diff --git a/bin/import.ml b/bin/import.ml
index ee2fecf14a3..de113400199 100644
--- a/bin/import.ml
+++ b/bin/import.ml
@@ -6,6 +6,7 @@ include struct
   open Dune_engine
   module Build_config = Build_config
   module Build_system = Build_system
+  module Build_system_error = Build_system_error
   module Load_rules = Load_rules
   module Hooks = Hooks
   module Action_builder = Action_builder
@@ -157,7 +158,7 @@ module Scheduler = struct
         | Failure ->
           let failure_message =
             match
-              Build_system.Error.(
+              Build_system_error.(
                 Id.Map.cardinal (Set.current (Fiber.Svar.read Build_system.errors)))
             with
             | 1 -> Pp.textf "Had 1 error"
diff --git a/src/dune_engine/build_system.ml b/src/dune_engine/build_system.ml
index 11532a06062..ffdc270f25f 100644
--- a/src/dune_engine/build_system.ml
+++ b/src/dune_engine/build_system.ml
@@ -1,6 +1,7 @@
 open Import
 open Memo.O
 module Action_builder = Action_builder0
+module Error = Build_system_error
 
 module Progress = struct
   type t =
@@ -26,132 +27,6 @@ module Progress = struct
   ;;
 end
 
-module Error = struct
-  module Id = Id.Make ()
-
-  type t =
-    | Exn of
-        { id : Id.t
-        ; exn : Exn_with_backtrace.t
-        }
-    | Diagnostic of
-        { id : Id.t
-        ; diagnostic : Compound_user_error.t
-        ; dir : Path.t option
-        ; promotion : Diff_promotion.Annot.t option
-        }
-
-  module Event = struct
-    type nonrec t =
-      | Add of t
-      | Remove of t
-  end
-
-  let of_exn (exn : Exn_with_backtrace.t) =
-    let exn =
-      match exn.exn with
-      | Memo.Error.E e -> { exn with exn = Memo.Error.get e }
-      | _ -> exn
-    in
-    match exn.exn with
-    | User_error.E main ->
-      let dir = User_message.Annots.find main.annots Process.with_directory_annot in
-      let promotion = User_message.Annots.find main.annots Diff_promotion.Annot.annot in
-      (match User_message.Annots.find main.annots Compound_user_error.annot with
-       | None ->
-         [ Diagnostic
-             { dir
-             ; id = Id.gen ()
-             ; diagnostic = Compound_user_error.make ~main ~related:[]
-             ; promotion
-             }
-         ]
-       | Some diagnostics ->
-         List.map diagnostics ~f:(fun diagnostic ->
-           Diagnostic { id = Id.gen (); diagnostic; dir; promotion }))
-    | _ -> [ Exn { id = Id.gen (); exn } ]
-  ;;
-
-  let promotion = function
-    | Exn _ -> None
-    | Diagnostic d -> d.promotion
-  ;;
-
-  let id = function
-    | Exn d -> d.id
-    | Diagnostic d -> d.id
-  ;;
-
-  let dir = function
-    | Exn _ -> None
-    | Diagnostic d -> d.dir
-  ;;
-
-  let description = function
-    | Exn e -> `Exn e.exn
-    | Diagnostic d -> `Diagnostic d.diagnostic
-  ;;
-
-  module For_tests = struct
-    let make ~description ~dir ~promotion () =
-      let id = Id.gen () in
-      match description with
-      | `Exn exn -> Exn { id; exn }
-      | `Diagnostic diagnostic -> Diagnostic { id; diagnostic; dir; promotion }
-    ;;
-  end
-
-  module Set : sig
-    type error := t
-
-    type nonrec t = private
-      { current : t Id.Map.t
-      ; stamp : int
-      ; last_event : Event.t option
-      }
-
-    val add : t -> error -> t
-    val one_event_diff : prev:t -> next:t -> Event.t option
-    val equal : t -> t -> bool
-    val current : t -> error Id.Map.t
-    val empty : t
-  end = struct
-    type nonrec t =
-      { current : t Id.Map.t
-      ; stamp : int
-      ; last_event : Event.t option
-      }
-
-    let add t error =
-      let current = Id.Map.set t.current (id error) error in
-      { current; stamp = t.stamp + 1; last_event = Some (Add error) }
-    ;;
-
-    let equal t { current; stamp; last_event } =
-      Int.equal t.stamp stamp
-      &&
-      match t.last_event, last_event with
-      | None, None ->
-        assert (Id.Map.is_empty t.current && Id.Map.is_empty current);
-        true (* only possible when both sets are empty *)
-      | Some x, Some y ->
-        (match x, y with
-         | Add x, Add y -> Id.equal (id x) (id y)
-         | Add _, _ -> false
-         | Remove x, Remove y -> Id.equal (id x) (id y)
-         | Remove _, _ -> false)
-      | Some _, None | None, Some _ -> false
-    ;;
-
-    let one_event_diff ~prev ~next =
-      if prev.stamp + 1 = next.stamp then next.last_event else None
-    ;;
-
-    let current t = t.current
-    let empty = { current = Id.Map.empty; stamp = 0; last_event = None }
-  end
-end
-
 module State = struct
   module Svar = Fiber.Svar
 
