diff --git a/test/domains/dune b/test/domains/dune
new file mode 100644
index 0000000..88cfb01
--- /dev/null
+++ b/test/domains/dune
@@ -0,0 +1,28 @@
+; Marker library: only present on OCaml 5+, used as a proxy for Domain availability.
+
+(library
+ (name ocaml5)
+ (modules ocaml5)
+ (enabled_if
+  (>= %{ocaml_version} 5)))
+
+(executable
+ (name t_domains)
+ (modules t_domains)
+ (libraries
+  trace
+  ; use the marker library to pick between the real test and the quine fallback.
+  (select
+   t_domains.ml
+   from
+   (ocaml5 threads -> t_domains.real.ml)
+   (-> t_domains.quine.ml))))
+
+(rule
+ (alias runtest)
+ (action
+  (progn
+   (with-stdout-to
+    t_domains.output
+    (run %{exe:t_domains.exe} %{dep:t_domains.expected}))
+   (diff t_domains.expected t_domains.output))))
diff --git a/test/domains/ocaml5.ml b/test/domains/ocaml5.ml
new file mode 100644
index 0000000..cac7be6
--- /dev/null
+++ b/test/domains/ocaml5.ml
@@ -0,0 +1 @@
+(* Marker module: presence indicates OCaml 5+ (and thus Domain availability). *)
diff --git a/test/domains/t_domains.expected b/test/domains/t_domains.expected
new file mode 100644
index 0000000..aee8761
--- /dev/null
+++ b/test/domains/t_domains.expected
@@ -0,0 +1,5 @@
+=== domain concurrency ===
+domain-0: enter=10000 exit=10000 msg=10000
+domain-1: enter=10000 exit=10000 msg=10000
+domain-2: enter=10000 exit=10000 msg=10000
+domain-3: enter=10000 exit=10000 msg=10000
diff --git a/test/domains/t_domains.quine.ml b/test/domains/t_domains.quine.ml
new file mode 100644
index 0000000..060ba6c
--- /dev/null
+++ b/test/domains/t_domains.quine.ml
@@ -0,0 +1,10 @@
+(* When Domain is unavailable (OCaml < 5), echo the expected file
+   so the diff always passes. The file is passed as argv[1] by the dune rule. *)
+let () =
+  let ic = open_in Sys.argv.(1) in
+  (try
+     while true do
+       print_char (input_char ic)
+     done
+   with End_of_file -> ());
+  close_in ic
diff --git a/test/domains/t_domains.real.ml b/test/domains/t_domains.real.ml
new file mode 100644
index 0000000..a51b48f
--- /dev/null
+++ b/test/domains/t_domains.real.ml
@@ -0,0 +1,89 @@
+(* Test concurrent span recording across multiple domains (OCaml 5+).
+   Each domain runs [iters] spans.  We embed the recording thread's id into
+   the span value so that exit events can be attributed to the right domain,
+   then verify that every domain has exactly [iters] enters and exits. *)
+
+let ( let@ ) = ( @@ )
+let iters = 10_000
+
+(* Custom span type that carries the thread id of the recording domain. *)
+type Trace_core.span += Thread_span of int
+
+let make_recorder () =
+  let log = Queue.create () in
+  let mu = Mutex.create () in
+  let add x =
+    Mutex.lock mu;
+    Queue.add x log;
+    Mutex.unlock mu
+  in
+  let enter_span () ~__FUNCTION__:_ ~__FILE__:_ ~__LINE__:_ ~level:_ ~params:_
+      ~data:_ ~parent:_ _name =
+    let tid = Thread.id (Thread.self ()) in
+    add (`Enter tid);
+    Thread_span tid
+  in
+  let exit_span () sp =
+    match sp with
+    | Thread_span tid -> add (`Exit tid)
+    | _ -> ()
+  in
+  let message () ~level:_ ~params:_ ~data:_ ~span:_ _msg =
+    add (`Msg (Thread.id (Thread.self ())))
+  in
+  let metric () ~level:_ ~params:_ ~data:_ _name _m = () in
+  let add_data_to_span () _sp _data = () in
+  let coll =
+    Trace_core.Collector.(
+      C_some
+        ( (),
+          Callbacks.make ~enter_span ~exit_span ~add_data_to_span ~message
+            ~metric () ))
+  in
+  coll, fun () -> Queue.fold (fun acc x -> x :: acc) [] log |> List.rev
+
+let () =
+  print_endline "=== domain concurrency ===";
+  let coll, get = make_recorder () in
+  let@ () = Trace_core.with_setup_collector coll in
+  let n = 4 in
+  (* Each domain returns its own thread id so we can check per-domain counts. *)
+  let domains =
+    Array.init n (fun _i ->
+        Domain.spawn (fun () ->
+            let tid = Thread.id (Thread.self ()) in
+            for j = 1 to iters do
+              Trace_core.with_span ~__FILE__ ~__LINE__ "" @@ fun _ ->
+              Trace_core.message "";
+              if j mod 1000 = 0 then Thread.yield ()
+            done;
+            tid))
+  in
+  let tids = Array.map Domain.join domains in
+  let events = get () in
+  let count pred = List.length (List.filter pred events) in
+  (* For each domain, verify exactly [iters] enters and [iters] exits. *)
+  Array.iteri
+    (fun i tid ->
+      let n_enter =
+        count (function
+          | `Enter t -> t = tid
+          | _ -> false)
+      in
+      let n_exit =
+        count (function
+          | `Exit t -> t = tid
+          | _ -> false)
+      in
+      let n_msg =
+        count (function
+          | `Msg t -> t = tid
+          | _ -> false)
+      in
+      Printf.printf "domain-%d: enter=%d exit=%d msg=%d%s\n" i n_enter n_exit
+        n_msg
+        (if n_enter = iters && n_exit = iters && n_msg = iters then
+           ""
+         else
+           " FAIL"))
+    tids