diff --git a/examples/rnaseq.ml b/examples/rnaseq.ml
index 9ef358bf..01c80c48 100644
--- a/examples/rnaseq.ml
+++ b/examples/rnaseq.ml
@@ -63,6 +63,7 @@ let differential_analysis =
 let () =
   let open Bistro_utils.Repo in
   [
+    item ["delme"] (counts `WT_BHI_3) ;
     item ["deseq2"] differential_analysis#directory ;
   ]
   |> build_main ~np:4 ~mem:(`GB 4) ~outdir:"res" ~loggers:[Bistro_utils.Console_logger.create ()]
diff --git a/examples/zhou2011.ml b/examples/zhou2011.ml
index 8bbf742a..e4e8d1bb 100644
--- a/examples/zhou2011.ml
+++ b/examples/zhou2011.ml
@@ -1,9 +1,8 @@
-(** 
+(**
    Paper: https://www.ncbi.nlm.nih.gov/pubmed/21700227
    Datasets: https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE29506
 *)
 open Core
-open Bistro
 open Bistro_bioinfo
 open Bistro_utils
 
@@ -15,7 +14,7 @@ type chIP_sample = [ `ChIP_Pho4_noPi ]
 
 type factor = [ `Pho4 ]
 [@@deriving show, enumerate]
-          
+
 let factor = function
   | `ChIP_Pho4_noPi -> `Pho4
 
@@ -87,5 +86,5 @@ let () =
   Repo.build_main
     ~np ~mem:(`GB 4)
     ~outdir:"res"
-    ~loggers:[ Console_logger.create () ] 
+    ~loggers:[ Console_logger.create () ]
     repo
diff --git a/examples/zhou2018.ml b/examples/zhou2018.ml
index 4f0068f5..1d1c36ef 100644
--- a/examples/zhou2018.ml
+++ b/examples/zhou2018.ml
@@ -15,15 +15,13 @@ module Dataset = struct
     | `SongD1 -> "SongD1"
 
   let alignments d =
-    Workflow.string "https://ndownloader.figshare.com/files/9473962"
-    |> Bistro_unix.wget
+    Bistro_unix.wget "https://ndownloader.figshare.com/files/9473962"
     |> Bistro_unix.tar_xfj
     |> Fn.flip Workflow.select ["single-gene_alignments" ; to_string d ]
     |> Workflow.glob ~pattern:"*"
 
   let best_trees d =
-    Workflow.string "https://ndownloader.figshare.com/files/9473953"
-    |> Bistro_unix.wget
+    Bistro_unix.wget "https://ndownloader.figshare.com/files/9473953"
     |> Bistro_unix.tar_xfj
     |> Fn.flip Workflow.select ["single-gene_trees" ; to_string d ; "Best_observed"]
     |> Workflow.glob ~pattern:"*"
diff --git a/lib/engine/db.ml b/lib/engine/db.ml
index 7920df80..21d99a5f 100644
--- a/lib/engine/db.ml
+++ b/lib/engine/db.ml
@@ -135,8 +135,8 @@ let rec workflow_path db (Bistro_internals.Workflow.Any w) =
     workflow_path db (Any dir)
     |> Option.map ~f:(fun d -> Cd (d, sel))
   | Shell { id ; _ } -> Some (Cache_id id)
-  | Value { id ; _ } -> Some (Cache_id id)
-  | Path { id ; _ } -> Some (Cache_id id)
+  | Plugin { id ; task = Path_plugin _ ; _ } -> Some (Cache_id id)
+  | Plugin { id ; task = Value_plugin _ ; _ } -> Some (Cache_id id)
   | _ -> None
 
 let is_in_cache db u =
diff --git a/lib/engine/execution_trace.ml b/lib/engine/execution_trace.ml
index 93d1ef32..40a21b4f 100644
--- a/lib/engine/execution_trace.ml
+++ b/lib/engine/execution_trace.ml
@@ -74,3 +74,5 @@ let error_report trace db buf =
   | (Done_already _ | Canceled _) -> ()
 