diff --git a/src/dune_engine/build_system.mli b/src/dune_engine/build_system.mli
index eca27f9df71..1a3dc81a0b4 100644
--- a/src/dune_engine/build_system.mli
+++ b/src/dune_engine/build_system.mli
@@ -81,65 +81,5 @@ end
 
 val state : State.t Fiber.Svar.t
 
-(** Errors found when building targets. *)
-module Error : sig
-  module Id : sig
-    type t
-
-    module Map : Map.S with type key = t
-
-    val compare : t -> t -> Ordering.t
-    val to_int : t -> int
-    val to_dyn : t -> Dyn.t
-  end
-
-  type t
-
-  val id : t -> Id.t
-
-  (** the directory where the rule the error is originating from *)
-  val dir : t -> Path.t option
-
-  (** The description of the error. Errors from build rules contain useful
-      metadata that are extracted into [`Diagnostic] *)
-  val description
-    :  t
-    -> [ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
-
-  val promotion : t -> Diff_promotion.Annot.t option
-
-  module Event : sig
-    type nonrec t =
-      | Add of t
-      | Remove of t
-  end
-
-  module Set : sig
-    type error := t
-    type t
-
-    (** [one_event_diff ~prev ~next] returns the event that constructs [next]
-        from [prev] if [next] is in the successive "generation" of [prev] *)
-    val one_event_diff : prev:t -> next:t -> Event.t option
-
-    val equal : t -> t -> bool
-    val current : t -> error Id.Map.t
-    val empty : t
-  end
-
-  module For_tests : sig
-    (** Internal helpers for testing purposes. Do not use. *)
-
-    (** Construct an [Error.t] *)
-    val make
-      :  description:
-           [ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
-      -> dir:Path.t option
-      -> promotion:Diff_promotion.Annot.t option
-      -> unit
-      -> t
-  end
-end
-
 (** The current set of active errors. *)
-val errors : Error.Set.t Fiber.Svar.t
+val errors : Build_system_error.Set.t Fiber.Svar.t
diff --git a/src/dune_engine/build_system_error.ml b/src/dune_engine/build_system_error.ml
new file mode 100644
index 00000000000..f34f83398b4
--- /dev/null
+++ b/src/dune_engine/build_system_error.ml
@@ -0,0 +1,112 @@
+open Import
+module Id = Id.Make ()
+
+type t =
+  | Exn of
+      { id : Id.t
+      ; exn : Exn_with_backtrace.t
+      }
+  | Diagnostic of
+      { id : Id.t
+      ; diagnostic : Compound_user_error.t
+      ; dir : Path.t option
+      ; promotion : Diff_promotion.Annot.t option
+      }
+
+module Event = struct
+  type nonrec t =
+    | Add of t
+    | Remove of t
+end
+
+let of_exn (exn : Exn_with_backtrace.t) =
+  let exn =
+    match exn.exn with
+    | Memo.Error.E e -> { exn with exn = Memo.Error.get e }
+    | _ -> exn
+  in
+  match exn.exn with
+  | User_error.E main ->
+    let dir = User_message.Annots.find main.annots Process.with_directory_annot in
+    let promotion = User_message.Annots.find main.annots Diff_promotion.Annot.annot in
+    (match User_message.Annots.find main.annots Compound_user_error.annot with
+     | None ->
+       [ Diagnostic
+           { dir
+           ; id = Id.gen ()
+           ; diagnostic = Compound_user_error.make ~main ~related:[]
+           ; promotion
+           }
+       ]
+     | Some diagnostics ->
+       List.map diagnostics ~f:(fun diagnostic ->
+         Diagnostic { id = Id.gen (); diagnostic; dir; promotion }))
+  | _ -> [ Exn { id = Id.gen (); exn } ]
+;;
+
+let promotion = function
+  | Exn _ -> None
+  | Diagnostic d -> d.promotion
+;;
+
+let id = function
+  | Exn d -> d.id
+  | Diagnostic d -> d.id
+;;
+
+let dir = function
+  | Exn _ -> None
+  | Diagnostic d -> d.dir
+;;
+
+let description = function
+  | Exn e -> `Exn e.exn
+  | Diagnostic d -> `Diagnostic d.diagnostic
+;;
+
+module Set = struct
+  type error = t
+
+  type t =
+    { current : error Id.Map.t
