diff --git a/appservice/appservice.go b/appservice/appservice.go
index b33d7b701..3e19e09b2 100644
--- a/appservice/appservice.go
+++ b/appservice/appservice.go
@@ -59,7 +59,7 @@ func NewInternalAPI(
 			},
 		},
 	}
-	js := jetstream.Prepare(&base.Cfg.Global.JetStream)
+	js, _ := jetstream.Prepare(&base.Cfg.Global.JetStream)
 
 	// Create a connection to the appservice postgres DB
 	appserviceDB, err := storage.NewDatabase(&base.Cfg.AppServiceAPI.Database)
diff --git a/clientapi/clientapi.go b/clientapi/clientapi.go
index 918476674..e4279c220 100644
--- a/clientapi/clientapi.go
+++ b/clientapi/clientapi.go
@@ -49,7 +49,7 @@ func AddPublicRoutes(
 	extRoomsProvider api.ExtraPublicRoomsProvider,
 	mscCfg *config.MSCs,
 ) {
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	syncProducer := &producers.SyncAPIProducer{
 		JetStream: js,
diff --git a/eduserver/eduserver.go b/eduserver/eduserver.go
index 9b7e21651..6882399da 100644
--- a/eduserver/eduserver.go
+++ b/eduserver/eduserver.go
@@ -42,7 +42,7 @@ func NewInternalAPI(
 ) api.EDUServerInputAPI {
 	cfg := &base.Cfg.EDUServer
 
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	return &input.EDUServerInputAPI{
 		Cache:                        eduCache,
diff --git a/federationapi/federationapi.go b/federationapi/federationapi.go
index a982d8009..9f149d973 100644
--- a/federationapi/federationapi.go
+++ b/federationapi/federationapi.go
@@ -92,7 +92,7 @@ func NewInternalAPI(
 		FailuresUntilBlacklist: cfg.FederationMaxRetries,
 	}
 
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	queues := queue.NewOutgoingQueues(
 		federationDB, base.ProcessContext,
diff --git a/keyserver/keyserver.go b/keyserver/keyserver.go
index bd36fd9f9..8a0ce6178 100644
--- a/keyserver/keyserver.go
+++ b/keyserver/keyserver.go
@@ -39,7 +39,7 @@ func AddInternalRoutes(router *mux.Router, intAPI api.KeyInternalAPI) {
 func NewInternalAPI(
 	base *base.BaseDendrite, cfg *config.KeyServer, fedClient fedsenderapi.FederationClient,
 ) api.KeyInternalAPI {
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	db, err := storage.NewDatabase(&cfg.Database)
 	if err != nil {
diff --git a/roomserver/internal/api.go b/roomserver/internal/api.go
index 10c8c844e..91001e418 100644
--- a/roomserver/internal/api.go
+++ b/roomserver/internal/api.go
@@ -43,6 +43,7 @@ type RoomserverInternalAPI struct {
 	ServerACLs             *acls.ServerACLs
 	fsAPI                  fsAPI.FederationInternalAPI
 	asAPI                  asAPI.AppServiceQueryAPI
+	NATSClient             *nats.Conn
 	JetStream              nats.JetStreamContext
 	Durable                string
 	InputRoomEventTopic    string // JetStream topic for new input room events
@@ -52,7 +53,8 @@ type RoomserverInternalAPI struct {
 
