Handle event_format federation in /sync responses (#3192)

12 files changed, 734 insertions, 79 deletions

diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index 1e87aee9..9df5e0f9 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -33,6 +33,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/notifier"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/streams"
+	"github.com/matrix-org/dendrite/syncapi/synctypes"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 	"github.com/nats-io/nats.go"
@@ -592,16 +593,10 @@ func (s *OutputRoomEventConsumer) updateStateEvent(event *rstypes.HeaderedEvent)
 		return event, nil
 	}
 
-	prevEventSender := string(prevEvent.SenderID())
-	prevUser, err := s.rsAPI.QueryUserIDForSender(s.ctx, *validRoomID, prevEvent.SenderID())
-	if err == nil && prevUser != nil {
-		prevEventSender = prevUser.String()
-	}
-
-	prev := types.PrevEventRef{
+	prev := synctypes.PrevEventRef{
 		PrevContent:   prevEvent.Content(),
 		ReplacesState: prevEvent.EventID(),
-		PrevSenderID:  prevEventSender,
+		PrevSenderID:  string(prevEvent.SenderID()),
 	}
 
 	event.PDU, err = event.SetUnsigned(prev)
diff --git a/syncapi/routing/getevent.go b/syncapi/routing/getevent.go
index 4fa282f3..bf0f9bf8 100644
--- a/syncapi/routing/getevent.go
+++ b/syncapi/routing/getevent.go
@@ -144,6 +144,6 @@ func GetEvent(
 	}
 	return util.JSONResponse{
 		Code: http.StatusOK,
-		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll, *senderUserID, sk),
+		JSON: synctypes.ToClientEvent(events[0], synctypes.FormatAll, senderUserID.String(), sk, events[0].Unsigned()),
 	}
 }
diff --git a/syncapi/routing/relations.go b/syncapi/routing/relations.go
index e3d1069a..b451a7e2 100644
--- a/syncapi/routing/relations.go
+++ b/syncapi/routing/relations.go
@@ -146,7 +146,7 @@ func Relations(
 		}
 		res.Chunk = append(
 			res.Chunk,
-			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender, sk),
+			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll, sender.String(), sk, event.Unsigned()),
 		)
 	}
 
diff --git a/syncapi/routing/search.go b/syncapi/routing/search.go
index d892b604..7d5c061b 100644
--- a/syncapi/routing/search.go
+++ b/syncapi/routing/search.go
@@ -267,7 +267,7 @@ func Search(req *http.Request, device *api.Device, syncDB storage.Database, fts
 				ProfileInfo: profileInfos,
 			},
 			Rank:   eventScore[event.EventID()].Score,
-			Result: synctypes.ToClientEvent(event, synctypes.FormatAll, sender, sk),
+			Result: synctypes.ToClientEvent(event, synctypes.FormatAll, sender.String(), sk, event.Unsigned()),
 		})
 		roomGroup := groups[event.RoomID()]
 		roomGroup.Results = append(roomGroup.Results, event.EventID())
diff --git a/syncapi/streams/stream_invite.go b/syncapi/streams/stream_invite.go
index 7c29d84a..1ce3346f 100644
--- a/syncapi/streams/stream_invite.go
+++ b/syncapi/streams/stream_invite.go
@@ -63,6 +63,11 @@ func (p *InviteStreamProvider) IncrementalSync(
 		return from
 	}
 
+	eventFormat := synctypes.FormatSync
+	if req.Filter.EventFormat == synctypes.EventFormatFederation {
+		eventFormat = synctypes.FormatSyncFederation
+	}
+
 	for roomID, inviteEvent := range invites {
 		user := spec.UserID{}
 		validRoomID, err := spec.NewRoomID(inviteEvent.RoomID())
@@ -87,7 +92,7 @@ func (p *InviteStreamProvider) IncrementalSync(
 		if _, ok := req.IgnoredUsers.List[user.String()]; ok {
 			continue
 		}
-		ir := types.NewInviteResponse(inviteEvent, user, sk)
+		ir := types.NewInviteResponse(inviteEvent, user, sk, eventFormat)
 		req.Response.Rooms.Invite[roomID] = ir
 	}
 
diff --git a/syncapi/streams/stream_pdu.go b/syncapi/streams/stream_pdu.go
index 4622c21a..ee524f72 100644
--- a/syncapi/streams/stream_pdu.go
+++ b/syncapi/streams/stream_pdu.go
@@ -88,6 +88,11 @@ func (p *PDUStreamProvider) CompleteSync(
 		req.Log.WithError(err).Error("unable to update event filter with ignored users")
 	}
 
+	eventFormat := synctypes.FormatSync
+	if req.Filter.EventFormat == synctypes.EventFormatFederation {
+		eventFormat = synctypes.FormatSyncFederation
+	}
+
 	recentEvents, err := snapshot.RecentEvents(ctx, joinedRoomIDs, r, &eventFilter, true, true)
 	if err != nil {
 		return from
@@ -105,7 +110,7 @@ func (p *PDUStreamProvider) CompleteSync(
 		// get the join response for each room
 		jr, jerr := p.getJoinResponseForCompleteSync(
 			ctx, snapshot, roomID, &stateFilter, req.WantFullState, req.Device, false,
-			events.Events, events.Limited,
+			events.Events, events.Limited, eventFormat,
 		)
 		if jerr != nil {
 			req.Log.WithError(jerr).Error("p.getJoinResponseForCompleteSync failed")
@@ -142,7 +147,7 @@ func (p *PDUStreamProvider) CompleteSync(
 			events := recentEvents[roomID]
 			jr, err = p.getJoinResponseForCompleteSync(
 				ctx, snapshot, roomID, &stateFilter, req.WantFullState, req.Device, true,
-				events.Events, events.Limited,
+				events.Events, events.Limited, eventFormat,
 			)
 			if err != nil {
 				req.Log.WithError(err).Error("p.getJoinResponseForCompleteSync failed")
@@ -346,6 +351,11 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 		return r.From, fmt.Errorf("p.DB.GetBackwardTopologyPos: %w", err)
 	}
 