+    ; stamp : int
+    ; last_event : Event.t option
+    }
+
+  let add t error =
+    let current = Id.Map.set t.current (id error) error in
+    { current; stamp = t.stamp + 1; last_event = Some (Add error) }
+  ;;
+
+  let equal t { current; stamp; last_event } =
+    Int.equal t.stamp stamp
+    &&
+    match t.last_event, last_event with
+    | None, None ->
+      assert (Id.Map.is_empty t.current && Id.Map.is_empty current);
+      true (* only possible when both sets are empty *)
+    | Some x, Some y ->
+      (match x, y with
+       | Add x, Add y -> Id.equal (id x) (id y)
+       | Add _, _ -> false
+       | Remove x, Remove y -> Id.equal (id x) (id y)
+       | Remove _, _ -> false)
+    | Some _, None | None, Some _ -> false
+  ;;
+
+  let one_event_diff ~prev ~next =
+    if prev.stamp + 1 = next.stamp then next.last_event else None
+  ;;
+
+  let current t = t.current
+  let empty = { current = Id.Map.empty; stamp = 0; last_event = None }
+end
+
+module For_tests = struct
+  let make ~description ~dir ~promotion () =
+    let id = Id.gen () in
+    match description with
+    | `Exn exn -> Exn { id; exn }
+    | `Diagnostic diagnostic -> Diagnostic { id; diagnostic; dir; promotion }
+  ;;
+end
diff --git a/src/dune_engine/build_system_error.mli b/src/dune_engine/build_system_error.mli
new file mode 100644
index 00000000000..9ffafb837c3
--- /dev/null
+++ b/src/dune_engine/build_system_error.mli
@@ -0,0 +1,64 @@
+(** Errors found when building targets. *)
+
+open Import
+
+module Id : sig
+  type t
+
+  module Map : Map.S with type key = t
+
+  val compare : t -> t -> Ordering.t
+  val to_int : t -> int
+  val to_dyn : t -> Dyn.t
+end
+
+type t
+
+(** Construct a list of errors from an exception. *)
+val of_exn : Exn_with_backtrace.t -> t list
+
+val id : t -> Id.t
+
+(** the directory where the rule the error is originating from *)
+val dir : t -> Path.t option
+
+(** The description of the error. Errors from build rules contain useful
+    metadata that are extracted into [`Diagnostic] *)
+val description
+  :  t
+  -> [ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
+
+val promotion : t -> Diff_promotion.Annot.t option
+
+module Event : sig
+  type nonrec t =
+    | Add of t
+    | Remove of t
+end
+
+module Set : sig
+  type error := t
+  type t
+
+  val add : t -> error -> t
+
+  (** [one_event_diff ~prev ~next] returns the event that constructs [next]
+      from [prev] if [next] is in the successive "generation" of [prev] *)
+  val one_event_diff : prev:t -> next:t -> Event.t option
+
+  val equal : t -> t -> bool
+  val current : t -> error Id.Map.t
+  val empty : t
+end
+
+module For_tests : sig
+  (** Internal helpers for testing purposes. Do not use. *)
+
+  (** Construct an [Error.t] *)
+  val make
+    :  description:[ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
+    -> dir:Path.t option
+    -> promotion:Diff_promotion.Annot.t option
+    -> unit
+    -> t
+end
diff --git a/src/dune_engine/dune_engine.ml b/src/dune_engine/dune_engine.ml
index 5203fa0a273..e40b7de3b65 100644
--- a/src/dune_engine/dune_engine.ml
+++ b/src/dune_engine/dune_engine.ml
@@ -18,6 +18,7 @@ module Target_promotion = Target_promotion
 module Build_context = Build_context
 module Build_config = Build_config
 module Build_system = Build_system
+module Build_system_error = Build_system_error
 module Load_rules = Load_rules
 module Clflags = Clflags
 module Response_file = Response_file
diff --git a/src/dune_rpc_impl/diagnostics.ml b/src/dune_rpc_impl/diagnostics.ml
index 443259780ae..d828c2686e3 100644
--- a/src/dune_rpc_impl/diagnostics.ml
+++ b/src/dune_rpc_impl/diagnostics.ml
@@ -14,10 +14,10 @@ let absolutize_paths ~dir (loc : Loc.t) =
   |> Loc.to_lexbuf_loc
 ;;
 
