diff --git a/test/fuchsia/write/dune b/test/fuchsia/write/dune
new file mode 100644
index 0000000..e9f181a
--- /dev/null
+++ b/test/fuchsia/write/dune
@@ -0,0 +1,4 @@
+
+(tests
+  (names t1 t2)
+  (libraries trace-fuchsia.write))
diff --git a/test/fuchsia/write/t1.ml b/test/fuchsia/write/t1.ml
new file mode 100644
index 0000000..8b59e85
--- /dev/null
+++ b/test/fuchsia/write/t1.ml
@@ -0,0 +1,65 @@
+open Trace_fuchsia_write
+
+module Str_ = struct
+  open String
+
+  let to_hex (s : string) : string =
+    let i_to_hex (i : int) =
+      if i < 10 then
+        Char.chr (i + Char.code '0')
+      else
+        Char.chr (i - 10 + Char.code 'a')
+    in
+
+    let res = Bytes.create (2 * length s) in
+    for i = 0 to length s - 1 do
+      let n = Char.code (get s i) in
+      Bytes.set res (2 * i) (i_to_hex ((n land 0xf0) lsr 4));
+      Bytes.set res ((2 * i) + 1) (i_to_hex (n land 0x0f))
+    done;
+    Bytes.unsafe_to_string res
+
+  let of_hex_exn (s : string) : string =
+    let n_of_c = function
+      | '0' .. '9' as c -> Char.code c - Char.code '0'
+      | 'a' .. 'f' as c -> 10 + Char.code c - Char.code 'a'
+      | 'A' .. 'F' as c -> 10 + Char.code c - Char.code 'A'
+      | _ -> invalid_arg "string: invalid hex"
+    in
+    if String.length s mod 2 <> 0 then
+      invalid_arg "string: hex sequence must be of even length";
+    let res = Bytes.make (String.length s / 2) '\x00' in
+    for i = 0 to (String.length s / 2) - 1 do
+      let n1 = n_of_c (String.get s (2 * i)) in
+      let n2 = n_of_c (String.get s ((2 * i) + 1)) in
+      let n = (n1 lsl 4) lor n2 in
+      Bytes.set res i (Char.chr n)
+    done;
+    Bytes.unsafe_to_string res
+end
+
+let () =
+  let l = List.init 100 (fun i -> Util.round_to_word i) in
+  assert (List.for_all (fun x -> x mod 8 = 0) l)
+
+let () =
+  assert (Str_ref.inline 0 = 0b0000_0000_0000_0000);
+  assert (Str_ref.inline 1 = 0b1000_0000_0000_0001);
+  assert (Str_ref.inline 6 = 0b1000_0000_0000_0110);
+  assert (Str_ref.inline 31999 = 0b1111_1100_1111_1111);
+  ()
+
+let () =
+  let buf = Buf.create 128 in
+  Buf.add_i64 buf 42L;
+  assert (Buf.to_string buf = "\x2a\x00\x00\x00\x00\x00\x00\x00")
+
+let () =
+  let buf = Buf.create 128 in
+  Buf.add_string buf "";
+  assert (Buf.to_string buf = "")
+
+let () =
+  let buf = Buf.create 128 in
+  Buf.add_string buf "hello";
+  assert (Buf.to_string buf = "hello\x00\x00\x00")
diff --git a/test/fuchsia/write/t2.expected b/test/fuchsia/write/t2.expected
new file mode 100644
index 0000000..24c3856
--- /dev/null
+++ b/test/fuchsia/write/t2.expected
@@ -0,0 +1,4 @@
+first trace
+100004467854160033000500000000000100000000000000560000000000000054001005000005804e61bc000000000068656c6c6f000000210001802a0000007800000000000000
+second trace