+	eventFormat := synctypes.FormatSync
+	if req.Filter.EventFormat == synctypes.EventFormatFederation {
+		eventFormat = synctypes.FormatSyncFederation
+	}
+
 	// Now that we've filtered the timeline, work out which state events are still
 	// left. Anything that appears in the filtered timeline will be removed from the
 	// "state" section and kept in "timeline".
@@ -359,7 +369,7 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 			continue
 		}
 		var newEvent gomatrixserverlib.PDU
-		newEvent, err = p.updatePowerLevelEvent(ctx, ev)
+		newEvent, err = p.updatePowerLevelEvent(ctx, ev, eventFormat)
 		if err != nil {
 			return r.From, err
 		}
@@ -383,7 +393,7 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 		// update the powerlevel event for state events
 		if ev.Version() == gomatrixserverlib.RoomVersionPseudoIDs && ev.Type() == spec.MRoomPowerLevels && ev.StateKeyEquals("") {
 			var newEvent gomatrixserverlib.PDU
-			newEvent, err = p.updatePowerLevelEvent(ctx, he)
+			newEvent, err = p.updatePowerLevelEvent(ctx, he, eventFormat)
 			if err != nil {
 				return r.From, err
 			}
@@ -413,13 +423,13 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 			}
 		}
 		jr.Timeline.PrevBatch = &prevBatch
-		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		// If we are limited by the filter AND the history visibility filter
 		// didn't "remove" events, return that the response is limited.
 		jr.Timeline.Limited = (limited && len(events) == len(recentEvents)) || delta.NewlyJoined
-		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		req.Response.Rooms.Join[delta.RoomID] = jr
@@ -428,11 +438,11 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 		jr := types.NewJoinResponse()
 		jr.Timeline.PrevBatch = &prevBatch
 		// TODO: Apply history visibility on peeked rooms
-		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(recentEvents), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(recentEvents), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		jr.Timeline.Limited = limited
-		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		req.Response.Rooms.Peek[delta.RoomID] = jr
@@ -443,13 +453,13 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 	case spec.Ban:
 		lr := types.NewLeaveResponse()
 		lr.Timeline.PrevBatch = &prevBatch
-		lr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		lr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		// If we are limited by the filter AND the history visibility filter
 		// didn't "remove" events, return that the response is limited.
 		lr.Timeline.Limited = limited && len(events) == len(recentEvents)
-		lr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+		lr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(delta.StateEvents), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 			return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 		})
 		req.Response.Rooms.Leave[delta.RoomID] = lr
@@ -458,7 +468,7 @@ func (p *PDUStreamProvider) addRoomDeltaToResponse(
 	return latestPosition, nil
 }
 
-func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstypes.HeaderedEvent) (gomatrixserverlib.PDU, error) {
+func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstypes.HeaderedEvent, eventFormat synctypes.ClientEventFormat) (gomatrixserverlib.PDU, error) {
 	pls, err := gomatrixserverlib.NewPowerLevelContentFromEvent(ev)
 	if err != nil {
 		return nil, err
@@ -467,11 +477,14 @@ func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstyp
 	var userID *spec.UserID
 	for user, level := range pls.Users {
 		validRoomID, _ := spec.NewRoomID(ev.RoomID())
-		userID, err = p.rsAPI.QueryUserIDForSender(ctx, *validRoomID, spec.SenderID(user))
-		if err != nil {
-			return nil, err
+		if eventFormat != synctypes.FormatSyncFederation {
+			userID, err = p.rsAPI.QueryUserIDForSender(ctx, *validRoomID, spec.SenderID(user))
+			if err != nil {
+				return nil, err
+			}
+			user = userID.String()
 		}
-		newPls[userID.String()] = level
+		newPls[user] = level
 	}
 	var newPlBytes, newEv []byte
 	newPlBytes, err = json.Marshal(newPls)
@@ -487,7 +500,7 @@ func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstyp
 	prevContent := gjson.GetBytes(ev.JSON(), "unsigned.prev_content")
 	if !prevContent.Exists() {
 		var evNew gomatrixserverlib.PDU
-		evNew, err = gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON(newEv, false)
+		evNew, err = gomatrixserverlib.MustGetRoomVersion(ev.Version()).NewEventFromTrustedJSON(newEv, false)
 		if err != nil {
 			return nil, err
 		}
@@ -503,11 +516,14 @@ func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstyp
 	newPls = make(map[string]int64)
 	for user, level := range pls.Users {
 		validRoomID, _ := spec.NewRoomID(ev.RoomID())
-		userID, err = p.rsAPI.QueryUserIDForSender(ctx, *validRoomID, spec.SenderID(user))
-		if err != nil {
-			return nil, err
+		if eventFormat != synctypes.FormatSyncFederation {
+			userID, err = p.rsAPI.QueryUserIDForSender(ctx, *validRoomID, spec.SenderID(user))
+			if err != nil {
+				return nil, err
+			}
+			user = userID.String()
 		}
-		newPls[userID.String()] = level
+		newPls[user] = level
 	}
 	newPlBytes, err = json.Marshal(newPls)
 	if err != nil {
@@ -519,7 +535,7 @@ func (p *PDUStreamProvider) updatePowerLevelEvent(ctx context.Context, ev *rstyp
 	}
 
