diff --git a/tests/unit/dune b/tests/unit/dune
index 4bc4f509..329becc8 100644
--- a/tests/unit/dune
+++ b/tests/unit/dune
@@ -1,5 +1,5 @@
 
-(test
-  (name t_util)
+(tests
+  (names t_util t_buf)
   (package tiny_httpd)
-  (libraries tiny_httpd qcheck-core qcheck-core.runner))
+  (libraries tiny_httpd qcheck-core qcheck-core.runner test_util))
diff --git a/tests/unit/t_buf.ml b/tests/unit/t_buf.ml
new file mode 100644
index 00000000..9ee0f685
--- /dev/null
+++ b/tests/unit/t_buf.ml
@@ -0,0 +1,27 @@
+open Test_util
+open Tiny_httpd_buf
+
+let spf = Printf.sprintf
+
+let () =
+  let b = create ~size:4 () in
+  add_string b "hello";
+  assert_eq ~to_string:(spf "%S") "hello" (contents b);
+
+  add_string b " world";
+  assert_eq ~to_string:(spf "%S") "hello world" (contents b);
+  ()
+
+let buffer_of_string str =
+  let buf = Buffer.create 32 in
+  Buffer.add_string buf str;
+  buf
+
+let () =
+  let b = create ~size:4 () in
+  add_buffer b (buffer_of_string "hello");
+  assert_eq ~to_string:(spf "%S") "hello" (contents b);
+
+  add_buffer b (buffer_of_string " world");
+  assert_eq ~to_string:(spf "%S") "hello world" (contents b);
+  ()
diff --git a/tests/unit/t_util.ml b/tests/unit/t_util.ml
index 556c5e6f..3ae913a4 100644
--- a/tests/unit/t_util.ml
+++ b/tests/unit/t_util.ml
@@ -1,25 +1,6 @@
-module Q = QCheck
-
-(* test utils *)
-let pp_res f = function
-  | Ok x -> f x
-  | Error e -> e
-
-let pp_res_query = Q.Print.(pp_res (list (pair string string)))
-
-let err_map f = function
-  | Ok x -> Ok (f x)
-  | Error e -> Error e
-
-let sort_l l = List.sort compare l
-let eq_sorted a b = err_map sort_l a = err_map sort_l b
-let is_ascii_char c = Char.code c < 128
-let assert_eq ?(cmp = ( = )) a b = assert (cmp a b)
-
+open Test_util
 open Tiny_httpd_util
 
-let qchecks = ref []
-let add_qcheck f = qchecks := f :: !qchecks
 let () = assert_eq "hello%20world" (percent_encode "hello world")
 let () = assert_eq "%23%25^%24%40^%40" (percent_encode "#%^$@^@")
 
@@ -67,4 +48,4 @@ let () =
          in
          eq_sorted (Ok l) (parse_query s))
 
-let () = exit @@ QCheck_base_runner.run_tests ~colors:false !qchecks
+let () = run_qcheck_and_exit ()
diff --git a/tests/unit/util/dune b/tests/unit/util/dune
new file mode 100644
index 00000000..fb97b15e
--- /dev/null
+++ b/tests/unit/util/dune
@@ -0,0 +1,5 @@
+
+(library
+  (name test_util)
+  (modules test_util)
+  (libraries qcheck-core qcheck-core.runner))
diff --git a/tests/unit/util/test_util.ml b/tests/unit/util/test_util.ml
new file mode 100644
index 00000000..ae225b00
--- /dev/null
+++ b/tests/unit/util/test_util.ml
@@ -0,0 +1,31 @@
+module Q = QCheck
+
+(* test utils *)
+let pp_res f = function
+  | Ok x -> f x
+  | Error e -> e
+
+let pp_res_query = Q.Print.(pp_res (list (pair string string)))
+
+let err_map f = function
+  | Ok x -> Ok (f x)
+  | Error e -> Error e
+
+let sort_l l = List.sort compare l
+let eq_sorted a b = err_map sort_l a = err_map sort_l b
+let is_ascii_char c = Char.code c < 128
+
+let assert_eq ?to_string ?(cmp = ( = )) a b =
+  let ok = cmp a b in
+  if not ok then (
+    (match to_string with
+    | Some f -> Printf.eprintf "failed: %s != %s\n%!" (f a) (f b)
+    | None -> ());
+    failwith "test failed"
+  )
+
+let qchecks = ref []
+let add_qcheck f = qchecks := f :: !qchecks
+
+let run_qcheck_and_exit () : 'a =
+  exit @@ QCheck_base_runner.run_tests ~colors:false !qchecks