diff --git a/src/tef/collector_tef.ml b/src/tef/collector_tef.ml
new file mode 100644
index 0000000..fd390b0
--- /dev/null
+++ b/src/tef/collector_tef.ml
@@ -0,0 +1,195 @@
+open Common_
+open Types
+open Trace_core
+
+module Buf_pool = struct
+  type t = Buffer.t Trace_util.Rpool.t
+
+  let create ?(max_size = 32) ?(buf_size = 256) () : t =
+    Trace_util.Rpool.create ~max_size ~clear:Buffer.reset
+      ~create:(fun () -> Buffer.create buf_size)
+      ()
+end
+
+open struct
+  let[@inline] time_us_of_time_ns (t : int64) : float =
+    Int64.div t 1_000L |> Int64.to_float
+end
+
+let on_tracing_error = ref (fun s -> Printf.eprintf "%s\n%!" s)
+
+(*
+type span_info = {
+  tid: int;
+  name: string;
+  start_us: float;
+  mutable data: (string * user_data) list;
+      (* NOTE: thread safety: this is supposed to only be modified by the thread
+that's running this (synchronous, stack-abiding) span. *)
+}
+(** Information we store about a span begin event, to emit a complete event when
+    we meet the corresponding span end event *)
+*)
+
+type t = {
+  active: bool A.t;
+  pid: int;
+  buf_pool: Buf_pool.t;
+  exporter: Exporter.t;
+  span_id_gen: Trace_util.Span_id64.Gen.t;
+  trace_id_gen: Trace_util.Trace_id64.Gen.t;
+}
+(** Subscriber state *)
+
+(* TODO: this is nice to have, can we make it optional?
+open struct
+  let print_non_closed_spans_warning spans =
+    let module Str_set = Set.Make (String) in
+    let spans = Span_tbl.to_list spans in
+    if spans <> [] then (
+      !on_tracing_error
+      @@ Printf.sprintf "trace-tef: warning: %d spans were not closed"
+           (List.length spans);
+      let names =
+        List.fold_left
+          (fun set (_, span) -> Str_set.add span.name set)
+          Str_set.empty spans
+      in
+      Str_set.iter
+        (fun name ->
+          !on_tracing_error @@ Printf.sprintf "  span %S was not closed" name)
+        names;
+      flush stderr
+    )
+end
+*)
+
+let close (self : t) : unit =
+  if A.exchange self.active false then
+    (* FIXME: print_non_closed_spans_warning self.spans; *)
+    self.exporter.close ()
+
+let[@inline] active self = A.get self.active
+let[@inline] flush (self : t) : unit = self.exporter.flush ()
+
+let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
+  {
+    active = A.make true;
+    exporter;
+    buf_pool;
+    pid;
+    span_id_gen = Trace_util.Span_id64.Gen.create ();
+    trace_id_gen = Trace_util.Trace_id64.Gen.create ();
+  }
+
+open struct
+  type st = t
+
+  let rec flavor_of_params = function
+    | [] -> `Sync
+    | Core_ext.Extension_span_flavor f :: _ -> f
+    | _ :: tl -> flavor_of_params tl
+
+  let new_span_id (self : st) = Trace_util.Span_id64.Gen.gen self.span_id_gen
+  let new_trace_id (self : st) = Trace_util.Trace_id64.Gen.gen self.trace_id_gen
+  let init _ = ()
+  let shutdown (self : st) = close self
+
+  (* add function name, if provided, to the metadata *)
+  let add_fun_name_ fun_name data : _ list =
+    match fun_name with
+    | None -> data
+    | Some f -> ("function", `String f) :: data
+
+  let enter_span (self : st) ~__FUNCTION__ ~__FILE__ ~__LINE__ ~params ~data
+      ~parent name : span =
+    let start_us = time_us_of_time_ns @@ Trace_util.Mock_.now_ns () in
+    let flavor = flavor_of_params params in
+
+    let pid = self.pid in
+    let tid = Trace_util.Mock_.get_tid () in
+    match flavor with
+    | `Sync -> Span_tef_sync { name; pid; tid; args = data; start_us }