 	var evNew gomatrixserverlib.PDU
-	evNew, err = gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSONWithEventID(ev.EventID(), newEv, false)
+	evNew, err = gomatrixserverlib.MustGetRoomVersion(ev.Version()).NewEventFromTrustedJSONWithEventID(ev.EventID(), newEv, false)
 	if err != nil {
 		return nil, err
 	}
@@ -592,6 +608,7 @@ func (p *PDUStreamProvider) getJoinResponseForCompleteSync(
 	isPeek bool,
 	recentStreamEvents []types.StreamEvent,
 	limited bool,
+	eventFormat synctypes.ClientEventFormat,
 ) (jr *types.JoinResponse, err error) {
 	jr = types.NewJoinResponse()
 	// TODO: When filters are added, we may need to call this multiple times to get enough events.
@@ -683,7 +700,7 @@ func (p *PDUStreamProvider) getJoinResponseForCompleteSync(
 		if ev.Type() != spec.MRoomPowerLevels || !ev.StateKeyEquals("") {
 			continue
 		}
-		newEvent, err := p.updatePowerLevelEvent(ctx, ev)
+		newEvent, err := p.updatePowerLevelEvent(ctx, ev, eventFormat)
 		if err != nil {
 			return nil, err
 		}
@@ -697,7 +714,7 @@ func (p *PDUStreamProvider) getJoinResponseForCompleteSync(
 		if ev.Type() != spec.MRoomPowerLevels || !ev.StateKeyEquals("") {
 			continue
 		}
-		newEvent, err := p.updatePowerLevelEvent(ctx, ev)
+		newEvent, err := p.updatePowerLevelEvent(ctx, ev, eventFormat)
 		if err != nil {
 			return nil, err
 		}
@@ -705,13 +722,13 @@ func (p *PDUStreamProvider) getJoinResponseForCompleteSync(
 	}
 
 	jr.Timeline.PrevBatch = prevBatch
-	jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+	jr.Timeline.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(events), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 		return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 	})
 	// If we are limited by the filter AND the history visibility filter
 	// didn't "remove" events, return that the response is limited.
 	jr.Timeline.Limited = limited && len(events) == len(recentEvents)
-	jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(stateEvents), synctypes.FormatSync, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+	jr.State.Events = synctypes.ToClientEvents(gomatrixserverlib.ToPDUs(stateEvents), eventFormat, func(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
 		return p.rsAPI.QueryUserIDForSender(ctx, roomID, senderID)
 	})
 	return jr, nil
diff --git a/syncapi/syncapi_test.go b/syncapi/syncapi_test.go
index ea1183cd..f2971995 100644
--- a/syncapi/syncapi_test.go
+++ b/syncapi/syncapi_test.go
@@ -209,6 +209,156 @@ func testSyncAccessTokens(t *testing.T, dbType test.DBType) {
 	}
 }
 
