diff --git a/src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml b/src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml
index b6266ad9..189b341e 100644
--- a/src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml
+++ b/src/client-cohttp-eio/opentelemetry_client_cohttp_eio.ml
@@ -15,8 +15,6 @@ let ( let@ ) = ( @@ )
 
 let spf = Printf.sprintf
 
-let tid () = Thread.id @@ Thread.self ()
-
 let set_headers = Config.Env.set_headers
 
 let get_headers = Config.Env.get_headers
@@ -330,7 +328,7 @@ let mk_emitter ~stop ~net (config : Config.t) : (module EMITTER) =
 
     let tick () =
       if Config.Env.get_debug () then
-        Printf.eprintf "tick (from %d)\n%!" (tid ());
+        Printf.eprintf "tick (from domain %d)\n%!" (Domain.self () :> int);
       run_tick_callbacks ();
       sample_gc_metrics_if_needed ();
       emit_all ~force:false
diff --git a/src/client/batch.ml b/src/client/batch.ml
index ba17e090..550479ba 100644
--- a/src/client/batch.ml
+++ b/src/client/batch.ml
@@ -1,13 +1,20 @@
 type 'a t = {
   mutable size: int;
   mutable q: 'a list;
-      (* The queue is a FIFO represented as a list in reverse order *)
-  batch: int; (* Minimum size to batch before popping *)
-  high_watermark: int;
+      (** The queue is a FIFO represented as a list in reverse order *)
+  batch: int;  (** Minimum size to batch before popping *)
+  high_watermark: int;  (** Size above which we start dropping signals *)
   timeout: Mtime.span option;
   mutable start: Mtime.t;
+  mutex: Mutex.t;
 }
 
+(* Mutex.protect was added in OCaml 5.1, but we want support back to 4.08.
+   cannot inline, otherwise flambda might move code around. (as per Stdlib) *)
+let[@inline never] protect_mutex m f =
+  Mutex.lock m;
+  Fun.protect f ~finally:(fun () -> Mutex.unlock m)
+
 let default_high_watermark batch_size =
   if batch_size = 1 then
     100
@@ -25,8 +32,9 @@ let make ?(batch = 1) ?high_watermark ?now ?timeout () : _ t =
     | Some x -> x
     | None -> Mtime_clock.now ()
   in
+  let mutex = Mutex.create () in
   assert (batch > 0);
-  { size = 0; q = []; start; batch; timeout; high_watermark }
+  { size = 0; q = []; start; batch; timeout; high_watermark; mutex }
 
 let timeout_expired_ ~now self : bool =
   match self.timeout with
@@ -42,24 +50,38 @@ let ready_to_pop ~force ~now self =
   self.size > 0 && (force || is_full_ self || timeout_expired_ ~now self)
 
 let pop_if_ready ?(force = false) ~now (self : _ t) : _ list option =
-  if ready_to_pop ~force ~now self then (
-    assert (self.q <> []);
-    let batch =
-      (* Reverse the list to retrieve the FIFO order. *)
-      List.rev self.q
-    in
-    self.q <- [];
-    self.size <- 0;
-    Some batch
-  ) else
-    None
+  let rev_batch_opt =
+    protect_mutex self.mutex @@ fun () ->
+    if ready_to_pop ~force ~now self then (
+      assert (self.q <> []);
+      let batch = self.q in
+      self.q <- [];
+      self.size <- 0;
+      Some batch
+    ) else
+      None
+  in
+  match rev_batch_opt with
+  | None -> None
+  | Some batch ->
+    (* Reverse the list to retrieve the FIFO order. *)
+    Some (List.rev batch)
 
 (* Helper so we can count new elements and prepend them onto the existing [q] in
    one pass. *)
 let append_with_count ~(elems : 'a list) ~(q : 'a list) : int * 'a list =
   elems |> List.fold_left (fun (count, q') x -> succ count, x :: q') (0, q)
 
+let rec push_unprotected (self : _ t) ~(elems : _ list) : unit =
+  match elems with
+  | [] -> ()
+  | x :: xs ->
+    self.q <- x :: self.q;
+    self.size <- 1 + self.size;
+    push_unprotected self ~elems:xs
+
 let push (self : _ t) elems : [ `Dropped | `Ok ] =
+  protect_mutex self.mutex @@ fun () ->
   if self.size >= self.high_watermark then
     (* drop this to prevent queue from growing too fast *)
     `Dropped
@@ -68,9 +90,7 @@ let push (self : _ t) elems : [ `Dropped | `Ok ] =
       (* current batch starts now *)
       self.start <- Mtime_clock.now ();
 
-    let count, q' = append_with_count ~elems ~q:self.q in
     (* add to queue *)
-    self.size <- self.size + count;
-    self.q <- q';
+    push_unprotected self ~elems;
     `Ok
   )
diff --git a/src/client/batch.mli b/src/client/batch.mli
index def675b1..8bd410fd 100644
--- a/src/client/batch.mli
+++ b/src/client/batch.mli
@@ -49,4 +49,4 @@ val pop_if_ready : ?force:bool -> now:Mtime.t -> 'a t -> 'a list option
 val push : 'a t -> 'a list -> [ `Dropped | `Ok ]
 (** [push b xs] is [`Ok] if it succeeds in pushing the values in [xs] into the batch
     [b], or [`Dropped] if the current size of the batch has exceeded the high water
-    mark determined by the [batch] argument to {!make}]. ) *)
+    mark determined by the [batch] argument to [{!make}]. ) *)
diff --git a/tests/bin/emit1_eio.ml b/tests/bin/emit1_eio.ml
index 83a9481e..9990b227 100644
--- a/tests/bin/emit1_eio.ml
+++ b/tests/bin/emit1_eio.ml
@@ -11,22 +11,22 @@ let sleep_outer = ref 2.0
 
 let n_jobs = ref 1
 
-let num_sleep = Atomic.make 0
-
 let stress_alloc_ = ref true
 
+let num_sleep = Atomic.make 0
+
 let stop = Atomic.make false
 
 let num_tr = Atomic.make 0
 
 (* Counter used to mark simulated failures *)
-let i = ref 0
+let i = Atomic.make 0
 
 let run_job clock _job_id iterations : unit =
   let@ scope =
     Atomic.incr num_tr;
     OT.Trace.with_ ~kind:OT.Span.Span_kind_producer "loop.outer"
-      ~attrs:[ "i", `Int !i ]
+      ~attrs:[ "i", `Int (Atomic.get i) ]
   in
 
   for j = 0 to iterations do
@@ -52,7 +52,7 @@ let run_job clock _job_id iterations : unit =
               ~severity:Severity_number_info "inner at %d" j;
           ]);
 
-      incr i;
+      Atomic.incr i;
 
       try
         Atomic.incr num_tr;
@@ -68,7 +68,7 @@ let run_job clock _job_id iterations : unit =
         let () = Eio.Time.sleep clock !sleep_inner in
         Atomic.incr num_sleep;
 
-        if j = 4 && !i mod 13 = 0 then failwith "oh no";
+        if j = 4 && Atomic.get i mod 13 = 0 then failwith "oh no";
 
         (* simulate a failure *)
         Opentelemetry.Scope.add_event scope (fun () ->