 func NewRoomserverAPI(
 	processCtx *process.ProcessContext, cfg *config.RoomServer, roomserverDB storage.Database,
-	consumer nats.JetStreamContext, inputRoomEventTopic, outputRoomEventTopic string,
+	consumer nats.JetStreamContext, nc *nats.Conn,
+	inputRoomEventTopic, outputRoomEventTopic string,
 	caches caching.RoomServerCaches, perspectiveServerNames []gomatrixserverlib.ServerName,
 ) *RoomserverInternalAPI {
 	serverACLs := acls.NewServerACLs(roomserverDB)
@@ -66,6 +68,7 @@ func NewRoomserverAPI(
 		InputRoomEventTopic:    inputRoomEventTopic,
 		OutputRoomEventTopic:   outputRoomEventTopic,
 		JetStream:              consumer,
+		NATSClient:             nc,
 		Durable:                cfg.Matrix.JetStream.Durable("RoomserverInputConsumer"),
 		ServerACLs:             serverACLs,
 		Queryer: &query.Queryer{
@@ -92,6 +95,7 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.FederationInternalA
 		InputRoomEventTopic:  r.InputRoomEventTopic,
 		OutputRoomEventTopic: r.OutputRoomEventTopic,
 		JetStream:            r.JetStream,
+		NATSClient:           r.NATSClient,
 		Durable:              nats.Durable(r.Durable),
 		ServerName:           r.Cfg.Matrix.ServerName,
 		FSAPI:                fsAPI,
diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index 178533ded..c6e354611 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -48,6 +48,7 @@ var keyContentFields = map[string]string{
 type Inputer struct {
 	ProcessContext       *process.ProcessContext
 	DB                   storage.Database
+	NATSClient           *nats.Conn
 	JetStream            nats.JetStreamContext
 	Durable              nats.SubOpt
 	ServerName           gomatrixserverlib.ServerName
@@ -103,6 +104,7 @@ func (r *Inputer) Start() error {
 				_ = msg.InProgress() // resets the acknowledgement wait timer
 				defer eventsInProgress.Delete(index)
 				defer roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Dec()
+				var errString string
 				if err := r.processRoomEvent(r.ProcessContext.Context(), &inputRoomEvent); err != nil {
 					if !errors.Is(err, context.DeadlineExceeded) && !errors.Is(err, context.Canceled) {
 						sentry.CaptureException(err)
@@ -113,9 +115,19 @@ func (r *Inputer) Start() error {
 						"type":     inputRoomEvent.Event.Type(),
 					}).Warn("Roomserver failed to process async event")
 					_ = msg.Term()
+					errString = err.Error()
 				} else {
 					_ = msg.Ack()
 				}
+				if replyTo := msg.Header.Get("sync"); replyTo != "" {
+					if err := r.NATSClient.Publish(replyTo, []byte(errString)); err != nil {
+						logrus.WithError(err).WithFields(logrus.Fields{
+							"room_id":  roomID,
+							"event_id": inputRoomEvent.Event.EventID(),
+							"type":     inputRoomEvent.Event.Type(),
+						}).Warn("Roomserver failed to respond for sync event")
+					}
+				}
 			})
 		},
 		// NATS wants to acknowledge automatically by default when the message is
@@ -131,6 +143,9 @@ func (r *Inputer) Start() error {
 		// Ensure that NATS doesn't try to resend us something that wasn't done
 		// within the period of time that we might still be processing it.
 		nats.AckWait(MaximumMissingProcessingTime+(time.Second*10)),
+		// It is recommended to disable this for pull consumers as per the docs:
+		// https://docs.nats.io/nats-concepts/jetstream/consumers#note-about-push-and-pull-consumers
+		nats.MaxAckPending(-1),
 	)
 	return err
 }
@@ -141,74 +156,58 @@ func (r *Inputer) InputRoomEvents(
 	request *api.InputRoomEventsRequest,
 	response *api.InputRoomEventsResponse,
 ) {
-	if request.Asynchronous {
+	var replyTo string
+	var replySub *nats.Subscription
+	if !request.Asynchronous {
 		var err error
-		for _, e := range request.InputRoomEvents {
-			msg := &nats.Msg{
-				Subject: r.InputRoomEventTopic,
-				Header:  nats.Header{},
-			}
-			roomID := e.Event.RoomID()
-			msg.Header.Set("room_id", roomID)
-			msg.Data, err = json.Marshal(e)
-			if err != nil {
-				response.ErrMsg = err.Error()
-				return
-			}
-			if _, err = r.JetStream.PublishMsg(msg); err != nil {
-				logrus.WithError(err).WithFields(logrus.Fields{
-					"room_id":  roomID,
-					"event_id": e.Event.EventID(),
-				}).Error("Roomserver failed to queue async event")
-				return
-			}
+		replyTo = nats.NewInbox()
+		replySub, err = r.NATSClient.SubscribeSync(replyTo)
+		if err != nil {
+			response.ErrMsg = err.Error()
+			return
 		}
-	} else {
-		responses := make(chan error, len(request.InputRoomEvents))
-		for _, e := range request.InputRoomEvents {
-			inputRoomEvent := e
-			roomID := inputRoomEvent.Event.RoomID()
-			index := roomID + "\000" + inputRoomEvent.Event.EventID()
-			if _, ok := eventsInProgress.LoadOrStore(index, struct{}{}); ok {
-				// We're already waiting to deal with this event, so there's no
-				// point in queuing it up again. We've notified NATS that we're
-				// working on the message still, so that will have deferred the
-				// redelivery by a bit.
-				return
-			}
-			roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Inc()
-			worker := r.workerForRoom(roomID)
-			worker.Act(nil, func() {
-				defer eventsInProgress.Delete(index)
-				defer roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Dec()
-				err := r.processRoomEvent(ctx, &inputRoomEvent)
-				if err != nil {
-					if !errors.Is(err, context.DeadlineExceeded) && !errors.Is(err, context.Canceled) {
-						sentry.CaptureException(err)
-					}
-					logrus.WithError(err).WithFields(logrus.Fields{
-						"room_id":  roomID,
-						"event_id": inputRoomEvent.Event.EventID(),
-					}).Warn("Roomserver failed to process sync event")
-				}
-				select {
-				case <-ctx.Done():
-				default:
-					responses <- err
-				}
-			})
+	}
+
+	var err error
+	for _, e := range request.InputRoomEvents {
+		msg := &nats.Msg{
+			Subject: r.InputRoomEventTopic,
+			Header:  nats.Header{},
+			Reply:   replyTo,
 		}
-		for i := 0; i < len(request.InputRoomEvents); i++ {
-			select {
-			case <-ctx.Done():
-				response.ErrMsg = context.DeadlineExceeded.Error()
-				return
-			case err := <-responses:
-				if err != nil {
-					response.ErrMsg = err.Error()
-					return
-				}
-			}
+		roomID := e.Event.RoomID()
+		msg.Header.Set("room_id", roomID)
+		if replyTo != "" {
+			msg.Header.Set("sync", replyTo)
+		}
+		msg.Data, err = json.Marshal(e)
+		if err != nil {
+			response.ErrMsg = err.Error()
+			return
+		}
+		if _, err = r.JetStream.PublishMsg(msg); err != nil {
+			logrus.WithError(err).WithFields(logrus.Fields{
+				"room_id":  roomID,
+				"event_id": e.Event.EventID(),
+			}).Error("Roomserver failed to queue async event")
+			return
+		}
+	}
+
+	if request.Asynchronous || replySub == nil {
+		return
+	}
+
+	defer replySub.Drain() // nolint:errcheck
+	for i := 0; i < len(request.InputRoomEvents); i++ {
+		msg, err := replySub.NextMsgWithContext(ctx)
+		if err != nil {
+			response.ErrMsg = err.Error()
+			return
+		}
+		if len(msg.Data) > 0 {
+			response.ErrMsg = string(msg.Data)
+			return
 		}
 	}
 }
diff --git a/roomserver/roomserver.go b/roomserver/roomserver.go
index 950c6b4e7..1992ac335 100644
--- a/roomserver/roomserver.go
+++ b/roomserver/roomserver.go
@@ -50,10 +50,10 @@ func NewInternalAPI(
 		logrus.WithError(err).Panicf("failed to connect to room server db")
 	}
 