+func TestSyncAPIEventFormatPowerLevels(t *testing.T) {
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		testSyncEventFormatPowerLevels(t, dbType)
+	})
+}
+
+func testSyncEventFormatPowerLevels(t *testing.T, dbType test.DBType) {
+	user := test.NewUser(t)
+	setRoomVersion := func(t *testing.T, r *test.Room) { r.Version = gomatrixserverlib.RoomVersionPseudoIDs }
+	room := test.NewRoom(t, user, setRoomVersion)
+	alice := userapi.Device{
+		ID:          "ALICEID",
+		UserID:      user.ID,
+		AccessToken: "ALICE_BEARER_TOKEN",
+		DisplayName: "Alice",
+		AccountType: userapi.AccountTypeUser,
+	}
+
+	room.CreateAndInsert(t, user, spec.MRoomPowerLevels, gomatrixserverlib.PowerLevelContent{
+		Users: map[string]int64{
+			user.ID: 100,
+		},
+	}, test.WithStateKey(""))
+
+	cfg, processCtx, close := testrig.CreateConfig(t, dbType)
+	routers := httputil.NewRouters()
+	cm := sqlutil.NewConnectionManager(processCtx, cfg.Global.DatabaseOptions)
+	caches := caching.NewRistrettoCache(128*1024*1024, time.Hour, caching.DisableMetrics)
+	natsInstance := jetstream.NATSInstance{}
+	defer close()
+
+	jsctx, _ := natsInstance.Prepare(processCtx, &cfg.Global.JetStream)
+	defer jetstream.DeleteAllStreams(jsctx, &cfg.Global.JetStream)
+	msgs := toNATSMsgs(t, cfg, room.Events()...)
+	AddPublicRoutes(processCtx, routers, cfg, cm, &natsInstance, &syncUserAPI{accounts: []userapi.Device{alice}}, &syncRoomserverAPI{rooms: []*test.Room{room}}, caches, caching.DisableMetrics)
+	testrig.MustPublishMsgs(t, jsctx, msgs...)
+
+	testCases := []struct {
+		name            string
+		wantCode        int
+		wantJoinedRooms []string
+		eventFormat     synctypes.ClientEventFormat
+	}{
+		{
+			name:            "Client format",
+			wantCode:        200,
+			wantJoinedRooms: []string{room.ID},
+			eventFormat:     synctypes.FormatSync,
+		},
+		{
+			name:            "Federation format",
+			wantCode:        200,
+			wantJoinedRooms: []string{room.ID},
+			eventFormat:     synctypes.FormatSyncFederation,
+		},
+	}
+
+	syncUntil(t, routers, alice.AccessToken, false, func(syncBody string) bool {
+		// wait for the last sent eventID to come down sync
+		path := fmt.Sprintf(`rooms.join.%s.timeline.events.#(event_id=="%s")`, room.ID, room.Events()[len(room.Events())-1].EventID())
+		return gjson.Get(syncBody, path).Exists()
+	})
+
+	for _, tc := range testCases {
+		format := ""
+		if tc.eventFormat == synctypes.FormatSyncFederation {
+			format = "federation"
+		}
+
+		w := httptest.NewRecorder()
+		routers.Client.ServeHTTP(w, test.NewRequest(t, "GET", "/_matrix/client/v3/sync", test.WithQueryParams(map[string]string{
+			"access_token": alice.AccessToken,
+			"timeout":      "0",
+			"filter":       fmt.Sprintf(`{"event_format":"%s"}`, format),
+		})))
+		if w.Code != tc.wantCode {
+			t.Fatalf("%s: got HTTP %d want %d", tc.name, w.Code, tc.wantCode)
+		}
+		if tc.wantJoinedRooms != nil {
+			var res types.Response
+			if err := json.NewDecoder(w.Body).Decode(&res); err != nil {
+				t.Fatalf("%s: failed to decode response body: %s", tc.name, err)
+			}
+			if len(res.Rooms.Join) != len(tc.wantJoinedRooms) {
+				t.Errorf("%s: got %v joined rooms, want %v.\nResponse: %+v", tc.name, len(res.Rooms.Join), len(tc.wantJoinedRooms), res)
+			}
+			t.Logf("res: %+v", res.Rooms.Join[room.ID])
+
+			gotEventIDs := make([]string, len(res.Rooms.Join[room.ID].Timeline.Events))
+			for i, ev := range res.Rooms.Join[room.ID].Timeline.Events {
+				gotEventIDs[i] = ev.EventID
+			}
+			test.AssertEventIDsEqual(t, gotEventIDs, room.Events())
+
+			event := room.CreateAndInsert(t, user, spec.MRoomPowerLevels, gomatrixserverlib.PowerLevelContent{
+				Users: map[string]int64{
+					user.ID:                100,
+					"@otheruser:localhost": 50,
+				},
+			}, test.WithStateKey(""))
+
+			msgs := toNATSMsgs(t, cfg, event)
+			testrig.MustPublishMsgs(t, jsctx, msgs...)
+
+			syncUntil(t, routers, alice.AccessToken, false, func(syncBody string) bool {
+				// wait for the last sent eventID to come down sync
+				path := fmt.Sprintf(`rooms.join.%s.timeline.events.#(event_id=="%s")`, room.ID, room.Events()[len(room.Events())-1].EventID())
+				return gjson.Get(syncBody, path).Exists()
+			})
+
+			since := res.NextBatch.String()
+			w := httptest.NewRecorder()
+			routers.Client.ServeHTTP(w, test.NewRequest(t, "GET", "/_matrix/client/v3/sync", test.WithQueryParams(map[string]string{
+				"access_token": alice.AccessToken,
+				"timeout":      "0",
+				"filter":       fmt.Sprintf(`{"event_format":"%s"}`, format),
+				"since":        since,
+			})))
+			if w.Code != 200 {
+				t.Errorf("since=%s got HTTP %d want 200", since, w.Code)
+			}
+
+			res = *types.NewResponse()
+			if err := json.NewDecoder(w.Body).Decode(&res); err != nil {
+				t.Errorf("failed to decode response body: %s", err)
+			}
+			if len(res.Rooms.Join) != 1 {
+				t.Fatalf("since=%s got %d joined rooms, want 1", since, len(res.Rooms.Join))
+			}
+			gotEventIDs = make([]string, len(res.Rooms.Join[room.ID].Timeline.Events))
+			for j, ev := range res.Rooms.Join[room.ID].Timeline.Events {
+				gotEventIDs[j] = ev.EventID
+				if ev.Type == spec.MRoomPowerLevels {
+					content := gomatrixserverlib.PowerLevelContent{}
+					err := json.Unmarshal(ev.Content, &content)
+					if err != nil {
+						t.Errorf("failed to unmarshal power level content: %s", err)
+					}
+					otherUserLevel := content.UserLevel("@otheruser:localhost")
+					if otherUserLevel != 50 {
+						t.Errorf("Expected user PL of %d but got %d", 50, otherUserLevel)
+					}
+				}
+			}
+			events := []*rstypes.HeaderedEvent{room.Events()[len(room.Events())-1]}
+			test.AssertEventIDsEqual(t, gotEventIDs, events)
+		}
+	}
+}
+
 // Tests what happens when we create a room and then /sync before all events from /createRoom have
 // been sent to the syncapi
 func TestSyncAPICreateRoomSyncEarly(t *testing.T) {
diff --git a/syncapi/synctypes/clientevent.go b/syncapi/synctypes/clientevent.go
index a78aea1c..7e5b1c1b 100644
--- a/syncapi/synctypes/clientevent.go
+++ b/syncapi/synctypes/clientevent.go
@@ -16,12 +16,21 @@
 package synctypes
 
 import (
+	"encoding/json"
 	"fmt"
 
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
+	"github.com/sirupsen/logrus"
 )
 
+// PrevEventRef represents a reference to a previous event in a state event upgrade
+type PrevEventRef struct {
+	PrevContent   json.RawMessage `json:"prev_content"`
+	ReplacesState string          `json:"replaces_state"`
+	PrevSenderID  string          `json:"prev_sender"`
+}
+
 type ClientEventFormat int
 
 const (
@@ -30,8 +39,21 @@ const (
 	// FormatSync will include only the event keys required by the /sync API. Notably, this
 	// means the 'room_id' will be missing from the events.
 	FormatSync
+	// FormatSyncFederation will include all event keys normally included in federated events.
+	// This allows clients to request federated formatted events via the /sync API.
+	FormatSyncFederation
 )
 
+// ClientFederationFields extends a ClientEvent to contain the additional fields present in a
+// federation event. Used when the client requests `event_format` of type `federation`.
+type ClientFederationFields struct {
+	Depth      int64        `json:"depth,omitempty"`
+	PrevEvents []string     `json:"prev_events,omitempty"`
+	AuthEvents []string     `json:"auth_events,omitempty"`
+	Signatures spec.RawJSON `json:"signatures,omitempty"`
+	Hashes     spec.RawJSON `json:"hashes,omitempty"`
+}
+
 // ClientEvent is an event which is fit for consumption by clients, in accordance with the specification.
 type ClientEvent struct {
 	Content        spec.RawJSON   `json:"content"`
@@ -44,6 +66,9 @@ type ClientEvent struct {
 	Type           string         `json:"type"`
 	Unsigned       spec.RawJSON   `json:"unsigned,omitempty"`
 	Redacts        string         `json:"redacts,omitempty"`
+
+	// Only sent to clients when `event_format` == `federation`.
+	ClientFederationFields
 }
 