 let all_ok xs = not (List.exists ~f:is_errored xs)
+
+module Set = S
diff --git a/lib/engine/execution_trace.mli b/lib/engine/execution_trace.mli
index b41940e7..3d713791 100644
--- a/lib/engine/execution_trace.mli
+++ b/lib/engine/execution_trace.mli
@@ -27,3 +27,5 @@ val error_report :
 val all_ok : t list -> bool
 
 val gather_failures : t list -> t list
+
+module Set : Set.S with type elt = t
diff --git a/lib/engine/scheduler.ml b/lib/engine/scheduler.ml
index 3ad62f1a..ecc5f528 100644
--- a/lib/engine/scheduler.ml
+++ b/lib/engine/scheduler.ml
@@ -9,14 +9,9 @@ type error = [
 
 module Table = String.Table
 
-module Traces = Caml.Set.Make(struct
-    type t = Execution_trace.t
-    let compare = compare
-  end)
-
 (* Lwt threads that accumulate errors *)
 module Eval_thread : sig
-  type 'a t = ('a, Traces.t) Lwt_result.t
+  type 'a t = ('a, Execution_trace.Set.t) Lwt_result.t
   val return : 'a -> 'a t
   (* val fail : Traces.t -> 'a t *)
   val fail1 : Execution_trace.t -> 'a t
@@ -38,7 +33,7 @@ module Eval_thread : sig
 end
 =
 struct
-  type 'a t = ('a, Traces.t) Lwt_result.t
+  type 'a t = ('a, Execution_trace.Set.t) Lwt_result.t
 
   module Infix = struct
     let ( >> ) = Lwt.( >>= )
@@ -48,13 +43,13 @@ struct
 
   let return = Lwt_result.return
   (* let fail = Lwt_result.fail *)
-  let fail1 e = Lwt_result.fail (Traces.singleton e)
+  let fail1 e = Lwt_result.fail (Execution_trace.Set.singleton e)
   let result_both x y =
     match x, y with
     | Ok x, Ok y -> Ok (x, y)
     | Ok _, Error e -> Error e
     | Error e, Ok _ -> Error e
-    | Error e, Error e' -> Error (Traces.union e e')
+    | Error e, Error e' -> Error (Execution_trace.Set.union e e')
 
   let both x y =
     Lwt.(x >>= fun x ->
@@ -263,14 +258,14 @@ struct
       register gc ?target l.elts
     | Input _ -> Lwt.return ()
     | Select x -> register gc ?target x.dir
-    | Value v ->
+    | Plugin { task = Value_plugin v ; _ } ->
       uses gc target w ;
       if stop_register gc w then Lwt.return ()
-      else register gc ~target:w v.task
-    | Path p ->
+      else register gc ~target:w v
+    | Plugin { task = Path_plugin p ; _ } ->
       uses gc target w ;
       if stop_register gc w then Lwt.return ()
-      else register gc ~target:w p.task
+      else register gc ~target:w p
     | Shell s ->
       uses gc target w ;
       if stop_register gc w then Lwt.return ()
@@ -332,9 +327,24 @@ struct
     | None -> Lwt.return ()
 end
 
+module Synchro : sig
+  type 'a t
+  val create : unit -> 'a t
+  val signal : 'a t -> 'a -> unit
+  val wait : 'a t -> 'a Lwt.t
+end
+=
+struct
+  type 'a t = ('a Lwt.t * 'a Lwt.u)
+  let create () = Lwt.wait ()
+  let signal (_, u) x = Lwt.wakeup u x
+  let wait = fst
+end
 