+    | `Async ->
+      let trace_id =
+        match parent with
+        | P_some (Span_tef_async sp) -> sp.trace_id
+        | _ -> new_trace_id self
+      in
+
+      (let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+       Writer.emit_begin_async buf ~name ~pid ~tid ~trace_id ~ts:start_us
+         ~args:data;
+       self.exporter.on_json buf);
+
+      Span_tef_async { pid; tid; trace_id; name; args = data }
+
+  let exit_span (self : st) sp =
+    let end_time_us = time_us_of_time_ns @@ Trace_util.Mock_.now_ns () in
+
+    let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+    let did_write =
+      match sp with
+      | Span_tef_sync { name; pid; tid; args; start_us } ->
+        (* emit full event *)
+        Writer.emit_duration_event buf ~pid ~tid ~name ~start:start_us
+          ~end_:end_time_us ~args;
+        true
+      | Span_tef_async { name; trace_id; pid; tid; args } ->
+        Writer.emit_end_async buf ~pid ~tid ~name ~trace_id ~ts:end_time_us
+          ~args;
+        true
+      | _ -> false
+    in
+
+    if did_write then self.exporter.on_json buf
+
+  let message (self : st) ~params:_ ~data ~span:_ msg : unit =
+    let tid = Trace_util.Mock_.get_tid () in
+    let time_us = time_us_of_time_ns @@ Trace_util.Mock_.now_ns () in
+    let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+    Writer.emit_instant_event buf ~pid:self.pid ~tid ~name:msg ~ts:time_us
+      ~args:data;
+    self.exporter.on_json buf
+
+  let counter_float (self : st) ~params:_ ~data:_ name n : unit =
+    let tid = Trace_util.Mock_.get_tid () in
+    let time_us = time_us_of_time_ns @@ Trace_util.Mock_.now_ns () in
+    let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+    Writer.emit_counter buf ~pid:self.pid ~name ~tid ~ts:time_us n;
+    self.exporter.on_json buf
+
+  let counter_int (self : st) ~params ~data name n : unit =
+    counter_float self ~params ~data name (float_of_int n)
+
+  let add_data_to_span _st sp data =
+    match sp with
+    | Span_tef_sync sp -> sp.args <- List.rev_append data sp.args
+    | Span_tef_async sp -> sp.args <- List.rev_append data sp.args
+    | _ -> ()
+
+  let on_name_thread_ (self : st) ~tid name : unit =
+    let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+    Writer.emit_name_thread buf ~pid:self.pid ~tid ~name;
+    self.exporter.on_json buf
+
+  let on_name_process_ (self : st) name : unit =
+    let@ buf = Trace_util.Rpool.with_ self.buf_pool in
+    Writer.emit_name_process ~pid:self.pid ~name buf;
+    self.exporter.on_json buf
+
+  let extension (self : st) ev =
+    match ev with
+    | Core_ext.Extension_set_thread_name name ->
+      let tid = Trace_util.Mock_.get_tid () in
+      on_name_thread_ self ~tid name
+    | Core_ext.Extension_set_process_name name -> on_name_process_ self name
+    | _ -> ()
+end
+
+let callbacks_collector : _ Collector.Callbacks.t =
+  Collector.Callbacks.make ~init ~shutdown ~enter_span ~exit_span ~message
+    ~add_data_to_span ~counter_int ~counter_float ~extension ()
+
+let collector (self : t) : Collector.t =
+  Collector.C_some (self, callbacks_collector)
diff --git a/src/tef/subscriber.mli b/src/tef/collector_tef.mli
similarity index 74%
rename from src/tef/subscriber.mli
rename to src/tef/collector_tef.mli
index 1cc1af3..1d94a5e 100644
--- a/src/tef/subscriber.mli
+++ b/src/tef/collector_tef.mli
@@ -1,3 +1,4 @@
+open Trace_core
 open Common_
 
