diff --git a/Makefile b/Makefile
index 04690e74d..8afdbd1b7 100644
--- a/Makefile
+++ b/Makefile
@@ -42,8 +42,7 @@ bench: ##
 
 .PHONY: test-ocaml
 test-ocaml: ## Run the unit tests
-	# FIXME: Find another approach to prevent competing test runs from causing errors
-	dune build  -j 1 @lsp/test/runtest @lsp-fiber/runtest @jsonrpc-fiber/runtest @ocaml-lsp-server/runtest
+	dune build @lsp/test/runtest @lsp-fiber/runtest @jsonrpc-fiber/runtest @ocaml-lsp-server/runtest
 
 .PHONY: promote
 promote:
@@ -106,6 +105,5 @@ coverage-deps:
 
 .PHONY: test-coverage
 test-coverage:
-	# FIXME: Find another approach to prevent competing test runs from causing errors
-	dune build -j 1 --instrument-with bisect_ppx --force @lsp/test/runtest @lsp-fiber/runtest @jsonrpc-fiber/runtest @ocaml-lsp-server/runtest
+	dune build --instrument-with bisect_ppx --force @lsp/test/runtest @lsp-fiber/runtest @jsonrpc-fiber/runtest @ocaml-lsp-server/runtest
 	bisect-ppx-report send-to Coveralls
diff --git a/ocaml-lsp-server/test/e2e-new/helpers.ml b/ocaml-lsp-server/test/e2e-new/helpers.ml
index 9e856e6a7..5039f1bb2 100644
--- a/ocaml-lsp-server/test/e2e-new/helpers.ml
+++ b/ocaml-lsp-server/test/e2e-new/helpers.ml
@@ -5,13 +5,8 @@ let client_capabilities = ClientCapabilities.create ()
 let uri = DocumentUri.of_path "test.ml"
 
 let test ?extra_env text req =
-  let handler =
-    Client.Handler.make
-      ~on_notification:(fun client _notification ->
-        Client.state client;
-        Fiber.return ())
-      ()
-  in
+  let on_notification, diagnostics = Test.drain_diagnostics () in
+  let handler = Client.Handler.make ~on_notification () in
   Test.run ~handler ?extra_env (fun client ->
       let run_client () =
         Client.start
@@ -31,6 +26,7 @@ let test ?extra_env text req =
         in
         let* () = req client in
         let* () = Client.request client Shutdown in
+        let* () = Fiber.Ivar.read diagnostics in
         Client.stop client
       in
       Fiber.fork_and_join_unit run_client run)
diff --git a/ocaml-lsp-server/test/e2e-new/test.ml b/ocaml-lsp-server/test/e2e-new/test.ml
index de646efc4..5a4f71b4b 100644
--- a/ocaml-lsp-server/test/e2e-new/test.ml
+++ b/ocaml-lsp-server/test/e2e-new/test.ml
@@ -168,19 +168,21 @@ end
 
 include T
 
-let run_request ?(prep = fun _ -> Fiber.return ()) ?settings request =
+let drain_diagnostics () =
   let diagnostics = Fiber.Ivar.create () in
-  let handler =
-    Client.Handler.make
-      ~on_notification:(fun _ -> function
-        | PublishDiagnostics _ -> (
-          let* diag = Fiber.Ivar.peek diagnostics in
-          match diag with
-          | Some _ -> Fiber.return ()
-          | None -> Fiber.Ivar.fill diagnostics ())
-        | _ -> Fiber.return ())
-      ()
+  let on_notification _ = function
+    | Lsp.Server_notification.PublishDiagnostics _ -> (
+      let* diag = Fiber.Ivar.peek diagnostics in
+      match diag with
+      | Some _ -> Fiber.return ()
+      | None -> Fiber.Ivar.fill diagnostics ())
+    | _ -> Fiber.return ()
   in
+  (on_notification, diagnostics)
+
+let run_request ?(prep = fun _ -> Fiber.return ()) ?settings request =
+  let on_notification, diagnostics = drain_diagnostics () in
+  let handler = Client.Handler.make ~on_notification () in
   run ~handler @@ fun client ->
   let run_client () =
     let capabilities =