-type 'a t = {
-  target : 'a W.t ;
+type t = {
+  start : unit Synchro.t ;
+  _end_ : unit Synchro.t ;
+  mutable closed : bool ;
   allocator : Allocator.t ;
   db : Db.t ;
   logger : Logger.t ;
@@ -347,10 +357,12 @@ let create
     ?(np = 1) ?mem:(`GB mem = `GB 1)
     ?(use_docker = true)
     ?(loggers = [])
-    ?(collect = false) db target =
+    ?(collect = false) db =
   let logger = Logger.tee loggers in
   {
-    target = Bistro.Private.reveal target ;
+    start = Synchro.create () ;
+    _end_ = Synchro.create () ;
+    closed = false ;
     allocator = Allocator.create ~np ~mem:(mem * 1024) ;
     db ;
     use_docker ;
@@ -486,9 +498,9 @@ let rec blocking_evaluator
       let dir = blocking_evaluator db s.dir in
       fun () -> W.cd (dir ()) s.sel
     | W.Input { path ; _ } -> fun () -> W.FS_path path
-    | W.Value { id ; _ } ->
+    | W.Plugin { id ; task = Value_plugin _ ; _ } ->
       fun () -> (load_value (Db.cache db id))
-    | W.Path p -> fun () -> W.Cache_id p.id
+    | W.Plugin { id ; task = Path_plugin _ ; _ } -> fun () -> W.Cache_id id
     | W.Spawn s ->
       let elts = blocking_evaluator db s.elts in
       fun () ->
@@ -512,7 +524,7 @@ let rec blocking_evaluator
         |> List.map ~f:(fun fn -> W.cd dir_path [fn])
 
 let rec shallow_eval
-  : type s. _ t -> s W.t -> s Lwt.t
+  : type s. t -> s W.t -> s Lwt.t
   = fun sched w ->
     match w with
     | W.Pure { value ; _ } -> Lwt.return value
@@ -529,13 +541,13 @@ let rec shallow_eval
       shallow_eval sched s.dir >>= fun dir ->
       Lwt.return (W.cd dir s.sel)
     | W.Input { path ; _ } -> Lwt.return (W.FS_path path)
-    | W.Value { id ; _ } ->
+    | W.Plugin { id ; task = Value_plugin _ ; _ } ->
       Lwt.return (load_value (Db.cache sched.db id)) (* FIXME: blocking call *)
     | W.Spawn s -> (* FIXME: much room for improvement *)
       shallow_eval sched s.elts >>= fun elts ->
       let targets = List.init (List.length elts) ~f:(fun i -> s.f (W.list_nth s.elts i)) in
       Lwt_list.map_p (shallow_eval sched) targets
-    | W.Path s -> Lwt.return (W.Cache_id s.id)
+    | W.Plugin { id ; task = Path_plugin _ ; _ } -> Lwt.return (W.Cache_id id)
     | W.Shell s -> Lwt.return (W.Cache_id s.id)
     | W.List l ->
       Lwt_list.map_p (shallow_eval sched) l.elts
@@ -604,12 +616,11 @@ let np_requirement
     | List _ -> 0
     | List_nth _ -> 0
     | Glob _ -> 0
-    | Value x -> x.np
-    | Path x -> x.np
+    | Plugin x -> x.np
     | Shell x -> x.np
 
 let mem_requirement
-  : type u v. u t -> v Workflow.t -> int Lwt.t
+  : type u. t -> u Workflow.t -> int Lwt.t
   = fun sched -> function
     | Pure _ -> Lwt.return 0
     | App _  -> Lwt.return 0
@@ -621,8 +632,7 @@ let mem_requirement
     | List _ -> Lwt.return 0
     | List_nth _ -> Lwt.return 0
     | Glob _ -> Lwt.return 0
-    | Value x -> shallow_eval sched x.mem
-    | Path x -> shallow_eval sched x.mem
+    | Plugin x -> shallow_eval sched x.mem
     | Shell x -> shallow_eval sched x.mem
 
 let build_trace sched w perform =
@@ -668,7 +678,7 @@ let schedule_cached_workflow sched ~id w ~deps ~perform =
   |> Eval_thread.ignore
 
 let rec build
-  : type u v. _ t -> ?target:v W.t -> u W.t -> unit thread
+  : type u v. t -> ?target:v W.t -> u W.t -> unit thread
   = fun sched ?target w ->
     let open Eval_thread.Infix in
     match w with
@@ -703,7 +713,7 @@ let rec build
         )
       |> Eval_thread.ignore
 
-    | W.Value { task = workflow ; id ; descr ; _ } ->
+    | W.Plugin { task = Value_plugin workflow ; id ; descr ; _ } ->
       schedule_cached_workflow sched ~id w
         ~deps:(fun () -> build sched ~target:w workflow)
         ~perform:(fun _ ->
@@ -718,7 +728,7 @@ let rec build
           | Error msg -> Ok (Task_result.Plugin { id ; outcome = `Failed ; msg = Some msg ; descr })
         )
 