 module Buf_pool : sig
@@ -7,19 +8,19 @@ module Buf_pool : sig
 end
 
 type t
-(** Main subscriber state. *)
+(** Main state. *)
 
 val create : ?buf_pool:Buf_pool.t -> pid:int -> exporter:Exporter.t -> unit -> t
-(** Create a subscriber state. *)
+(** Create a fresh state. *)
 
 val flush : t -> unit
 val close : t -> unit
 val active : t -> bool
 
-val sub_callbacks : t Sub.Callbacks.t
+val callbacks_collector : t Collector.Callbacks.t
 (** Callbacks used for the subscriber *)
 
-val subscriber : t -> Sub.t
+val collector : t -> Collector.t
 (** Subscriber that writes json into this writer *)
 
 (**/**)
diff --git a/src/tef/common_.ml b/src/tef/common_.ml
index 2148902..169190b 100644
--- a/src/tef/common_.ml
+++ b/src/tef/common_.ml
@@ -1,6 +1,3 @@
-module Sub = Trace_subscriber
 module A = Trace_core.Internal_.Atomic_
 
 let ( let@ ) = ( @@ )
-
-type span_id = Sub.Span_sub.span_id
diff --git a/src/tef/dune b/src/tef/dune
index 2e036f9..2f3b5a8 100644
--- a/src/tef/dune
+++ b/src/tef/dune
@@ -3,11 +3,4 @@
  (public_name trace-tef)
  (synopsis
   "Simple and lightweight tracing using TEF/Catapult format, in-process")
- (libraries
-  trace.core
-  trace.private.util
-  trace.subscriber
-  mtime
-  mtime.clock.os
-  unix
-  threads))
+ (libraries trace.core trace.util mtime mtime.clock.os unix threads))
diff --git a/src/tef/emit_tef.ml b/src/tef/emit_tef.ml
deleted file mode 100644
index e69de29..0000000
diff --git a/src/tef/exporter.ml b/src/tef/exporter.ml
index 561d80d..8507c0e 100644
--- a/src/tef/exporter.ml
+++ b/src/tef/exporter.ml
@@ -1,17 +1,10 @@
-(** An exporter, takes JSON objects and writes them somewhere *)
-
 open Common_
 
 type t = {
   on_json: Buffer.t -> unit;
-      (** Takes a buffer and writes it somewhere. The buffer is only valid
-          during this call and must not be stored. *)
-  flush: unit -> unit;  (** Force write *)
-  close: unit -> unit;  (** Close underlying resources *)
+  flush: unit -> unit;
+  close: unit -> unit;
 }
-(** An exporter, takes JSON objects and writes them somewhere.
-
-    This should be thread-safe if used in a threaded environment. *)
 
 open struct
   let with_lock lock f =
@@ -26,11 +19,6 @@ open struct
       Printexc.raise_with_backtrace e bt
 end
 
