diff --git a/lib/request.ml b/lib/request.ml
index 977b714c7d..92f810f219 100644
--- a/lib/request.ml
+++ b/lib/request.ml
@@ -18,11 +18,11 @@
 open Sexplib.Std
 
 type t = {
-  mutable headers: Header.t;
-  mutable meth: Code.meth;
-  mutable uri: Uri.t;
-  mutable version: Code.version;
-  mutable encoding: Transfer.encoding;
+  headers: Header.t;
+  meth: Code.meth;
+  uri: Uri.t;
+  version: Code.version;
+  encoding: Transfer.encoding;
 } with fields, sexp
 
 let make ?(meth=`GET) ?(version=`HTTP_1_1) ?encoding ?headers uri =
diff --git a/lib/response.ml b/lib/response.ml
index bfaeb30e19..46ee84621c 100644
--- a/lib/response.ml
+++ b/lib/response.ml
@@ -18,11 +18,11 @@
 open Sexplib.Std
 
 type t = {
-  mutable encoding: Transfer.encoding;
-  mutable headers: Header.t;
-  mutable version: Code.version;
-  mutable status: Code.status_code;
-  mutable flush: bool;
+  encoding: Transfer.encoding;
+  headers: Header.t;
+  version: Code.version;
+  status: Code.status_code;
+  flush: bool;
 } with fields, sexp
 
 let make ?(version=`HTTP_1_1) ?(status=`OK) ?(flush=false) ?(encoding=Transfer.Chunked) ?headers () =
diff --git a/lib/s.mli b/lib/s.mli
index cd9e8eff3a..81c2a0af9d 100644
--- a/lib/s.mli
+++ b/lib/s.mli
@@ -98,11 +98,11 @@ end
 
 module type Request = sig
   type t = {
-    mutable headers: Header.t;    (** HTTP request headers *)
-    mutable meth: Code.meth;      (** HTTP request method *)
-    mutable uri: Uri.t;           (** Full HTTP request uri *)
-    mutable version: Code.version; (** HTTP version, usually 1.1 *)
-    mutable encoding: Transfer.encoding; (** transfer encoding of this HTTP request *)
+    headers: Header.t;    (** HTTP request headers *)
+    meth: Code.meth;      (** HTTP request method *)
+    uri: Uri.t;           (** Full HTTP request uri *)
+    version: Code.version; (** HTTP version, usually 1.1 *)
+    encoding: Transfer.encoding; (** transfer encoding of this HTTP request *)
   } with fields, sexp
 
   val make : ?meth:Code.meth -> ?version:Code.version ->
@@ -120,11 +120,11 @@ end
 
 module type Response = sig
   type t = {
-    mutable encoding: Transfer.encoding; (** Transfer encoding of this HTTP response *)
-    mutable headers: Header.t;    (** response HTTP headers *)
-    mutable version: Code.version; (** (** HTTP version, usually 1.1 *) *)
-    mutable status: Code.status_code; (** HTTP status code of the response *)
-    mutable flush: bool;
+    encoding: Transfer.encoding; (** Transfer encoding of this HTTP response *)
+    headers: Header.t;    (** response HTTP headers *)
+    version: Code.version; (** (** HTTP version, usually 1.1 *) *)
+    status: Code.status_code; (** HTTP status code of the response *)
+    flush: bool;
   } with fields, sexp
 
   val make :
diff --git a/lib_test/test_parser.ml b/lib_test/test_parser.ml
index 0eb4075bfe..a189d6d67a 100644
--- a/lib_test/test_parser.ml
+++ b/lib_test/test_parser.ml
@@ -268,7 +268,7 @@ let mutate_simple_req () =
   let open Cohttp_lwt_unix in
   let expected = "POST /foo/bar HTTP/1.1\r\nfoo: bar\r\nhost: localhost\r\ntransfer-encoding: chunked\r\n\r\n6\r\nfoobar\r\n0\r\n\r\n" in
   let req = Request.make ~encoding:Transfer.Chunked ~headers:(Header.init_with "foo" "bar") (Uri.of_string "/foo/bar") in
-  Request.set_meth req `POST;
+  let req = Fieldslib.Field.fset Request.Fields.meth req `POST in
   write_req expected req
 
 let make_simple_res () =