diff --git a/src/tef/event.ml b/src/tef/event.ml
new file mode 100644
index 0000000..17d8fff
--- /dev/null
+++ b/src/tef/event.ml
@@ -0,0 +1,54 @@
+open Trace_core
+
+type t =
+  | E_tick
+  | E_message of {
+      tid: int;
+      msg: string;
+      time_us: float;
+      data: (string * user_data) list;
+    }
+  | E_define_span of {
+      tid: int;
+      name: string;
+      time_us: float;
+      id: span;
+      fun_name: string option;
+      data: (string * user_data) list;
+    }
+  | E_exit_span of {
+      id: span;
+      time_us: float;
+    }
+  | E_add_data of {
+      id: span;
+      data: (string * user_data) list;
+    }
+  | E_enter_manual_span of {
+      tid: int;
+      name: string;
+      time_us: float;
+      id: int;
+      flavor: [ `Sync | `Async ] option;
+      fun_name: string option;
+      data: (string * user_data) list;
+    }
+  | E_exit_manual_span of {
+      tid: int;
+      name: string;
+      time_us: float;
+      flavor: [ `Sync | `Async ] option;
+      data: (string * user_data) list;
+      id: int;
+    }
+  | E_counter of {
+      name: string;
+      tid: int;
+      time_us: float;
+      n: float;
+    }
+  | E_name_process of { name: string }
+  | E_name_thread of {
+      tid: int;
+      name: string;
+    }
diff --git a/src/tef/trace_tef.ml b/src/tef/trace_tef.ml
index f4ffddd..03824ba 100644
--- a/src/tef/trace_tef.ml
+++ b/src/tef/trace_tef.ml
@@ -1,7 +1,10 @@
 open Trace_core
 open Trace_private_util
+open Event
 module A = Trace_core.Internal_.Atomic_
 
+let ( let@ ) = ( @@ )
+
 module Mock_ = struct
   let enabled = ref false
   let now = ref 0
@@ -25,59 +28,6 @@ let[@inline] now_us () : float =
 
 let on_tracing_error = ref (fun s -> Printf.eprintf "trace-tef error: %s\n%!" s)
 
-type event =
-  | E_tick
-  | E_message of {
-      tid: int;
-      msg: string;
-      time_us: float;
-      data: (string * user_data) list;
-    }
-  | E_define_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      id: span;
-      fun_name: string option;
-      data: (string * user_data) list;
-    }
-  | E_exit_span of {
-      id: span;
-      time_us: float;
-    }
-  | E_add_data of {
-      id: span;
-      data: (string * user_data) list;
-    }
-  | E_enter_manual_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      id: int;
-      flavor: [ `Sync | `Async ] option;
-      fun_name: string option;
-      data: (string * user_data) list;
-    }
-  | E_exit_manual_span of {
-      tid: int;
-      name: string;
-      time_us: float;
-      flavor: [ `Sync | `Async ] option;
-      data: (string * user_data) list;
-      id: int;
-    }
-  | E_counter of {
-      name: string;
-      tid: int;
-      time_us: float;
-      n: float;
-    }
-  | E_name_process of { name: string }
-  | E_name_thread of {
-      tid: int;
-      name: string;
-    }
-
 module Span_tbl = Hashtbl.Make (struct
   include Int64
 
@@ -104,7 +54,8 @@ let key_data : (string * user_data) list ref Meta_map.key =
 module Writer = struct
   type t = {
     oc: out_channel;
-    mutable first: bool;  (** first event? *)
+    jsonl: bool;  (** JSONL mode, one json event per line *)
+    mutable first: bool;  (** first event? useful in json mode *)
     buf: Buffer.t;  (** Buffer to write into *)
     must_close: bool;  (** Do we have to close the underlying channel [oc]? *)
     pid: int;
@@ -112,12 +63,15 @@ module Writer = struct
   (** A writer to a [out_channel]. It writes JSON entries in an array
       and closes the array at the end. *)
 
-  let create ~out () : t =
+  let create ~(mode : [ `Single | `Jsonl ]) ~out () : t =
+    let jsonl = mode = `Jsonl in
     let oc, must_close =
       match out with
       | `Stdout -> stdout, false
       | `Stderr -> stderr, false
       | `File path -> open_out path, true
+      | `File_append path ->
+        open_out_gen [ Open_creat; Open_wronly; Open_append ] 0o644 path, true
     in
     let pid =
       if !Mock_.enabled then
@@ -125,16 +79,19 @@ module Writer = struct
       else
         Unix.getpid ()
     in
-    output_char oc '[';
-    { oc; first = true; pid; must_close; buf = Buffer.create 2_048 }
+    if not jsonl then output_char oc '[';
+    { oc; jsonl; first = true; pid; must_close; buf = Buffer.create 2_048 }
 
   let close (self : t) : unit =
-    output_char self.oc ']';
+    if self.jsonl then
+      output_char self.oc '\n'
+    else
+      output_char self.oc ']';
     flush self.oc;
     if self.must_close then close_out self.oc
 