-(** Export to the channel
-    @param jsonl
-      if true, export as a JSON object per line, otherwise export as a single
-      big JSON array.
-    @param close_channel if true, closing the exporter will close the channel *)
 let of_out_channel ~close_channel ~jsonl oc : t =
   let lock = Mutex.create () in
   let first = ref true in
diff --git a/src/tef/exporter.mli b/src/tef/exporter.mli
new file mode 100644
index 0000000..6414f9c
--- /dev/null
+++ b/src/tef/exporter.mli
@@ -0,0 +1,22 @@
+(** An exporter, takes JSON objects and writes them somewhere *)
+
+type t = {
+  on_json: Buffer.t -> unit;
+      (** Takes a buffer and writes it somewhere. The buffer is only valid
+          during this call and must not be stored. *)
+  flush: unit -> unit;  (** Force write *)
+  close: unit -> unit;  (** Close underlying resources *)
+}
+(** An exporter, takes JSON objects and writes them somewhere.
+
+    This should be thread-safe if used in a threaded environment. *)
+
+val of_out_channel : close_channel:bool -> jsonl:bool -> out_channel -> t
+(** Export to the channel
+    @param jsonl
+      if true, export as a JSON object per line, otherwise export as a single
+      big JSON array.
+    @param close_channel if true, closing the exporter will close the channel *)
+
+val of_buffer : jsonl:bool -> Buffer.t -> t
+(** Emit into the buffer *)
diff --git a/src/tef/subscriber.ml b/src/tef/subscriber.ml
deleted file mode 100644
index 886a255..0000000
--- a/src/tef/subscriber.ml
+++ /dev/null
@@ -1,164 +0,0 @@
-open Common_
-open Trace_core
-open Trace_private_util
-module Span_sub = Sub.Span_sub
-
-module Buf_pool = struct
-  type t = Buffer.t Rpool.t
-
-  let create ?(max_size = 32) ?(buf_size = 256) () : t =
-    Rpool.create ~max_size ~clear:Buffer.reset
-      ~create:(fun () -> Buffer.create buf_size)
-      ()
-end
-
-open struct
-  let[@inline] time_us_of_time_ns (t : int64) : float =
-    Int64.div t 1_000L |> Int64.to_float
-end
-
-let on_tracing_error = ref (fun s -> Printf.eprintf "%s\n%!" s)
-
-(*
-type span_info = {
-  tid: int;
-  name: string;
-  start_us: float;
-  mutable data: (string * user_data) list;
-      (* NOTE: thread safety: this is supposed to only be modified by the thread
-that's running this (synchronous, stack-abiding) span. *)
-}
-(** Information we store about a span begin event, to emit a complete event when
-    we meet the corresponding span end event *)
-*)
-
-type t = {
-  active: bool A.t;
-  pid: int;
-  buf_pool: Buf_pool.t;
-  exporter: Exporter.t;
-  span_id_gen: Sub.Span_id_generator.t;
-  trace_id_gen: Sub.Trace_id_generator.t;
-}
-(** Subscriber state *)
-
-(* TODO: this is nice to have, can we make it optional?
-open struct
-  let print_non_closed_spans_warning spans =
-    let module Str_set = Set.Make (String) in
-    let spans = Span_tbl.to_list spans in
-    if spans <> [] then (
-      !on_tracing_error
-      @@ Printf.sprintf "trace-tef: warning: %d spans were not closed"
-           (List.length spans);
-      let names =
-        List.fold_left
-          (fun set (_, span) -> Str_set.add span.name set)
-          Str_set.empty spans
-      in
-      Str_set.iter
-        (fun name ->
-          !on_tracing_error @@ Printf.sprintf "  span %S was not closed" name)
-        names;
-      flush stderr
-    )
-end
-*)
-
-let close (self : t) : unit =
-  if A.exchange self.active false then
-    (* FIXME: print_non_closed_spans_warning self.spans; *)
-    self.exporter.close ()
-
-let[@inline] active self = A.get self.active
-let[@inline] flush (self : t) : unit = self.exporter.flush ()
-
-let create ?(buf_pool = Buf_pool.create ()) ~pid ~exporter () : t =
