diff --git a/src/dune_engine/build_system.ml b/src/dune_engine/build_system.ml
index 8b59841ee042..102febcb0587 100644
--- a/src/dune_engine/build_system.ml
+++ b/src/dune_engine/build_system.ml
@@ -92,6 +92,15 @@ module Error = struct
     | Diagnostic d -> `Diagnostic d.diagnostic
   ;;
 
+  module For_tests = struct
+    let make ~id ~description () =
+      match description with
+      | `Exn exn -> Exn { id; exn }
+      | `Diagnostic diagnostic ->
+        Diagnostic { id; diagnostic; dir = None; promotion = None }
+    ;;
+  end
+
   module Set : sig
     type error := t
 
diff --git a/src/dune_engine/build_system.mli b/src/dune_engine/build_system.mli
index c86e939d053a..27c14d57a93a 100644
--- a/src/dune_engine/build_system.mli
+++ b/src/dune_engine/build_system.mli
@@ -88,6 +88,7 @@ module Error : sig
 
     module Map : Map.S with type key = t
 
+    val gen : unit -> t
     val compare : t -> t -> Ordering.t
     val to_int : t -> int
     val to_dyn : t -> Dyn.t
@@ -126,6 +127,18 @@ module Error : sig
     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
+      :  id:Id.t
+      -> description:
+           [ `Exn of Exn_with_backtrace.t | `Diagnostic of Compound_user_error.t ]
+      -> unit
+      -> t
+  end
 end
 
 (** The current set of active errors. *)
diff --git a/src/dune_rpc_impl/dune_rpc_impl.ml b/src/dune_rpc_impl/dune_rpc_impl.ml
index a6eb06bdde2a..c685ca918576 100644
--- a/src/dune_rpc_impl/dune_rpc_impl.ml
+++ b/src/dune_rpc_impl/dune_rpc_impl.ml
@@ -6,6 +6,12 @@ module Private = Dune_rpc_client.Private
 module Watch_mode_config = Watch_mode_config
 module Where = Dune_rpc_client.Where
 
+module Diagnostics = struct
+  module For_tests = struct
+    let diagnostic_of_error = Diagnostics.diagnostic_of_error
+  end
+end
+
 module Poll_active =
   Dune_rpc_private.Registry.Poll
     (Fiber)
diff --git a/test/expect-tests/dune_rpc/dune b/test/expect-tests/dune_rpc/dune
index fb5593916ae5..77b3d7fcf356 100644
--- a/test/expect-tests/dune_rpc/dune
+++ b/test/expect-tests/dune_rpc/dune
@@ -5,10 +5,12 @@
  (libraries
   ocaml_config
   dune_util
+  dune_console
   dune_rpc_private
   dune_rpc_server
   dune_rpc_impl
   dune_rpc_client
+  dune_engine
   stdune
   test_scheduler
   csexp
diff --git a/test/expect-tests/dune_rpc/dune_rpc_tests.ml b/test/expect-tests/dune_rpc/dune_rpc_tests.ml
index 1dcacfc169e9..fe5e7cf7e2da 100644
--- a/test/expect-tests/dune_rpc/dune_rpc_tests.ml
+++ b/test/expect-tests/dune_rpc/dune_rpc_tests.ml
@@ -925,3 +925,19 @@ let%expect_test "print digests for all public RPCs" =
       Request: Sexp
       Response: 33528f248084297d123a6ebd4c3ddee0 |}]
 ;;
+
+let%expect_test "serialize and then deserialize error messages" =
+  let id = Dune_engine.Build_system.Error.Id.gen () in
+  let main = User_error.make [ Pp.verbatim "Oh no!" ] in
+  Dune_console.print_user_message main;
+  let description =
+    `Diagnostic (Dune_engine.Compound_user_error.make ~main ~related:[])
+  in
+  Dune_engine.Build_system.Error.For_tests.make ~id ~description ()
+  |> Dune_rpc_impl.Diagnostics.For_tests.diagnostic_of_error
+  |> Dune_rpc_private.Diagnostic.to_user_message
+  |> Dune_console.print_user_message;
+  [%expect {|
+    Error: Oh no!
+    Error: Error: Oh no! |}]
+;;