 // ToClientEvents converts server events to client events.
@@ -53,6 +78,11 @@ func ToClientEvents(serverEvs []gomatrixserverlib.PDU, format ClientEventFormat,
 		if se == nil {
 			continue // TODO: shouldn't happen?
 		}
+		if format == FormatSyncFederation {
+			evs = append(evs, ToClientEvent(se, format, string(se.SenderID()), se.StateKey(), spec.RawJSON(se.Unsigned())))
+			continue
+		}
+
 		sender := spec.UserID{}
 		validRoomID, err := spec.NewRoomID(se.RoomID())
 		if err != nil {
@@ -71,28 +101,61 @@ func ToClientEvents(serverEvs []gomatrixserverlib.PDU, format ClientEventFormat,
 				sk = &skString
 			}
 		}
-		evs = append(evs, ToClientEvent(se, format, sender, sk))
+
+		unsigned := se.Unsigned()
+		var prev PrevEventRef
+		if err := json.Unmarshal(se.Unsigned(), &prev); err == nil && prev.PrevSenderID != "" {
+			prevUserID, err := userIDForSender(*validRoomID, spec.SenderID(prev.PrevSenderID))
+			if err == nil && userID != nil {
+				prev.PrevSenderID = prevUserID.String()
+			} else {
+				errString := "userID unknown"
+				if err != nil {
+					errString = err.Error()
+				}
+				logrus.Warnf("Failed to find userID for prev_sender in ClientEvent: %s", errString)
+				// NOTE: Not much can be done here, so leave the previous value in place.
+			}
+			unsigned, err = json.Marshal(prev)
+			if err != nil {
+				logrus.Errorf("Failed to marshal unsigned content for ClientEvent: %s", err.Error())
+				continue
+			}
+		}
+		evs = append(evs, ToClientEvent(se, format, sender.String(), sk, spec.RawJSON(unsigned)))
 	}
 	return evs
 }
 
 // ToClientEvent converts a single server event to a client event.
-func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender spec.UserID, stateKey *string) ClientEvent {
+func ToClientEvent(se gomatrixserverlib.PDU, format ClientEventFormat, sender string, stateKey *string, unsigned spec.RawJSON) ClientEvent {
 	ce := ClientEvent{
 		Content:        spec.RawJSON(se.Content()),
-		Sender:         sender.String(),
+		Sender:         sender,
 		Type:           se.Type(),
 		StateKey:       stateKey,
-		Unsigned:       spec.RawJSON(se.Unsigned()),
+		Unsigned:       unsigned,
 		OriginServerTS: se.OriginServerTS(),
 		EventID:        se.EventID(),
 		Redacts:        se.Redacts(),
 	}
-	if format == FormatAll {
+
+	switch format {
+	case FormatAll:
+		ce.RoomID = se.RoomID()
+	case FormatSync:
+	case FormatSyncFederation:
 		ce.RoomID = se.RoomID()
+		ce.AuthEvents = se.AuthEventIDs()
+		ce.PrevEvents = se.PrevEventIDs()
+		ce.Depth = se.Depth()
+		// TODO: Set Signatures & Hashes fields
 	}
-	if se.Version() == gomatrixserverlib.RoomVersionPseudoIDs {
-		ce.SenderKey = se.SenderID()
+
+	if format != FormatSyncFederation {
+		if se.Version() == gomatrixserverlib.RoomVersionPseudoIDs {
+			ce.SenderKey = se.SenderID()
+		}
 	}
 	return ce
 }
@@ -118,7 +181,7 @@ func ToClientEventDefault(userIDQuery spec.UserIDForSender, event gomatrixserver
 			sk = &skString
 		}
 	}
-	return ToClientEvent(event, FormatAll, sender, sk)
+	return ToClientEvent(event, FormatAll, sender.String(), sk, event.Unsigned())
 }
 
 // If provided state key is a user ID (state keys beginning with @ are reserved for this purpose)
diff --git a/syncapi/synctypes/clientevent_test.go b/syncapi/synctypes/clientevent_test.go
index 63c65b2a..202c185f 100644
--- a/syncapi/synctypes/clientevent_test.go
+++ b/syncapi/synctypes/clientevent_test.go
@@ -18,12 +18,69 @@ package synctypes
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
+	"reflect"
 	"testing"
 
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/gomatrixserverlib/spec"
 )
 
