diff --git a/src/stdlib-variants/thread-counter/thread_counter.ml b/src/stdlib-variants/thread-counter/thread_counter.ml
index c97511b..d8e9ccc 100644
--- a/src/stdlib-variants/thread-counter/thread_counter.ml
+++ b/src/stdlib-variants/thread-counter/thread_counter.ml
@@ -29,6 +29,7 @@ end)
 module Counter = struct
 
   open Ctx_util
+  open Ctx_util.Syntax
 
   let lock_if b m = if b then lock_mutex m else empty_context' ()
 
@@ -37,7 +38,7 @@ module Counter = struct
      able to report stray exceptions outweighs the slim chance anyone
      would rely on being able to ignore exceptions in threads. *)
 
-  type 'a finished = Return of 'a | Uncaught of exn_info | Overflow of int
+  type 'a finished = Return of 'a | Uncaught of Util.exn_info | Overflow of int
   type 'a state = Running | Finished of 'a finished
 
   type 'a group = {
@@ -119,7 +120,10 @@ module Counter = struct
       owner = cnt;
     }
 
-  let join_group ~timeout ?(leftover_thread_limit = 0) group =
+  let get_thread_count group =
+    let< _ = lock_mutex group.owner.mut in group.thread_count
+
+  let join_group ~leftover_thread_limit ~timeout group =
     (* group must be stopped first; busy waits to implement timeout *)
     d_ "join_group";
 
@@ -130,9 +134,9 @@ module Counter = struct
 
     let time0 = Mtime_clock.counter () in
     let rec loop () =
-      let remaining = (let< _ = lock_mutex group.owner.mut in group.thread_count) in
+      let remaining = get_thread_count group in
       if remaining <= leftover_thread_limit then remaining
-      else if Mtime.Span.compare (Mtime_clock.count time0) timeout > 0 then remaining
+      else if Mtime.Span.compare (Mtime_clock.count time0) timeout >= 0 then remaining
       else (Thread.yield (); loop ())
     in
     loop ()
@@ -140,9 +144,9 @@ module Counter = struct
   type thread_group_err =
     | ThreadLimitReached of int
     | ThreadsLeftOver of { left_over : int; limit : int }
-    | ExceptionRaised of { main : bool; exn_info : exn_info }
+    | ExceptionRaised of { main : bool; exn_info : Util.exn_info }
 
-  let spawn_thread_group ?thread_limit ~join_timeout ?leftover_thread_limit cnt f x =
+  let spawn_thread_group_no_check ?thread_limit ?leftover_limit cnt f x =
     let group = create_group ?thread_limit cnt in
 
     spawn_thread ~group cnt Util.(try_return group % try_to_result f) x |> ignore;
@@ -155,8 +159,15 @@ module Counter = struct
       let< _ = lock_mutex cnt.mut in loop ()
     in
 
-    let leftover_count = join_group ~timeout:join_timeout ?leftover_thread_limit group in
+    let leftover_count =
+      match leftover_limit with
+      | Some (n, t) -> join_group ~leftover_thread_limit:n ~timeout:t group
+      | None -> get_thread_count group
+    in
+
+    fin, leftover_count
 
+  let check_spawn_thread_group ?leftover_limit fin leftover_count =
     let r = match[@warning "-4"] fin with
       | Return (Ok x) -> Ok x
       | Return (Error e) -> Error [ExceptionRaised { main = true; exn_info = e }]
@@ -164,11 +175,15 @@ module Counter = struct
       | Overflow l -> Error [ThreadLimitReached l]
     in
 
-    match leftover_thread_limit with
-      | Some lim when leftover_count > lim ->
+    match leftover_limit with
+      | Some (lim, (_ : Mtime.span)) when leftover_count > lim ->
           r |> add_err (ThreadsLeftOver { left_over = leftover_count ; limit = lim })
       | _ -> r
 
+  let spawn_thread_group ?thread_limit ?leftover_limit cnt f x =
+    let fin, leftover_count = spawn_thread_group_no_check cnt f x ?thread_limit ?leftover_limit in
+    check_spawn_thread_group ?leftover_limit fin leftover_count
+
   (* the rest is for error reporting, could do with less code/abstraction... *)
 
   let string_of_thread_group_err = function
@@ -180,9 +195,10 @@ module Counter = struct
         Printf.sprintf "%s thread raised an exception: %s\n%s"
           (if main then "The main" else "A created")
           (Printexc.to_string exn) (Printexc.raw_backtrace_to_string backtrace)
+        |> String.trim
 
   let string_of_thread_group_errs = function
-    | [] -> "Unknown error in a thread group"
+    | [] -> "Unknown error in a thread group" (* this shouldn't happen *)
     | [err] -> "Error in a thread group: " ^ string_of_thread_group_err err
     | errs ->
         "Multiple errors in a thread group:\n" ^