-  let with_ ~out f =
-    let writer = create ~out () in
+  let with_ ~mode ~out f =
+    let writer = create ~mode ~out () in
     Fun.protect ~finally:(fun () -> close writer) (fun () -> f writer)
 
   let[@inline] flush (self : t) : unit = flush self.oc
@@ -142,7 +99,9 @@ module Writer = struct
   (** Emit "," if we need, and get the buffer ready *)
   let emit_sep_and_start_ (self : t) =
     Buffer.reset self.buf;
-    if self.first then
+    if self.jsonl then
+      Buffer.add_char self.buf '\n'
+    else if self.first then
       self.first <- false
     else
       Buffer.add_string self.buf ",\n"
@@ -263,9 +222,9 @@ end
 (** Background thread, takes events from the queue, puts them
     in context using local state, and writes fully resolved
     TEF events to [out]. *)
-let bg_thread ~out (events : event B_queue.t) : unit =
+let bg_thread ~mode ~out (events : Event.t B_queue.t) : unit =
   (* open a writer to [out] *)
-  Writer.with_ ~out @@ fun writer ->
+  Writer.with_ ~mode ~out @@ fun writer ->
   (* local state, to keep track of span information and implicit stack context *)
   let spans : span_info Span_tbl.t = Span_tbl.create 32 in
 
@@ -277,7 +236,7 @@ let bg_thread ~out (events : event B_queue.t) : unit =
   in
 
   (* how to deal with an event *)
-  let handle_ev (ev : event) : unit =
+  let handle_ev (ev : Event.t) : unit =
     match ev with
     | E_tick -> Writer.flush writer
     | E_message { tid; msg; time_us; data } ->
@@ -346,7 +305,8 @@ type output =
   | `File of string
   ]
 
-let collector ~out () : collector =
+let collector_ ~(finally : unit -> unit) ~(mode : [ `Single | `Jsonl ]) ~out ()
+    : collector =
   let module M = struct
     let active = A.make true
 
@@ -354,10 +314,15 @@ let collector ~out () : collector =
     let span_id_gen_ = A.make 0
 
     (* queue of messages to write *)
-    let events : event B_queue.t = B_queue.create ()
+    let events : Event.t B_queue.t = B_queue.create ()
 
     (** writer thread. It receives events and writes them to [oc]. *)
-    let t_write : Thread.t = Thread.create (fun () -> bg_thread ~out events) ()
+    let t_write : Thread.t =
+      Thread.create
+        (fun () ->
+          let@ () = Fun.protect ~finally in
+          bg_thread ~mode ~out events)
+        ()
 
     (** ticker thread, regularly sends a message to the writer thread.
          no need to join it. *)
@@ -473,6 +438,9 @@ let collector ~out () : collector =
   end in
   (module M)
 
+let[@inline] collector ~out () : collector =
+  collector_ ~finally:ignore ~mode:`Single ~out ()
+
 let setup ?(out = `Env) () =
   match out with
   | `Stderr -> Trace_core.setup_collector @@ collector ~out:`Stderr ()
@@ -498,4 +466,9 @@ let with_setup ?out () f =
 module Internal_ = struct
   let mock_all_ () = Mock_.enabled := true
   let on_tracing_error = on_tracing_error
+
+  let collector_jsonl ~finally ~out () : collector =
+    collector_ ~finally ~mode:`Jsonl ~out ()
+
+  module Event = Event
 end
diff --git a/src/tef/trace_tef.mli b/src/tef/trace_tef.mli
index 3aaf060..03866f0 100644
--- a/src/tef/trace_tef.mli
+++ b/src/tef/trace_tef.mli
@@ -44,6 +44,14 @@ module Internal_ : sig
   (** use fake, deterministic timestamps, TID, PID *)
 
   val on_tracing_error : (string -> unit) ref
+
+  val collector_jsonl :
+    finally:(unit -> unit) ->
+    out:[ `File_append of string ] ->
+    unit ->
+    Trace_core.collector
+
+  module Event = Event
 end
 
 (**/**)