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, nc := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	return internal.NewRoomserverAPI(
-		base.ProcessContext, cfg, roomserverDB, js,
+		base.ProcessContext, cfg, roomserverDB, js, nc,
 		cfg.Matrix.JetStream.TopicFor(jetstream.InputRoomEvent),
 		cfg.Matrix.JetStream.TopicFor(jetstream.OutputRoomEvent),
 		base.Caches, perspectiveServerNames,
diff --git a/setup/jetstream/helpers.go b/setup/jetstream/helpers.go
index d444272d2..78cecb6ae 100644
--- a/setup/jetstream/helpers.go
+++ b/setup/jetstream/helpers.go
@@ -71,8 +71,8 @@ func JetStreamConsumer(
 				continue
 			}
 			if f(ctx, msg) {
-				if err = msg.Ack(); err != nil {
-					logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.Ack: %w", err))
+				if err = msg.AckSync(); err != nil {
+					logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.AckSync: %w", err))
 					sentry.CaptureException(err)
 				}
 			} else {
diff --git a/setup/jetstream/nats.go b/setup/jetstream/nats.go
index 562b0131e..37597d584 100644
--- a/setup/jetstream/nats.go
+++ b/setup/jetstream/nats.go
@@ -15,7 +15,7 @@ import (
 var natsServer *natsserver.Server
 var natsServerMutex sync.Mutex
 