-  {
-    active = A.make true;
-    exporter;
-    buf_pool;
-    pid;
-    span_id_gen = Sub.Span_id_generator.create ();
-    trace_id_gen = Sub.Trace_id_generator.create ();
-  }
-
-open struct
-  type st = t
-
-  let new_span_id (self : st) = Sub.Span_id_generator.gen self.span_id_gen
-  let new_trace_id (self : st) = Sub.Trace_id_generator.gen self.trace_id_gen
-  let on_init _ ~time_ns:_ = ()
-  let on_shutdown (self : st) ~time_ns:_ = close self
-
-  (* add function name, if provided, to the metadata *)
-  let add_fun_name_ fun_name data : _ list =
-    match fun_name with
-    | None -> data
-    | Some f -> ("function", `String f) :: data
-
-  let on_enter_span (self : st) (span : Span_sub.t) : unit =
-    match span.flavor with
-    | `Async ->
-      let { Span_sub.time_ns; data; tid; name; _ } = span in
-      let time_us = time_us_of_time_ns time_ns in
-      let data = add_fun_name_ span.__FUNCTION__ data in
-      let@ buf = Rpool.with_ self.buf_pool in
-      Writer.emit_begin buf ~pid:self.pid ~tid ~name ~id:span.trace_id
-        ~ts:time_us ~args:data ~flavor:span.flavor;
-      self.exporter.on_json buf
-    | `Sync -> () (* done at exit *)
-
-  let on_exit_span (self : st) ~time_ns:exit_time_ns ~tid:_ span : unit =
-    let { Span_sub.data; tid; flavor; name; _ } = span in
-    let exit_time_us = time_us_of_time_ns exit_time_ns in
-
-    let@ buf = Rpool.with_ self.buf_pool in
-    (match flavor with
-    | `Sync ->
-      (* emit full event *)
-      let start_time_us = time_us_of_time_ns span.time_ns in
-      Writer.emit_duration_event buf ~pid:self.pid ~tid ~name
-        ~start:start_time_us ~end_:exit_time_us ~args:data
-    | `Async ->
-      Writer.emit_end buf ~pid:self.pid ~tid ~name ~id:span.trace_id
-        ~ts:exit_time_us ~flavor ~args:data);
-
-    self.exporter.on_json buf
-
-  let on_message (self : st) ~time_ns ~tid ~span:_ ~params:_ ~data msg : unit =
-    let time_us = time_us_of_time_ns @@ time_ns in
-    let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_instant_event buf ~pid:self.pid ~tid ~name:msg ~ts:time_us
-      ~args:data;
-    self.exporter.on_json buf
-
-  let on_counter (self : st) ~time_ns ~tid ~params:_ ~data:_ ~name n : unit =
-    let time_us = time_us_of_time_ns @@ time_ns in
-    let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_counter buf ~pid:self.pid ~name ~tid ~ts:time_us n;
-    self.exporter.on_json buf
-
-  let on_name_thread_ (self : st) ~tid name : unit =
-    let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_name_thread buf ~pid:self.pid ~tid ~name;
-    self.exporter.on_json buf
-
-  let on_name_process_ (self : st) name : unit =
-    let@ buf = Rpool.with_ self.buf_pool in
-    Writer.emit_name_process ~pid:self.pid ~name buf;
-    self.exporter.on_json buf
-
-  let on_extension_event (self : st) ~time_ns:_ ~tid ev =
-    match ev with
-    | Core_ext.Extension_set_thread_name name -> on_name_thread_ self ~tid name
-    | Core_ext.Extension_set_process_name name -> on_name_process_ self name
-    | _ -> ()
-end
-
-let sub_callbacks : _ Sub.Callbacks.t =
-  Sub.Callbacks.make ~new_span_id ~new_trace_id ~on_init ~on_shutdown
-    ~on_enter_span ~on_exit_span ~on_message ~on_counter ~on_extension_event ()
-
-let subscriber (self : t) : Sub.t =
-  Sub.Subscriber.Sub { st = self; callbacks = sub_callbacks }
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index 39b9c06..a3935df 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -1,7 +1,8 @@
 open Trace_core
-module Subscriber = Subscriber
+module Collector_tef = Collector_tef
 module Exporter = Exporter
 module Writer = Writer
+module Types = Types
 