+const testSenderID = "testSenderID"
+const testUserID = "@test:localhost"
+
+type EventFieldsToVerify struct {
+	EventID        string
+	Type           string
+	OriginServerTS spec.Timestamp
+	StateKey       *string
+	Content        spec.RawJSON
+	Unsigned       spec.RawJSON
+	Sender         string
+	Depth          int64
+	PrevEvents     []string
+	AuthEvents     []string
+}
+
+func verifyEventFields(t *testing.T, got EventFieldsToVerify, want EventFieldsToVerify) {
+	if got.EventID != want.EventID {
+		t.Errorf("ClientEvent.EventID: wanted %s, got %s", want.EventID, got.EventID)
+	}
+	if got.OriginServerTS != want.OriginServerTS {
+		t.Errorf("ClientEvent.OriginServerTS: wanted %d, got %d", want.OriginServerTS, got.OriginServerTS)
+	}
+	if got.StateKey == nil && want.StateKey != nil {
+		t.Errorf("ClientEvent.StateKey: no state key present when one was wanted: %s", *want.StateKey)
+	}
+	if got.StateKey != nil && want.StateKey == nil {
+		t.Errorf("ClientEvent.StateKey: state key present when one was not wanted: %s", *got.StateKey)
+	}
+	if got.StateKey != nil && want.StateKey != nil && *got.StateKey != *want.StateKey {
+		t.Errorf("ClientEvent.StateKey: wanted %s, got %s", *want.StateKey, *got.StateKey)
+	}
+	if got.Type != want.Type {
+		t.Errorf("ClientEvent.Type: wanted %s, got %s", want.Type, got.Type)
+	}
+	if !bytes.Equal(got.Content, want.Content) {
+		t.Errorf("ClientEvent.Content: wanted %s, got %s", string(want.Content), string(got.Content))
+	}
+	if !bytes.Equal(got.Unsigned, want.Unsigned) {
+		t.Errorf("ClientEvent.Unsigned: wanted %s, got %s", string(want.Unsigned), string(got.Unsigned))
+	}
+	if got.Sender != want.Sender {
+		t.Errorf("ClientEvent.Sender: wanted %s, got %s", want.Sender, got.Sender)
+	}
+	if got.Depth != want.Depth {
+		t.Errorf("ClientEvent.Depth: wanted %d, got %d", want.Depth, got.Depth)
+	}
+	if !reflect.DeepEqual(got.PrevEvents, want.PrevEvents) {
+		t.Errorf("ClientEvent.PrevEvents: wanted %v, got %v", want.PrevEvents, got.PrevEvents)
+	}
+	if !reflect.DeepEqual(got.AuthEvents, want.AuthEvents) {
+		t.Errorf("ClientEvent.AuthEvents: wanted %v, got %v", want.AuthEvents, got.AuthEvents)
+	}
+}
+
 func TestToClientEvent(t *testing.T) { // nolint: gocyclo
 	ev, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionV1).NewEventFromTrustedJSON([]byte(`{
 		"type": "m.room.name",
@@ -49,28 +106,28 @@ func TestToClientEvent(t *testing.T) { // nolint: gocyclo
 		t.Fatalf("failed to create userID: %s", err)
 	}
 	sk := ""
-	ce := ToClientEvent(ev, FormatAll, *userID, &sk)
-	if ce.EventID != ev.EventID() {
-		t.Errorf("ClientEvent.EventID: wanted %s, got %s", ev.EventID(), ce.EventID)
-	}
-	if ce.OriginServerTS != ev.OriginServerTS() {
-		t.Errorf("ClientEvent.OriginServerTS: wanted %d, got %d", ev.OriginServerTS(), ce.OriginServerTS)
-	}
-	if ce.StateKey == nil || *ce.StateKey != "" {
-		t.Errorf("ClientEvent.StateKey: wanted '', got %v", ce.StateKey)
-	}
-	if ce.Type != ev.Type() {
-		t.Errorf("ClientEvent.Type: wanted %s, got %s", ev.Type(), ce.Type)
-	}
-	if !bytes.Equal(ce.Content, ev.Content()) {
-		t.Errorf("ClientEvent.Content: wanted %s, got %s", string(ev.Content()), string(ce.Content))
-	}
-	if !bytes.Equal(ce.Unsigned, ev.Unsigned()) {
-		t.Errorf("ClientEvent.Unsigned: wanted %s, got %s", string(ev.Unsigned()), string(ce.Unsigned))
-	}
-	if ce.Sender != userID.String() {
-		t.Errorf("ClientEvent.Sender: wanted %s, got %s", userID.String(), ce.Sender)
-	}
+	ce := ToClientEvent(ev, FormatAll, userID.String(), &sk, ev.Unsigned())
+
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce.EventID,
+			Type:           ce.Type,
+			OriginServerTS: ce.OriginServerTS,
+			StateKey:       ce.StateKey,
+			Content:        ce.Content,
+			Unsigned:       ce.Unsigned,
+			Sender:         ce.Sender,
+		},
+		EventFieldsToVerify{
+			EventID:        ev.EventID(),
+			Type:           ev.Type(),
+			OriginServerTS: ev.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev.Content(),
+			Unsigned:       ev.Unsigned(),
+			Sender:         userID.String(),
+		})
+
 	j, err := json.Marshal(ce)
 	if err != nil {
 		t.Fatalf("failed to Marshal ClientEvent: %s", err)
@@ -109,8 +166,378 @@ func TestToClientFormatSync(t *testing.T) {
 		t.Fatalf("failed to create userID: %s", err)
 	}
 	sk := ""
-	ce := ToClientEvent(ev, FormatSync, *userID, &sk)
+	ce := ToClientEvent(ev, FormatSync, userID.String(), &sk, ev.Unsigned())
 	if ce.RoomID != "" {
 		t.Errorf("ClientEvent.RoomID: wanted '', got %s", ce.RoomID)
 	}
 }
+
+func TestToClientEventFormatSyncFederation(t *testing.T) { // nolint: gocyclo
+	ev, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionV10).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+		"state_key": "",
+		"event_id": "$test:localhost",
+		"room_id": "!test:localhost",
+		"sender": "@test:localhost",
+		"content": {
+			"name": "Hello World"
+		},
+		"origin_server_ts": 123456,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World"
+			}
+		},
+        "depth": 8,
+        "prev_events": [
+          "$f597Tp0Mm1PPxEgiprzJc2cZAjVhxCxACOGuwJb33Oo"
+        ],
+        "auth_events": [
+          "$Bj0ZGgX6VTqAQdqKH4ZG3l6rlbxY3rZlC5D3MeuK1OQ",
+          "$QsMs6A1PUVUhgSvmHBfpqEYJPgv4DXt96r8P2AK7iXQ",
+          "$tBteKtlnFiwlmPJsv0wkKTMEuUVWpQH89H7Xskxve1Q"
+        ]
+	}`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+	userID, err := spec.NewUserID("@test:localhost", true)
+	if err != nil {
+		t.Fatalf("failed to create userID: %s", err)
+	}
+	sk := ""
+	ce := ToClientEvent(ev, FormatSyncFederation, userID.String(), &sk, ev.Unsigned())
+
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce.EventID,
+			Type:           ce.Type,
+			OriginServerTS: ce.OriginServerTS,
+			StateKey:       ce.StateKey,
+			Content:        ce.Content,
+			Unsigned:       ce.Unsigned,
+			Sender:         ce.Sender,
+			Depth:          ce.Depth,
+			PrevEvents:     ce.PrevEvents,
+			AuthEvents:     ce.AuthEvents,
+		},
+		EventFieldsToVerify{
+			EventID:        ev.EventID(),
+			Type:           ev.Type(),
+			OriginServerTS: ev.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev.Content(),
+			Unsigned:       ev.Unsigned(),
+			Sender:         userID.String(),
+			Depth:          ev.Depth(),
+			PrevEvents:     ev.PrevEventIDs(),
+			AuthEvents:     ev.AuthEventIDs(),
+		})
+}
+
+func userIDForSender(roomID spec.RoomID, senderID spec.SenderID) (*spec.UserID, error) {
+	if senderID == "unknownSenderID" {
+		return nil, fmt.Errorf("Cannot find userID")
+	}
+	return spec.NewUserID(testUserID, true)
+}
+
+func TestToClientEventsFormatSyncFederation(t *testing.T) { // nolint: gocyclo
+	ev, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World"
+		},
+		"origin_server_ts": 123456,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World"
+			}
+		},
+        "depth": 8,
+        "prev_events": [
+          "$f597Tp0Mm1PPxEgiprzJc2cZAjVhxCxACOGuwJb33Oo"
+        ],
+        "auth_events": [
+          "$Bj0ZGgX6VTqAQdqKH4ZG3l6rlbxY3rZlC5D3MeuK1OQ",
+          "$QsMs6A1PUVUhgSvmHBfpqEYJPgv4DXt96r8P2AK7iXQ",
+          "$tBteKtlnFiwlmPJsv0wkKTMEuUVWpQH89H7Xskxve1Q"