-    | W.Path { id ; task = workflow ; descr ; _ } ->
+    | W.Plugin { id ; task = Path_plugin workflow ; descr ; _ } ->
       schedule_cached_workflow sched ~id w
         ~deps:(fun () -> build sched ~target:w workflow)
         ~perform:(fun _ ->
@@ -747,16 +757,17 @@ let rec build
     | List l ->
       Eval_thread.join l.elts ~f:(build ?target sched)
 
-let run sched =
-  Maybe_gc.register sched.gc sched.target >>= fun () ->
-  build sched sched.target
-  >>= (fun r -> Maybe_gc.stop sched.gc >|= fun () -> r)
-  |> Fn.flip Lwt_result.bind Lwt.(fun () -> shallow_eval sched sched.target >|= Result.return)
-  |> Lwt_result.map_err Traces.elements
+let start sched = Synchro.signal sched.start ()
 
-let eval ?np ?mem ?use_docker ?loggers ?collect db w =
-  let sched = create ?np ?mem ?use_docker ?loggers ?collect db w in
-  run sched
+let eval sched target =
+  if sched.closed then failwith "Scheduler is closed" ;
+  let target = Bistro.Private.reveal target in
+  Synchro.wait sched.start >>= fun () ->
+  Maybe_gc.register sched.gc target >>= fun () ->
+  build sched target
+  >>= (fun r -> Maybe_gc.stop sched.gc >|= fun () -> r)
+  |> Fn.flip Lwt_result.bind Lwt.(fun () -> shallow_eval sched target >|= Result.return)
+  |> Lwt_result.map_err Execution_trace.Set.elements
 
 let error_report db traces =
   let buf = Buffer.create 1024 in
@@ -765,7 +776,11 @@ let error_report db traces =
     ) ;
   Buffer.contents buf
 
-let eval_exn ?np ?mem ?use_docker ?loggers ?collect db w =
-  eval ?np ?mem ?use_docker ?loggers ?collect db w >|= function
+let eval_exn sched w =
+  eval sched w >|= function
   | Ok r -> r
-  | Error errors -> failwith (error_report db errors)
+  | Error errors -> failwith (error_report sched.db errors)
+
+let stop sched =
+  Maybe_gc.stop sched.gc >>= fun () ->
+  sched.logger#stop
diff --git a/lib/engine/scheduler.mli b/lib/engine/scheduler.mli
index e1af3e2b..39de68cb 100644
--- a/lib/engine/scheduler.mli
+++ b/lib/engine/scheduler.mli
@@ -12,7 +12,7 @@ module Gc : sig
   }
 end
 
-type 'a t
+type t
 
 val create :
   ?np:int ->
@@ -21,32 +21,19 @@ val create :
   ?loggers:Logger.t list ->
   ?collect:bool ->
   Db.t ->
-  'a Bistro.workflow ->
-  'a t
+  t
 
-val run :
-  'a t ->
-  ('a, Execution_trace.t list) Lwt_result.t
+val gc_state : t -> Gc.state option
 
-val gc_state : _ t -> Gc.state option
+val start : t -> unit
 
 val eval :
-  ?np:int ->
-  ?mem:[`GB of int] ->
-  ?use_docker:bool ->
-  ?loggers:Logger.t list ->
-  ?collect:bool ->
-  Db.t ->
+  t ->
   'a Bistro.workflow ->
   ('a, Execution_trace.t list) Lwt_result.t
 
 val eval_exn :
-  ?np:int ->
-  ?mem:[`GB of int] ->
-  ?use_docker:bool ->
-  ?loggers:Logger.t list ->
-  ?collect:bool ->
-  Db.t ->
+  t ->
   'a Bistro.workflow ->
   'a Lwt.t
 