+1000044678541600210000000000000000ca9a3b00000000330005000000000001000000000000005600000000000000300011000000b0006f63616d6c2d747261636500000000004400040500000580a0860100000000006f75746572000000404b4c0000000000440004050000058020bf020000000000696e6e657200000020aa44000000000054001005000005804e61bc000000000068656c6c6f000000210001802a0000007800000000000000
diff --git a/test/fuchsia/write/t2.ml b/test/fuchsia/write/t2.ml
new file mode 100644
index 0000000..9f08a70
--- /dev/null
+++ b/test/fuchsia/write/t2.ml
@@ -0,0 +1,75 @@
+open Trace_fuchsia_write
+
+let pf = Printf.printf
+
+module Str_ = struct
+  open String
+
+  let to_hex (s : string) : string =
+    let i_to_hex (i : int) =
+      if i < 10 then
+        Char.chr (i + Char.code '0')
+      else
+        Char.chr (i - 10 + Char.code 'a')
+    in
+
+    let res = Bytes.create (2 * length s) in
+    for i = 0 to length s - 1 do
+      let n = Char.code (get s i) in
+      Bytes.set res (2 * i) (i_to_hex ((n land 0xf0) lsr 4));
+      Bytes.set res ((2 * i) + 1) (i_to_hex (n land 0x0f))
+    done;
+    Bytes.unsafe_to_string res
+
+  let of_hex_exn (s : string) : string =
+    let n_of_c = function
+      | '0' .. '9' as c -> Char.code c - Char.code '0'
+      | 'a' .. 'f' as c -> 10 + Char.code c - Char.code 'a'
+      | 'A' .. 'F' as c -> 10 + Char.code c - Char.code 'A'
+      | _ -> invalid_arg "string: invalid hex"
+    in
+    if String.length s mod 2 <> 0 then
+      invalid_arg "string: hex sequence must be of even length";
+    let res = Bytes.make (String.length s / 2) '\x00' in
+    for i = 0 to (String.length s / 2) - 1 do
+      let n1 = n_of_c (String.get s (2 * i)) in
+      let n2 = n_of_c (String.get s ((2 * i) + 1)) in
+      let n = (n1 lsl 4) lor n2 in
+      Bytes.set res i (Char.chr n)
+    done;
+    Bytes.unsafe_to_string res
+end
+
+let () = pf "first trace\n"
+
+let () =
+  let buf = Buf.create 128 in
+  Metadata.Magic_record.encode buf;
+  Thread_record.encode buf ~as_ref:5 ~pid:1 ~tid:86 ();
+  Event.Instant.encode buf ~name:"hello" ~time_ns:1234_5678L
+    ~t_ref:(Thread_ref.Ref 5)
+    ~args:[ "x", `Int 42 ]
+    ();
+  pf "%s\n" (Buf.to_string buf |> Str_.to_hex)
+
+let () = pf "second trace\n"
+
+let () =
+  let buf = Buf.create 512 in
+  Metadata.Magic_record.encode buf;
+  Metadata.Initialization_record.(
+    encode buf ~ticks_per_secs:default_ticks_per_sec ());
+  Thread_record.encode buf ~as_ref:5 ~pid:1 ~tid:86 ();
+  Metadata.Provider_info.encode buf ~id:1 ~name:"ocaml-trace" ();
+  Event.Duration_complete.encode buf ~name:"outer" ~t_ref:(Thread_ref.Ref 5)
+    ~time_ns:100_000L ~end_time_ns:5_000_000L ~args:[] ();
+  Event.Duration_complete.encode buf ~name:"inner" ~t_ref:(Thread_ref.Ref 5)
+    ~time_ns:180_000L ~end_time_ns:4_500_000L ~args:[] ();
+  Event.Instant.encode buf ~name:"hello" ~time_ns:1_234_567L
+    ~t_ref:(Thread_ref.Ref 5)
+    ~args:[ "x", `Int 42 ]
+    ();
+  (let oc = open_out "foo.fxt" in
+   output_string oc (Buf.to_string buf);
+   close_out oc);
+  pf "%s\n" (Buf.to_string buf |> Str_.to_hex)