-let diagnostic_of_error : Build_system.Error.t -> Dune_rpc_private.Diagnostic.t =
+let diagnostic_of_error : Build_system_error.t -> Dune_rpc_private.Diagnostic.t =
   fun m ->
   let dir =
-    let dir = Build_system.Error.dir m in
+    let dir = Build_system_error.dir m in
     Option.map dir ~f:Path.drop_optional_build_context_maybe_sandboxed
   in
   let make_loc loc =
@@ -25,7 +25,7 @@ let diagnostic_of_error : Build_system.Error.t -> Dune_rpc_private.Diagnostic.t
     absolutize_paths ~dir loc
   in
   let message, related =
-    match Build_system.Error.description m with
+    match Build_system_error.description m with
     | `Exn e ->
       (* CR-someday jeremiedimino: Use [Report_error.get_user_message] here. *)
       User_message.make [ Pp.text (Printexc.to_string e.exn) ], []
@@ -34,10 +34,10 @@ let diagnostic_of_error : Build_system.Error.t -> Dune_rpc_private.Diagnostic.t
   let loc = Option.map message.loc ~f:make_loc in
   let make_message pars = Pp.map_tags (Pp.concat pars) ~f:(fun _ -> ()) in
   let id =
-    Build_system.Error.id m |> Build_system.Error.Id.to_int |> Diagnostic.Id.create
+    Build_system_error.id m |> Build_system_error.Id.to_int |> Diagnostic.Id.create
   in
   let promotion =
-    match Build_system.Error.promotion m with
+    match Build_system_error.promotion m with
     | None -> []
     | Some { in_source; in_build } ->
       [ { Diagnostic.Promotion.in_source =
@@ -63,7 +63,7 @@ let diagnostic_of_error : Build_system.Error.t -> Dune_rpc_private.Diagnostic.t
   }
 ;;
 