@@ -54,3 +41,5 @@ val error_report :
   Db.t ->
   Execution_trace.t list ->
   string
+
+val stop : t -> unit Lwt.t
diff --git a/lib/internals/workflow.ml b/lib/internals/workflow.ml
index 7aa4ad49..0f7ff3be 100644
--- a/lib/internals/workflow.ml
+++ b/lib/internals/workflow.ml
@@ -42,8 +42,7 @@ type _ t =
       dir : path t ;
       sel : string list ;
     } -> path t
-  | Value : ((unit -> 'a) t, any) step -> 'a t
-  | Path : ((string -> unit) t, any) step -> path t
+  | Plugin : ('a plugin, any) step -> 'a t
   | Shell : (shell_command, any) step -> path t
   | Glob : {
       id : string ;
@@ -61,6 +60,10 @@ and ('a, 'b) step = {
   deps : 'b list ;
 }
 
+and 'a plugin =
+  | Value_plugin : (unit -> 'a) t -> 'a plugin
+  | Path_plugin : (string -> unit) t -> path plugin
+
 and shell_command = token Command.t
 
 and token = Path_token of path t | String_token of string t
@@ -73,8 +76,7 @@ let digest x =
 let id : type s. s t -> string = function
   | Input { id ; _ } -> id
   | Select { id ; _ } -> id
-  | Value { id ; _ } -> id
-  | Path { id ; _ } -> id
+  | Plugin { id ; _ } -> id
   | Pure { id ; _ } -> id
   | App { id ; _ } -> id
   | Spawn { id ; _ } -> id
@@ -110,8 +112,7 @@ module Any = struct
     | List_nth l -> [ Any l.elts ]
     | Input _ -> []
     | Select sel -> [ any sel.dir ]
-    | Value v -> v.deps
-    | Path p -> p.deps
+    | Plugin v -> v.deps
     | Shell s -> s.deps
     | Glob g -> [ Any g.dir ]
 end
@@ -124,7 +125,7 @@ let select dir sel =
   let dir, sel =
     match dir with
     | Select { dir ; sel = root ; _ } -> dir, root @ sel
-    | Input _ | Path _ | Shell _ -> dir, sel
+    | Input _ | Plugin _ | Shell _ -> dir, sel
     | _ -> assert false
   in
   let id = digest ("select", id dir, sel) in
@@ -144,11 +145,11 @@ let both fst snd =
 
 let cached_value ?(descr = "") ?(np = 1) ?(mem = int 100) ?version workflow =
   let id = digest (`Value, id workflow, version) in
-  Value { id ; descr ; task = workflow ; np ; mem ; version ; deps = [ any mem ; any workflow ] }
+  Plugin { id ; descr ; task = Value_plugin workflow ; np ; mem ; version ; deps = [ any mem ; any workflow ] }
 
 let cached_path ?(descr = "") ?(np = 1) ?(mem = int 100) ?version workflow =
   let id = digest (`Value, id workflow, version) in
-  Path { id ; descr ; task = workflow ; np ; mem ; version ; deps = [ any mem ; any workflow ] }
+  Plugin { id ; descr ; task = Path_plugin workflow ; np ; mem ; version ; deps = [ any mem ; any workflow ] }
 
 let eval_path w = Eval_path { id = digest (`Eval_path, id w) ; workflow = w }
 
diff --git a/lib/utils/console_logger.ml b/lib/utils/console_logger.ml
index 1eb7da6b..79f1387d 100644
--- a/lib/utils/console_logger.ml
+++ b/lib/utils/console_logger.ml
@@ -35,13 +35,11 @@ let error_short_descr =
 let output_step_event t ~id ~descr =
   let id = String.prefix id 6 in
   msg t "started %s.%s" descr id
-  
+
 let output_event t = function
   | Logger.Workflow_started (Shell { id ; descr ; _ }, _) ->
     output_step_event t ~id ~descr
-  | Logger.Workflow_started (Value { id ; descr ; _ }, _) ->
-    output_step_event t ~id ~descr
-  | Logger.Workflow_started (Path { id ; descr ; _ }, _) ->
+  | Logger.Workflow_started (Plugin { id ; descr ; _ }, _) ->
     output_step_event t ~id ~descr
 
