {= "20230830"}
   "ppx_derivers" {= "1.2.1"}
   "ppx_deriving" {= "6.1.1"}
   "ppx_yojson_conv_lib" {= "v0.17.0" & with-dev-setup}
-  "ppxlib" {= "0.37.0"}
+  "ppxlib" {= "0.38.0"}
   "ptime" {= "1.2.0"}
-  "re" {= "1.12.0" & with-dev-setup}
-  "seq" {= "base" & with-dev-setup}
+  "re" {= "1.14.0" & with-dev-setup}
+  "sedlex" {= "3.7"}
+  "seq" {= "base"}
   "sexplib0" {= "v0.17.0"}
   "spawn" {= "v0.17.0" & with-dev-setup}
   "stdio" {= "v0.17.0" & with-dev-setup}
   "stdlib-shims" {= "0.3.0"}
-  "stdune" {= "3.21.0" & with-dev-setup}
-  "top-closure" {= "3.21.0" & with-dev-setup}
-  "topkg" {= "1.0.7"}
-  "uucp" {= "16.0.0" & with-dev-setup}
-  "uuseg" {= "16.0.0" & with-dev-setup}
-  "uutf" {= "1.0.3" & with-dev-setup}
-  "xdg" {= "3.17.2" & with-dev-setup}
-  "yojson" {= "2.2.2" & with-dev-setup}
+  "stdune" {= "3.23.1" & with-dev-setup}
+  "top-closure" {= "3.23.1" & with-dev-setup}
+  "topkg" {= "1.1.1"}
+  "uucp" {= "17.0.0" & with-dev-setup}
+  "uuseg" {= "17.0.0" & with-dev-setup}
+  "uutf" {= "1.0.4" & with-dev-setup}
+  "xdg" {= "3.23.1" & with-dev-setup}
+  "yojson" {= "3.0.0"}
 ]
 build: [
   ["dune" "subst"] {dev}