diff --git a/src/room/InCallView.test.tsx b/src/room/InCallView.test.tsx
index e4fc1d8bd..209fcc113 100644
--- a/src/room/InCallView.test.tsx
+++ b/src/room/InCallView.test.tsx
@@ -26,7 +26,7 @@ import {
   mockMuteStates,
   mockRtcMembership,
   type MockRTCSession,
-  mockMatrixRTCClient,
+  mockRTCSlot,
   mockRTCMember,
 } from "../utils/test";
 import { E2eeType } from "../e2ee/e2eeType";
@@ -69,11 +69,11 @@ vi.mock("../tile/GridTile");
 vi.mock("../tile/SpotlightTile");
 vi.mock("../e2ee/sharedKeyManagement");
 vi.mock("../tracks/MemberAudioRenderer");
-// ActiveCall builds the client itself; the view is what these tests are about
-const createMatrixRTCClientMock = vi.hoisted(() => vi.fn());
+// ActiveCall builds the slot itself; the view is what these tests are about
+const createRTCSlotMock = vi.hoisted(() => vi.fn());
 vi.mock("@element-hq/matrixrtc-sdk", async (importOriginal) => ({
   ...(await importOriginal<typeof MatrixRTCSdk>()),
-  createMatrixRTCClient: createMatrixRTCClientMock,
+  createRTCSlot: createRTCSlotMock,
 }));
 vi.mock("react-use-measure", () => ({
   default: (): [() => void, object] => [(): void => {}, {}],
@@ -97,8 +97,8 @@ let useRoomEncryptionSystemMock: MockedFunction<typeof useRoomEncryptionSystem>;
 beforeEach(() => {
   vi.clearAllMocks();
 
-  createMatrixRTCClientMock.mockImplementation((scope: ObservableScope) =>
-    mockMatrixRTCClient(scope, {
+  createRTCSlotMock.mockImplementation((scope: ObservableScope) =>
+    mockRTCSlot(scope, {
       localMember$: constant(
         mockRTCMember(true, {
           membership: localRtcMember,
diff --git a/src/room/InCallView.tsx b/src/room/InCallView.tsx
index 742fd04c3..ff8b3c3a5 100644
--- a/src/room/InCallView.tsx
+++ b/src/room/InCallView.tsx
@@ -69,8 +69,9 @@ import { ReactionsAudioRenderer } from "./ReactionAudioRenderer";
 import { ReactionsOverlay } from "./ReactionsOverlay";
 import { CallEventAudioRenderer } from "./CallEventAudioRenderer";
 import {
-  createMatrixRTCClient,
-  type MatrixRTCClientOptions,
+  createRTCSlot,
+  type RTCParticipationOptions,
+  type RTCSlotOptions,
   constant,
 } from "@element-hq/matrixrtc-sdk";
 import { map } from "rxjs";
@@ -161,22 +162,20 @@ export const ActiveCall: FC<ActiveCallProps> = (props) => {
       urlParams;
 
     const capture = captureSettings();
-    const rtcClient = createMatrixRTCClient(
+    const slot = createRTCSlot(
       scope,
       props.client,
       props.matrixRoom,
-      {
-        ...matrixRTCClientOptions(urlParams, props.e2eeSystem),
-        publish: initialPublishRequests(
-          props.muteStates,
-          mediaDevices,
-          capture,
-        ),
-      },
+      rtcSlotOptions(props.e2eeSystem),
     );
+    const rtcParticipation = slot.join({
+      ...rtcParticipationOptions(urlParams),
+      publish: initialPublishRequests(props.muteStates, mediaDevices, capture),
+    });
+
     const vm = createCallViewModel$(
       scope,
-      rtcClient,
+      rtcParticipation,
       props.matrixRoom,
       mediaDevices,
       props.muteStates,
@@ -199,8 +198,6 @@ export const ActiveCall: FC<ActiveCallProps> = (props) => {
       reactionsReader.raisedHands$,
       reactionsReader.reactions$,
     );
-    // TODO move this somewhere else once we use the callViewModel in the lobby as well!
-    vm.join();
     setVm(vm);
 
     vm.leave$.pipe(scope.bind()).subscribe(props.onLeft);
@@ -793,20 +790,26 @@ export function captureSettings(): CaptureSettings {
  * switches and the devices, so the caller adds that. Shared with the legacy
  * SDK bundle so that a widget built from it behaves like the app.
  */
-export function matrixRTCClientOptions(
-  urlParams: ReturnType<typeof useUrlParams>,
+export function rtcSlotOptions(
   encryptionSystem: EncryptionSystem,
-): Omit<MatrixRTCClientOptions, "publish"> {
-  const config = Config.get();
-  const session = config.matrix_rtc_session;
+): RTCSlotOptions {
   return {
     encryptionSystem,
     // matrix_rtc_mode in config.json overrides the user's Developer Settings
     // choice. We merely sample the current mode here, so the user would need
     // to manually rejoin to switch to a different one.
     matrixRTCMode:
-      (config.matrix_rtc_mode as MatrixRTCMode | undefined) ??
+      (Config.get().matrix_rtc_mode as MatrixRTCMode | undefined) ??
       matrixRTCModeSetting.value$.value,
+  };
+}
+
+export function rtcParticipationOptions(
+  urlParams: ReturnType<typeof useUrlParams>,
+): Omit<RTCParticipationOptions, "publish"> {
+  const config = Config.get();
+  const session = config.matrix_rtc_session;
+  return {
     sendNotificationType: urlParams.sendNotificationType,
     applicationData: urlParams.callIntent
       ? { "m.call.intent": urlParams.callIntent }
diff --git a/src/state/CallViewModel/CallViewModel.test.ts b/src/state/CallViewModel/CallViewModel.test.ts
index b3072befb..ce8d57c3f 100644
--- a/src/state/CallViewModel/CallViewModel.test.ts
+++ b/src/state/CallViewModel/CallViewModel.test.ts
@@ -261,16 +261,18 @@ describe.each(modes)("CallViewModel (%s mode)", (mode) => {
 