   | Workflow_ended { outcome = (Task_result.Shell { id ; descr ; _ } as outcome) ; _ } ->
diff --git a/lib/utils/dot_output.ml b/lib/utils/dot_output.ml
index f2bffbad..56ae2297 100644
--- a/lib/utils/dot_output.ml
+++ b/lib/utils/dot_output.ml
@@ -95,8 +95,7 @@ let dot_output ?db oc g ~needed =
       let label = Path.to_string s.sel in
       [ `Label label ; `Fontcolor color ; `Color color ; shape ]
     | Shell { descr ; _ } -> step_attributes ~descr u
-    | Value { descr ; _ } -> step_attributes ~descr u
-    | Path { descr ; _ } -> step_attributes ~descr u
+    | Plugin { descr ; _ } -> step_attributes ~descr u
     | Pure _ -> [ label "pure" u ; `Shape `Plaintext ]
     | App _ -> [ label "app" u ; `Shape `Plaintext ]
     | Spawn _ -> [ label "spawn" u ; `Shape `Ellipse ]
diff --git a/lib/utils/repo.ml b/lib/utils/repo.ml
index 2392a8ab..1de60387 100644
--- a/lib/utils/repo.ml
+++ b/lib/utils/repo.ml
@@ -80,6 +80,7 @@ let link dst p_u =
   ignore (Sys.command cmd)
 
 let generate outdir items =
+  let items = remove_redundancies items in
   List.iter items ~f:(fun item ->
       let repo_path = Filename.concat outdir item.repo_path in
       let file_path = Filename.concat outdir item.file_path in
@@ -90,32 +91,33 @@ let generate outdir items =
       link file_path cache_path
     )
 
+let item_to_workflow = function
+  | Item (path, w) ->
+    [%workflow [ normalized_repo_item ~repo_path:path ~id:(W.id (Private.reveal w)) ~cache_path:[%path w] ]]
+  | Item_list l ->
+    [%workflow
+      let id = W.id (Private.reveal l.elts) in
+      let elts = [%eval Workflow.spawn l.elts ~f:Workflow.eval_path] in
+      let n = List.length elts in
+      let m = Float.(n |> of_int |> log10 |> to_int) in
+      let ext = match l.ext with
+        | None -> ""
+        | Some s -> "." ^ s
+      in
+      let format =
+        Scanf.format_from_string
+          (sprintf {|%%s_%%0%dd%%s|} m)
+          "%s%d%s" in
+      let list_elt_path i =
+        l.path @ [ sprintf format l.prefix i ext ]
+      in
+      List.mapi elts ~f:(fun i path_w ->
+          normalized_repo_item ~repo_path:(list_elt_path i) ~id:(Misc.digest (id, i)) ~cache_path:path_w
+        )]
+
 let to_workflow ~outdir items =
   let normalized_items =
-    List.map items ~f:(function
-        | Item (path, w) ->
-          [%workflow [ normalized_repo_item ~repo_path:path ~id:(W.id (Private.reveal w)) ~cache_path:[%path w] ]]
-        | Item_list l ->
-          [%workflow
-            let id = W.id (Private.reveal l.elts) in
-            let elts = [%eval Workflow.spawn l.elts ~f:Workflow.eval_path] in
-            let n = List.length elts in
-            let m = Float.(n |> of_int |> log10 |> to_int) in
-            let ext = match l.ext with
-              | None -> ""
-              | Some s -> "." ^ s
-            in
-            let format =
-              Scanf.format_from_string
-                (sprintf {|%%s_%%0%dd%%s|} m)
-                "%s%d%s" in
-            let list_elt_path i =
-              l.path @ [ sprintf format l.prefix i ext ]
-            in
-            List.mapi elts ~f:(fun i path_w ->
-                normalized_repo_item ~repo_path:(list_elt_path i) ~id:(Misc.digest (id, i)) ~cache_path:path_w
-              )]
-      )
+    List.map items ~f:item_to_workflow
     |> Workflow.list
   in
   [%workflow
@@ -124,15 +126,31 @@ let to_workflow ~outdir items =
     |> remove_redundancies
     |> generate outdir]
 