 let block_signals () =
   try
@@ -21,11 +22,11 @@ let block_signals () =
 
 (** Thread that simply regularly "ticks", sending events to the background
     thread so it has a chance to write to the file *)
-let tick_thread (sub : Subscriber.t) : unit =
+let tick_thread (c : Collector_tef.t) : unit =
   block_signals ();
-  while Subscriber.active sub do
+  while Collector_tef.active c do
     Thread.delay 0.5;
-    Subscriber.flush sub
+    Collector_tef.flush c
   done
 
 type output =
@@ -34,8 +35,8 @@ type output =
   | `File of string
   ]
 
-let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () :
-    Trace_subscriber.t =
+let collector_ ~(finally : unit -> unit) ~out ~(mode : [ `Single | `Jsonl ]) ()
+    : Collector.t =
   let jsonl = mode = `Jsonl in
   let oc, must_close =
     match out with
@@ -46,12 +47,7 @@ let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () :
       open_out_gen [ Open_creat; Open_wronly; Open_append ] 0o644 path, true
     | `Output oc -> oc, false
   in
-  let pid =
-    if !Trace_subscriber.Private_.mock then
-      2
-    else
-      Unix.getpid ()
-  in
+  let pid = Trace_util.Mock_.get_pid () in
 
   let exporter = Exporter.of_out_channel oc ~jsonl ~close_channel:must_close in
   let exporter =
@@ -63,17 +59,9 @@ let subscriber_ ~finally ~out ~(mode : [ `Single | `Jsonl ]) () :
           finally ());
     }
   in
-  let sub = Subscriber.create ~pid ~exporter () in
-  let _t_tick : Thread.t = Thread.create tick_thread sub in
-  Subscriber.subscriber sub
-
-let collector_ ~(finally : unit -> unit) ~(mode : [ `Single | `Jsonl ]) ~out ()
-    : collector =
-  let sub = subscriber_ ~finally ~mode ~out () in
-  Trace_subscriber.collector sub
-
-let[@inline] subscriber ~out () : Trace_subscriber.t =
-  subscriber_ ~finally:ignore ~mode:`Single ~out ()
+  let coll_st = Collector_tef.create ~pid ~exporter () in
+  let _t_tick : Thread.t = Thread.create tick_thread coll_st in
+  Collector_tef.collector coll_st
 
 let[@inline] collector ~out () : collector =
   collector_ ~finally:ignore ~mode:`Single ~out ()
@@ -111,29 +99,12 @@ let with_setup ?out () f =
   setup ?out ();
   Fun.protect ~finally:Trace_core.shutdown f
 
-module Mock_ = struct
-  let now = ref 0
-
-  (* used to mock timing *)
-  let get_now_ns () : int64 =
-    let x = !now in
-    incr now;
-    Int64.(mul (of_int x) 1000L)
-
-  let get_tid_ () : int = 3
-end
-
 module Private_ = struct
   let mock_all_ () =
-    Trace_subscriber.Private_.mock := true;
-    Trace_subscriber.Private_.get_now_ns_ := Mock_.get_now_ns;
-    Trace_subscriber.Private_.get_tid_ := Mock_.get_tid_;
+    Trace_util.Mock_.mock_all ();
     ()
 
-  let on_tracing_error = Subscriber.on_tracing_error
-
-  let subscriber_jsonl ~finally ~out () =
-    subscriber_ ~finally ~mode:`Jsonl ~out ()
+  let on_tracing_error = Collector_tef.on_tracing_error
 
   let collector_jsonl ~finally ~out () : collector =
     collector_ ~finally ~mode:`Jsonl ~out ()
diff --git a/src/tef/trace_tef.mli b/src/tef/trace_tef.mli
index 9cf8dd1..d9cfb16 100644
--- a/src/tef/trace_tef.mli
+++ b/src/tef/trace_tef.mli
@@ -1,6 +1,7 @@
-module Subscriber = Subscriber
+module Collector_tef = Collector_tef
 module Exporter = Exporter
 module Writer = Writer
+module Types = Types
 
 type output =
   [ `Stdout
@@ -14,10 +15,6 @@ type output =
     - [`File "foo"] will enable tracing and print events into file named "foo"
 *)
 