+        ]
+	}`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+	ev2, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test2:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World 2"
+		},
+		"origin_server_ts": 1234567,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World 2"
+			},
+            "prev_sender": "testSenderID"
+		},
+        "depth": 9,
+        "prev_events": [
+          "$f597Tp0Mm1PPxEgiprzJc2cZAjVhxCxACOGuwJb33Oo"
+        ],
+        "auth_events": [
+          "$Bj0ZGgX6VTqAQdqKH4ZG3l6rlbxY3rZlC5D3MeuK1OQ",
+          "$QsMs6A1PUVUhgSvmHBfpqEYJPgv4DXt96r8P2AK7iXQ",
+          "$tBteKtlnFiwlmPJsv0wkKTMEuUVWpQH89H7Xskxve1Q"
+        ]
+    }`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+
+	clientEvents := ToClientEvents([]gomatrixserverlib.PDU{ev, ev2}, FormatSyncFederation, userIDForSender)
+	ce := clientEvents[0]
+	sk := testSenderID
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce.EventID,
+			Type:           ce.Type,
+			OriginServerTS: ce.OriginServerTS,
+			StateKey:       ce.StateKey,
+			Content:        ce.Content,
+			Unsigned:       ce.Unsigned,
+			Sender:         ce.Sender,
+			Depth:          ce.Depth,
+			PrevEvents:     ce.PrevEvents,
+			AuthEvents:     ce.AuthEvents,
+		},
+		EventFieldsToVerify{
+			EventID:        ev.EventID(),
+			Type:           ev.Type(),
+			OriginServerTS: ev.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev.Content(),
+			Unsigned:       ev.Unsigned(),
+			Sender:         testSenderID,
+			Depth:          ev.Depth(),
+			PrevEvents:     ev.PrevEventIDs(),
+			AuthEvents:     ev.AuthEventIDs(),
+		})
+
+	ce2 := clientEvents[1]
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce2.EventID,
+			Type:           ce2.Type,
+			OriginServerTS: ce2.OriginServerTS,
+			StateKey:       ce2.StateKey,
+			Content:        ce2.Content,
+			Unsigned:       ce2.Unsigned,
+			Sender:         ce2.Sender,
+			Depth:          ce2.Depth,
+			PrevEvents:     ce2.PrevEvents,
+			AuthEvents:     ce2.AuthEvents,
+		},
+		EventFieldsToVerify{
+			EventID:        ev2.EventID(),
+			Type:           ev2.Type(),
+			OriginServerTS: ev2.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev2.Content(),
+			Unsigned:       ev2.Unsigned(),
+			Sender:         testSenderID,
+			Depth:          ev2.Depth(),
+			PrevEvents:     ev2.PrevEventIDs(),
+			AuthEvents:     ev2.AuthEventIDs(),
+		})
+}
+
+func TestToClientEventsFormatSync(t *testing.T) { // nolint: gocyclo
+	ev, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World"
+		},
+		"origin_server_ts": 123456,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World"
+			}
+		}
+    }`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+	ev2, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test2:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World 2"
+		},
+		"origin_server_ts": 1234567,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World 2"
+			},
+            "prev_sender": "testSenderID"
+		},
+        "depth": 9	
+    }`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+
+	clientEvents := ToClientEvents([]gomatrixserverlib.PDU{ev, ev2}, FormatSync, userIDForSender)
+	ce := clientEvents[0]
+	sk := testUserID
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce.EventID,
+			Type:           ce.Type,
+			OriginServerTS: ce.OriginServerTS,
+			StateKey:       ce.StateKey,
+			Content:        ce.Content,
+			Unsigned:       ce.Unsigned,
+			Sender:         ce.Sender,
+		},
+		EventFieldsToVerify{
+			EventID:        ev.EventID(),
+			Type:           ev.Type(),
+			OriginServerTS: ev.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev.Content(),
+			Unsigned:       ev.Unsigned(),
+			Sender:         testUserID,
+		})
+
+	var prev PrevEventRef
+	prev.PrevContent = []byte(`{"name": "Goodbye World 2"}`)
+	prev.PrevSenderID = testUserID
+	expectedUnsigned, _ := json.Marshal(prev)
+
+	ce2 := clientEvents[1]
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce2.EventID,
+			Type:           ce2.Type,
+			OriginServerTS: ce2.OriginServerTS,
+			StateKey:       ce2.StateKey,
+			Content:        ce2.Content,
+			Unsigned:       ce2.Unsigned,
+			Sender:         ce2.Sender,
+		},
+		EventFieldsToVerify{
+			EventID:        ev2.EventID(),
+			Type:           ev2.Type(),
+			OriginServerTS: ev2.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev2.Content(),
+			Unsigned:       expectedUnsigned,
+			Sender:         testUserID,
+		})
+}
+
+func TestToClientEventsFormatSyncUnknownPrevSender(t *testing.T) { // nolint: gocyclo
+	ev, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World"
+		},
+		"origin_server_ts": 123456,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World"
+			}
+		}
+    }`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+	ev2, err := gomatrixserverlib.MustGetRoomVersion(gomatrixserverlib.RoomVersionPseudoIDs).NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.name",
+        "state_key": "testSenderID",
+		"event_id": "$test2:localhost",
+		"room_id": "!test:localhost",
+		"sender": "testSenderID",
+		"content": {
+			"name": "Hello World 2"
+		},
+		"origin_server_ts": 1234567,
+		"unsigned": {
+			"prev_content": {
+				"name": "Goodbye World 2"
+			},
+            "prev_sender": "unknownSenderID"
+		},
+        "depth": 9	
+    }`), false)
+	if err != nil {
+		t.Fatalf("failed to create Event: %s", err)
+	}
+
+	clientEvents := ToClientEvents([]gomatrixserverlib.PDU{ev, ev2}, FormatSync, userIDForSender)
+	ce := clientEvents[0]
+	sk := testUserID
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce.EventID,
+			Type:           ce.Type,
+			OriginServerTS: ce.OriginServerTS,
+			StateKey:       ce.StateKey,
+			Content:        ce.Content,
+			Unsigned:       ce.Unsigned,
+			Sender:         ce.Sender,
+		},
+		EventFieldsToVerify{
+			EventID:        ev.EventID(),
+			Type:           ev.Type(),
+			OriginServerTS: ev.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev.Content(),
+			Unsigned:       ev.Unsigned(),
+			Sender:         testUserID,
+		})
+
+	var prev PrevEventRef
+	prev.PrevContent = []byte(`{"name": "Goodbye World 2"}`)
+	prev.PrevSenderID = "unknownSenderID"
+	expectedUnsigned, _ := json.Marshal(prev)
+
+	ce2 := clientEvents[1]
+	verifyEventFields(t,
+		EventFieldsToVerify{
+			EventID:        ce2.EventID,
+			Type:           ce2.Type,
+			OriginServerTS: ce2.OriginServerTS,
+			StateKey:       ce2.StateKey,
+			Content:        ce2.Content,
+			Unsigned:       ce2.Unsigned,
+			Sender:         ce2.Sender,
+		},
+		EventFieldsToVerify{
+			EventID:        ev2.EventID(),
+			Type:           ev2.Type(),
+			OriginServerTS: ev2.OriginServerTS(),
+			StateKey:       &sk,
+			Content:        ev2.Content(),
+			Unsigned:       expectedUnsigned,
+			Sender:         testUserID,
+		})
+}
diff --git a/syncapi/synctypes/filter.go b/syncapi/synctypes/filter.go
index c994ddb9..8998d443 100644
--- a/syncapi/synctypes/filter.go
+++ b/syncapi/synctypes/filter.go
@@ -78,9 +78,14 @@ type RoomEventFilter struct {
 	ContainsURL               *bool     `json:"contains_url,omitempty"`
 }
 