+let partition_results xs =
+  let rec inner ok err = function
+    | [] -> ok, err
+    | Ok x :: t -> inner (x :: ok) err t
+    | Error e :: t -> inner ok (e :: err) t
+  in
+  inner [] [] xs
+
 let build ?np ?mem ?loggers ?use_docker ?(bistro_dir = "_bistro") ?collect ~outdir repo =
   let db = Db.init_exn bistro_dir in
-  let expr = to_workflow ~outdir repo in
-  Scheduler.eval ?np ?mem ?loggers ?use_docker ?collect db expr >|=
-  function
-  | Ok () -> ()
-  | Error errors ->
+  let expressions = List.map repo ~f:(item_to_workflow) in
+  let sched = Scheduler.create ?np ?mem ?loggers ?use_docker ?collect db in
+  let results = Lwt_list.map_p (Scheduler.eval sched) expressions in
+  Scheduler.start sched ;
+  Lwt.map partition_results results >>= fun (res, errors) ->
+  Scheduler.stop sched >|= fun () ->
+  generate outdir (List.concat res) ;
+  if errors <> [] then (
+    let errors =
+      List.concat errors
+      |> Execution_trace.gather_failures
+    in
     prerr_endline (Scheduler.error_report db errors) ;
     failwith "Some workflow failed!"
+  )
 
 let build_main ?np ?mem ?loggers ?use_docker ?bistro_dir ?collect ~outdir repo =
   build ?np ?mem ?loggers ?use_docker ?bistro_dir ?collect ~outdir repo
diff --git a/test/test1.ml b/test/test1.ml
index 8b1b1283..9c33ca22 100644
--- a/test/test1.ml
+++ b/test/test1.ml
@@ -12,8 +12,8 @@ let pipeline = add (Workflow.int 1) (Workflow.int 41)
 
 let _ =
   let open Bistro_engine in
-  let open Lwt_result.Infix in
   let db = Db.init_exn "_bistro" in
-  Scheduler.eval db pipeline
-  >|= Printf.printf "%d\n"
-  |> Lwt_main.run
+  let sched = Scheduler.create db in
+  let thread = Scheduler.eval_exn sched pipeline in
+  Scheduler.start sched ;
+  Printf.printf "%d\n" (Lwt_main.run thread)
diff --git a/test/test2.ml b/test/test2.ml
index a463d0b5..a6b9a6cf 100644
--- a/test/test2.ml
+++ b/test/test2.ml
@@ -35,6 +35,8 @@ let _ =
   let db = Db.init_exn "_bistro" in
   let pipeline = Repo.to_workflow repo ~outdir:"res" in
   Dot_output.workflow_to_file ~db "workflow.dot" pipeline ;
-  let sched = Scheduler.create ~np:4 ~loggers:[Bistro_utils.Console_logger.create ()] ~collect:true db pipeline in
-  ignore (Scheduler.run sched |> Lwt_main.run) ;
-  dump_gc_state sched db "gc_final.dot" ;
+  let sched = Scheduler.create ~np:4 ~loggers:[Bistro_utils.Console_logger.create ()] ~collect:true db in
+  let thread = Scheduler.eval_exn sched pipeline in
+  Scheduler.start sched ;
+  Lwt_main.run thread ;
+  dump_gc_state sched db "gc_final.dot"
diff --git a/test/test_gc.ml b/test/test_gc.ml
index c6b7d26d..2c9064e0 100644
--- a/test/test_gc.ml
+++ b/test/test_gc.ml
@@ -35,6 +35,8 @@ let dump_gc_state sched db fn =
 let _ =
   let open Bistro_engine in
   let db = Db.init_exn "_bistro" in
-  let sched = Scheduler.create ~np:4 ~loggers:[Bistro_utils.Console_logger.create ()] ~collect:true db pipeline in
-  ignore (Scheduler.run sched |> Lwt_main.run) ;
+  let sched = Scheduler.create ~np:4 ~loggers:[Bistro_utils.Console_logger.create ()] ~collect:true db in
+  let thread = Scheduler.eval_exn sched pipeline in
+  Scheduler.start sched ;
+  ignore (Lwt_main.run thread) ;
   dump_gc_state sched db "gc_final.dot"