-val subscriber : out:[< output ] -> unit -> Trace_subscriber.t
-(** A subscriber emitting TEF traces into [out].
-    @since 0.8 *)
-
 val collector : out:[< output ] -> unit -> Trace_core.collector
 (** Make a collector that writes into the given output. See {!setup} for more
     details. *)
@@ -48,12 +45,6 @@ module Private_ : sig
 
   val on_tracing_error : (string -> unit) ref
 
-  val subscriber_jsonl :
-    finally:(unit -> unit) ->
-    out:[ `File_append of string | `Output of out_channel ] ->
-    unit ->
-    Trace_subscriber.t
-
   val collector_jsonl :
     finally:(unit -> unit) ->
     out:[ `File_append of string | `Output of out_channel ] ->
diff --git a/src/tef/types.ml b/src/tef/types.ml
new file mode 100644
index 0000000..535e6d4
--- /dev/null
+++ b/src/tef/types.ml
@@ -0,0 +1,20 @@
+open Trace_core
+module Trace_id = Trace_util.Trace_id64
+
+type trace_id = Trace_id.t
+
+type Trace_core.span +=
+  | Span_tef_sync of {
+      pid: int;
+      tid: int;
+      name: string;
+      start_us: float;
+      mutable args: (string * Trace_core.user_data) list;
+    }
+  | Span_tef_async of {
+      pid: int;
+      tid: int;
+      name: string;
+      trace_id: trace_id;
+      mutable args: (string * Trace_core.user_data) list;
+    }
diff --git a/src/tef/writer.ml b/src/tef/writer.ml
index 85d9bbe..ea2b75b 100644
--- a/src/tef/writer.ml
+++ b/src/tef/writer.ml
@@ -48,23 +48,17 @@ let emit_duration_event ~pid ~tid ~name ~start ~end_ ~args buf : unit =
     (emit_args_o_ pp_user_data_)
     args
 
-let emit_begin ~pid ~tid ~name ~(id : int64) ~ts ~args ~flavor buf : unit =
+let emit_begin_async ~pid ~tid ~name ~trace_id ~ts ~args buf =
   Printf.bprintf buf
     {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
-    pid id tid ts str_val name
-    (match flavor with
-    | `Async -> 'b'
-    | `Sync -> 'B')
+    pid trace_id tid ts str_val name 'b'
     (emit_args_o_ pp_user_data_)
     args
 
-let emit_end ~pid ~tid ~name ~(id : int64) ~ts ~args ~flavor buf : unit =
+let emit_end_async ~pid ~tid ~name ~(trace_id : int64) ~ts ~args buf : unit =
   Printf.bprintf buf
     {json|{"pid":%d,"cat":"trace","id":%Ld,"tid": %d,"ts": %.2f,"name":%a,"ph":"%c"%a}|json}
-    pid id tid ts str_val name
-    (match flavor with
-    | `Async -> 'e'
-    | `Sync -> 'E')
+    pid trace_id tid ts str_val name 'e'
     (emit_args_o_ pp_user_data_)
     args
 
diff --git a/src/tef/writer.mli b/src/tef/writer.mli
index 465545c..bb53767 100644
--- a/src/tef/writer.mli
+++ b/src/tef/writer.mli
@@ -4,6 +4,7 @@
     raw event data. *)
 
 open Common_
+open Types
 
 val emit_duration_event :
   pid:int ->
@@ -15,25 +16,23 @@ val emit_duration_event :
   Buffer.t ->
   unit
 
-val emit_begin :
+val emit_begin_async :
   pid:int ->
   tid:int ->
   name:string ->
-  id:span_id ->
+  trace_id:trace_id ->
   ts:float ->
   args:(string * Trace_core.user_data) list ->
-  flavor:[ `Sync | `Async ] ->
   Buffer.t ->
   unit
 
-val emit_end :
+val emit_end_async :
   pid:int ->
   tid:int ->
   name:string ->
-  id:span_id ->
+  trace_id:trace_id ->
   ts:float ->
   args:(string * Trace_core.user_data) list ->
-  flavor:[ `Sync | `Async ] ->
   Buffer.t ->
   unit