-func Prepare(cfg *config.JetStream) natsclient.JetStreamContext {
+func Prepare(cfg *config.JetStream) (natsclient.JetStreamContext, *natsclient.Conn) {
 	// check if we need an in-process NATS Server
 	if len(cfg.Addresses) != 0 {
 		return setupNATS(cfg, nil)
@@ -48,20 +48,20 @@ func Prepare(cfg *config.JetStream) natsclient.JetStreamContext {
 	return setupNATS(cfg, nc)
 }
 
-func setupNATS(cfg *config.JetStream, nc *natsclient.Conn) natsclient.JetStreamContext {
+func setupNATS(cfg *config.JetStream, nc *natsclient.Conn) (natsclient.JetStreamContext, *natsclient.Conn) {
 	if nc == nil {
 		var err error
 		nc, err = natsclient.Connect(strings.Join(cfg.Addresses, ","))
 		if err != nil {
 			logrus.WithError(err).Panic("Unable to connect to NATS")
-			return nil
+			return nil, nil
 		}
 	}
 
 	s, err := nc.JetStream()
 	if err != nil {
 		logrus.WithError(err).Panic("Unable to get JetStream context")
-		return nil
+		return nil, nil
 	}
 
 	for _, stream := range streams { // streams are defined in streams.go
@@ -89,5 +89,5 @@ func setupNATS(cfg *config.JetStream, nc *natsclient.Conn) natsclient.JetStreamC
 		}
 	}
 
-	return s
+	return s, nc
 }
diff --git a/setup/jetstream/streams.go b/setup/jetstream/streams.go
index 3f07488f9..aa3e95cb8 100644
--- a/setup/jetstream/streams.go
+++ b/setup/jetstream/streams.go
@@ -42,7 +42,7 @@ var streams = []*nats.StreamConfig{
 	},
 	{
 		Name:      OutputKeyChangeEvent,
-		Retention: nats.LimitsPolicy,
+		Retention: nats.InterestPolicy,
 		Storage:   nats.FileStorage,
 	},
 	{
diff --git a/syncapi/syncapi.go b/syncapi/syncapi.go
index cb9890ff7..41635c911 100644
--- a/syncapi/syncapi.go
+++ b/syncapi/syncapi.go
@@ -49,7 +49,7 @@ func AddPublicRoutes(
 	federation *gomatrixserverlib.FederationClient,
 	cfg *config.SyncAPI,
 ) {
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	syncDB, err := storage.NewSyncServerDatasource(&cfg.Database)
 	if err != nil {
diff --git a/userapi/userapi.go b/userapi/userapi.go
index 251a4edad..1e4ebcb2e 100644
--- a/userapi/userapi.go
+++ b/userapi/userapi.go
@@ -46,7 +46,7 @@ func NewInternalAPI(
 	appServices []config.ApplicationService, keyAPI keyapi.KeyInternalAPI,
 	rsAPI rsapi.RoomserverInternalAPI, pgClient pushgateway.Client,
 ) api.UserInternalAPI {
-	js := jetstream.Prepare(&cfg.Matrix.JetStream)
+	js, _ := jetstream.Prepare(&cfg.Matrix.JetStream)
 
 	syncProducer := producers.NewSyncAPI(
 		db, js,