diff --git a/federationapi/routing/send_test.go b/federationapi/routing/send_test.go
index e1211ffe9..ba653c1e8 100644
--- a/federationapi/routing/send_test.go
+++ b/federationapi/routing/send_test.go
@@ -77,10 +77,11 @@ func (p *testEDUProducer) InputSendToDeviceEvent(
 }
 
 type testRoomserverAPI struct {
-	inputRoomEvents           []api.InputRoomEvent
-	queryStateAfterEvents     func(*api.QueryStateAfterEventsRequest) api.QueryStateAfterEventsResponse
-	queryEventsByID           func(req *api.QueryEventsByIDRequest) api.QueryEventsByIDResponse
-	queryLatestEventsAndState func(*api.QueryLatestEventsAndStateRequest) api.QueryLatestEventsAndStateResponse
+	inputRoomEvents            []api.InputRoomEvent
+	queryMissingAuthPrevEvents func(*api.QueryMissingAuthPrevEventsRequest) api.QueryMissingAuthPrevEventsResponse
+	queryStateAfterEvents      func(*api.QueryStateAfterEventsRequest) api.QueryStateAfterEventsResponse
+	queryEventsByID            func(req *api.QueryEventsByIDRequest) api.QueryEventsByIDResponse
+	queryLatestEventsAndState  func(*api.QueryLatestEventsAndStateRequest) api.QueryLatestEventsAndStateResponse
 }
 
 func (t *testRoomserverAPI) SetFederationSenderAPI(fsAPI fsAPI.FederationSenderInternalAPI) {}
@@ -162,6 +163,20 @@ func (t *testRoomserverAPI) QueryStateAfterEvents(
 	return nil
 }
 
+// Query the state after a list of events in a room from the room server.
+func (t *testRoomserverAPI) QueryMissingAuthPrevEvents(
+	ctx context.Context,
+	request *api.QueryMissingAuthPrevEventsRequest,
+	response *api.QueryMissingAuthPrevEventsResponse,
+) error {
+	response.RoomVersion = testRoomVersion
+	res := t.queryMissingAuthPrevEvents(request)
+	response.RoomExists = res.RoomExists
+	response.MissingAuthEventIDs = res.MissingAuthEventIDs
+	response.MissingPrevEventIDs = res.MissingPrevEventIDs
+	return nil
+}
+
 // Query a list of events by event ID.
 func (t *testRoomserverAPI) QueryEventsByID(
 	ctx context.Context,
@@ -453,11 +468,11 @@ func assertInputRoomEvents(t *testing.T, got []api.InputRoomEvent, want []gomatr
 // to the roomserver. It's the most basic test possible.
 func TestBasicTransaction(t *testing.T) {
 	rsAPI := &testRoomserverAPI{
-		queryStateAfterEvents: func(req *api.QueryStateAfterEventsRequest) api.QueryStateAfterEventsResponse {
-			return api.QueryStateAfterEventsResponse{
-				PrevEventsExist: true,
-				RoomExists:      true,
-				StateEvents:     fromStateTuples(req.StateToFetch, nil),
+		queryMissingAuthPrevEvents: func(req *api.QueryMissingAuthPrevEventsRequest) api.QueryMissingAuthPrevEventsResponse {
+			return api.QueryMissingAuthPrevEventsResponse{
+				RoomExists:          true,
+				MissingAuthEventIDs: []string{},
+				MissingPrevEventIDs: []string{},
 			}
 		},
 	}
@@ -473,14 +488,11 @@ func TestBasicTransaction(t *testing.T) {
 // as it does the auth check.
 func TestTransactionFailAuthChecks(t *testing.T) {
 	rsAPI := &testRoomserverAPI{
-		queryStateAfterEvents: func(req *api.QueryStateAfterEventsRequest) api.QueryStateAfterEventsResponse {
-			return api.QueryStateAfterEventsResponse{
-				PrevEventsExist: true,
-				RoomExists:      true,
-				// omit the create event so auth checks fail
-				StateEvents: fromStateTuples(req.StateToFetch, []gomatrixserverlib.StateKeyTuple{
-					{EventType: gomatrixserverlib.MRoomCreate, StateKey: ""},
-				}),
+		queryMissingAuthPrevEvents: func(req *api.QueryMissingAuthPrevEventsRequest) api.QueryMissingAuthPrevEventsResponse {
+			return api.QueryMissingAuthPrevEventsResponse{
+				RoomExists:          true,
+				MissingAuthEventIDs: []string{"create_event"},
+				MissingPrevEventIDs: []string{},
 			}
 		},
 	}
@@ -504,28 +516,24 @@ func TestTransactionFetchMissingPrevEvents(t *testing.T) {
 