+const (
+	EventFormatClient     = "client"
+	EventFormatFederation = "federation"
+)
+
 // Validate checks if the filter contains valid property values
 func (filter *Filter) Validate() error {
-	if filter.EventFormat != "" && filter.EventFormat != "client" && filter.EventFormat != "federation" {
+	if filter.EventFormat != "" && filter.EventFormat != EventFormatClient && filter.EventFormat != EventFormatFederation {
 		return errors.New("Bad event_format value. Must be one of [\"client\", \"federation\"]")
 	}
 	return nil
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index cb3c362d..b90c128c 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -339,13 +339,6 @@ func NewStreamTokenFromString(tok string) (token StreamingToken, err error) {
 	return token, nil
 }
 
-// PrevEventRef represents a reference to a previous event in a state event upgrade
-type PrevEventRef struct {
-	PrevContent   json.RawMessage `json:"prev_content"`
-	ReplacesState string          `json:"replaces_state"`
-	PrevSenderID  string          `json:"prev_sender"`
-}
-
 type DeviceLists struct {
 	Changed []string `json:"changed,omitempty"`
 	Left    []string `json:"left,omitempty"`
@@ -539,7 +532,7 @@ type InviteResponse struct {
 }
 
 // NewInviteResponse creates an empty response with initialised arrays.
-func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID, stateKey *string) *InviteResponse {
+func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID, stateKey *string, eventFormat synctypes.ClientEventFormat) *InviteResponse {
 	res := InviteResponse{}
 	res.InviteState.Events = []json.RawMessage{}
 
@@ -552,7 +545,7 @@ func NewInviteResponse(event *types.HeaderedEvent, userID spec.UserID, stateKey
 
 	// Then we'll see if we can create a partial of the invite event itself.
 	// This is needed for clients to work out *who* sent the invite.
-	inviteEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatSync, userID, stateKey)
+	inviteEvent := synctypes.ToClientEvent(event.PDU, eventFormat, userID.String(), stateKey, event.Unsigned())
 	inviteEvent.Unsigned = nil
 	if ev, err := json.Marshal(inviteEvent); err == nil {
 		res.InviteState.Events = append(res.InviteState.Events, ev)
diff --git a/syncapi/types/types_test.go b/syncapi/types/types_test.go
index c1b7f70b..a79b9fc5 100644
--- a/syncapi/types/types_test.go
+++ b/syncapi/types/types_test.go
@@ -72,7 +72,7 @@ func TestNewInviteResponse(t *testing.T) {
 	skString := skUserID.String()
 	sk := &skString
 
-	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev}, *sender, sk)
+	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev}, *sender, sk, synctypes.FormatSync)
 	j, err := json.Marshal(res)
 	if err != nil {
 		t.Fatal(err)