-let diagnostic_event_of_error_event (e : Build_system.Error.Event.t) : Diagnostic.Event.t =
+let diagnostic_event_of_error_event (e : Build_system_error.Event.t) : Diagnostic.Event.t =
   match e with
   | Remove e -> Remove (diagnostic_of_error e)
   | Add e -> Add (diagnostic_of_error e)
diff --git a/src/dune_rpc_impl/diagnostics.mli b/src/dune_rpc_impl/diagnostics.mli
index 38ba44b84ea..22782fdcb16 100644
--- a/src/dune_rpc_impl/diagnostics.mli
+++ b/src/dune_rpc_impl/diagnostics.mli
@@ -3,7 +3,7 @@
 open Import
 
 val diagnostic_event_of_error_event
-  :  Build_system.Error.Event.t
+  :  Build_system_error.Event.t
   -> Dune_rpc.Diagnostic.Event.t
 
-val diagnostic_of_error : Build_system.Error.t -> Dune_rpc_private.Diagnostic.t
+val diagnostic_of_error : Build_system_error.t -> Dune_rpc_private.Diagnostic.t
diff --git a/src/dune_rpc_impl/import.ml b/src/dune_rpc_impl/import.ml
index 21390b978d6..61961a8153e 100644
--- a/src/dune_rpc_impl/import.ml
+++ b/src/dune_rpc_impl/import.ml
@@ -5,6 +5,7 @@ module Dune_rpc = Dune_rpc_private
 include struct
   open Dune_engine
   module Build_system = Build_system
+  module Build_system_error = Build_system_error
   module Scheduler = Scheduler
   module Running_jobs = Running_jobs
 end
diff --git a/src/dune_rpc_impl/server.ml b/src/dune_rpc_impl/server.ml
index 003965cf306..ac3148bb957 100644
--- a/src/dune_rpc_impl/server.ml
+++ b/src/dune_rpc_impl/server.ml
@@ -229,7 +229,7 @@ let handler (t : _ t Fdecl.t) action_runner_server handle : 'a Dune_rpc_server.H
     Handler.create ~on_terminate ~on_init ~version:Dune_rpc_private.Version.latest ()
   in
   let () =
-    let module Error = Build_system.Error in
+    let module Error = Build_system_error in
     let diff ~last ~(now : Error.Set.t) =
       match last with
       | None ->
@@ -386,8 +386,8 @@ let handler (t : _ t Fdecl.t) action_runner_server handle : 'a Dune_rpc_server.H
   let () =
     let f _ () =
       let errors = Fiber.Svar.read Build_system.errors in
-      Build_system.Error.Set.current errors
-      |> Build_system.Error.Id.Map.values
+      Build_system_error.Set.current errors
+      |> Build_system_error.Id.Map.values
       |> List.map ~f:Diagnostics.diagnostic_of_error
       |> Fiber.return
     in
diff --git a/test/expect-tests/dune_rpc_impl/dune_rpc_impl_tests.ml b/test/expect-tests/dune_rpc_impl/dune_rpc_impl_tests.ml
index 1d172662600..bd4c24105fe 100644
--- a/test/expect-tests/dune_rpc_impl/dune_rpc_impl_tests.ml
+++ b/test/expect-tests/dune_rpc_impl/dune_rpc_impl_tests.ml
@@ -14,7 +14,7 @@ let test ~dir ~f main =
   Dune_console.printf "---- Original ----";
   f main;
   Dune_console.printf "------- RPC ------";
-  Dune_engine.Build_system.Error.For_tests.make ~description ~dir ~promotion:None ()
+  Dune_engine.Build_system_error.For_tests.make ~description ~dir ~promotion:None ()
   |> Dune_rpc_impl.Diagnostics.For_tests.diagnostic_of_error
   |> Dune_rpc_private.Diagnostic.to_user_message
   |> f