 	var rsAPI *testRoomserverAPI // ref here so we can refer to inputRoomEvents inside these functions
 	rsAPI = &testRoomserverAPI{
-		queryStateAfterEvents: func(req *api.QueryStateAfterEventsRequest) api.QueryStateAfterEventsResponse {
-			// we expect this to be called three times:
-			// - first with input event to realise there's a gap
-			// - second with the prevEvent to realise there is no gap
-			// - third with the input event to realise there is no longer a gap
-			prevEventsExist := false
+		queryMissingAuthPrevEvents: func(req *api.QueryMissingAuthPrevEventsRequest) api.QueryMissingAuthPrevEventsResponse {
+			missingPrevEvent := []string{"missing_prev_event"}
 			if len(req.PrevEventIDs) == 1 {
 				switch req.PrevEventIDs[0] {
 				case haveEvent.EventID():
-					prevEventsExist = true
+					missingPrevEvent = []string{}
 				case prevEvent.EventID():
 					// we only have this event if we've been send prevEvent
 					if len(rsAPI.inputRoomEvents) == 1 && rsAPI.inputRoomEvents[0].Event.EventID() == prevEvent.EventID() {
-						prevEventsExist = true
+						missingPrevEvent = []string{}
 					}
 				}
 			}
 
-			return api.QueryStateAfterEventsResponse{
-				PrevEventsExist: prevEventsExist,
-				RoomExists:      true,
-				StateEvents:     fromStateTuples(req.StateToFetch, nil),
+			return api.QueryMissingAuthPrevEventsResponse{
+				RoomExists:          true,
+				MissingAuthEventIDs: []string{},
+				MissingPrevEventIDs: missingPrevEvent,
 			}
 		},
 		queryLatestEventsAndState: func(req *api.QueryLatestEventsAndStateRequest) api.QueryLatestEventsAndStateResponse {
@@ -626,6 +634,38 @@ func TestTransactionFetchMissingStateByStateIDs(t *testing.T) {
 				StateEvents:     stateEvents,
 			}
 		},
+
+		queryMissingAuthPrevEvents: func(req *api.QueryMissingAuthPrevEventsRequest) api.QueryMissingAuthPrevEventsResponse {
+			askingForEvent := req.PrevEventIDs[0]
+			haveEventB := false
+			haveEventC := false
+			for _, ev := range rsAPI.inputRoomEvents {
+				switch ev.Event.EventID() {
+				case eventB.EventID():
+					haveEventB = true
+				case eventC.EventID():
+					haveEventC = true
+				}
+			}
+			prevEventExists := false
+			if askingForEvent == eventC.EventID() {
+				prevEventExists = haveEventC
+			} else if askingForEvent == eventB.EventID() {
+				prevEventExists = haveEventB
+			}
+
+			var missingPrevEvent []string
+			if !prevEventExists {
+				missingPrevEvent = []string{"test"}
+			}
+
+			return api.QueryMissingAuthPrevEventsResponse{
+				RoomExists:          true,
+				MissingAuthEventIDs: []string{},
+				MissingPrevEventIDs: missingPrevEvent,
+			}
+		},
+
 		queryLatestEventsAndState: func(req *api.QueryLatestEventsAndStateRequest) api.QueryLatestEventsAndStateResponse {
 			omitTuples := []gomatrixserverlib.StateKeyTuple{
 				{EventType: gomatrixserverlib.MRoomPowerLevels, StateKey: ""},