   test("the selected audio output is handed to the client as it changes", () => {
     const { mediaDevices, selected$ } = withAudioOutput();
-    withCallViewModel({ mediaDevices }, (_vm, _session, { rtcClient }) => {
+    withCallViewModel({ mediaDevices }, (_vm, _session, { rtcParticipation: participation }) => {
       // Nothing selected yet, so the browser's choice stands
-      expect(rtcClient.setAudioOutputDeviceId).not.toHaveBeenCalled();
+      expect(participation.setAudioOutputDeviceId).not.toHaveBeenCalled();
       selected$.next({ id: "speaker", virtualEarpiece: false });
-      expect(rtcClient.setAudioOutputDeviceId).toHaveBeenCalledWith("speaker");
+      expect(participation.setAudioOutputDeviceId).toHaveBeenCalledWith(
+        "speaker",
+      );
       selected$.next({ id: "headphones", virtualEarpiece: false });
-      expect(rtcClient.setAudioOutputDeviceId).toHaveBeenLastCalledWith(
+      expect(participation.setAudioOutputDeviceId).toHaveBeenLastCalledWith(
         "headphones",
       );
-      expect(rtcClient.setAudioOutputDeviceId).toHaveBeenCalledTimes(2);
+      expect(participation.setAudioOutputDeviceId).toHaveBeenCalledTimes(2);
     });
   });
 
@@ -278,9 +280,9 @@ describe.each(modes)("CallViewModel (%s mode)", (mode) => {
     const { mediaDevices, selected$ } = withAudioOutput();
     withCallViewModel(
       { mediaDevices },
-      (_vm, _session, { rtcClient }) => {
+      (_vm, _session, { rtcParticipation: participation }) => {
         selected$.next({ id: "earpiece", virtualEarpiece: true });
-        expect(rtcClient.setAudioOutputDeviceId).not.toHaveBeenCalled();
+        expect(participation.setAudioOutputDeviceId).not.toHaveBeenCalled();
       },
       { controlledAudioDevices: true },
     );
diff --git a/src/state/CallViewModel/CallViewModel.ts b/src/state/CallViewModel/CallViewModel.ts
index e494cba26..7cc2c7cf1 100644
--- a/src/state/CallViewModel/CallViewModel.ts
+++ b/src/state/CallViewModel/CallViewModel.ts
@@ -17,7 +17,7 @@ import {
   type LocalMediaTrack,
   type LocalRTCMember,
   type LocalVideoMediaTrack,
-  type MatrixRTCClient,
+  type RTCParticipation,
   type ObservableScope,
   pauseWhen,
   type PublishRequest,
@@ -158,7 +158,7 @@ export function publishRequest(
       };
 }
 
-/** What the client publishes at the join: the sources whose mute switch is on. */
+/** What the participation publishes at the join: the sources whose mute switch is on. */
 export function initialPublishRequests(
   muteStates: MuteStates,
   mediaDevices: MediaDevices,
@@ -295,12 +295,7 @@ export interface CallViewModel {
   /** Call to initiate hangup. Use in conbination with reconnection state track the async hangup process. */
   hangup: () => void;
 
-  // joining
-  join: () => void;
-
-  /**
-   * calls requestDisconnect. The async leave state can than be observed via connected$
-   */
+  /** Leaves the participation. The async leave can then be observed via connected$. */
   leave: () => void;
   // screen sharing
   /**
@@ -457,12 +452,12 @@ export interface CallViewModel {
  * UI (may eventually be expanded to cover the lobby and feedback screens in the
  * future).
  *
- * The session itself — memberships, transports, media — is the client's; this
- * adds everything that makes it a call.
+ * The slot and the participation — memberships, transports, media — are the
+ * SDK's; this adds everything that makes it a call.
  */
 export function createCallViewModel$(
   scope: ObservableScope,
-  rtcClient: MatrixRTCClient,
+  rtcParticipation: RTCParticipation,
   // A call is permanently tied to a single Matrix room
   matrixRoom: MatrixRoom,
   mediaDevices: MediaDevices,
@@ -492,7 +487,7 @@ export function createCallViewModel$(
   const localTracks$ = scope.behavior<
     (LocalAudioMediaTrack | LocalVideoMediaTrack)[]
   >(
-    rtcClient.localMember$.pipe(
+    rtcParticipation.localMember$.pipe(
       switchMap((member) => member?.tracks$ ?? of(null)),
       map((tracks) => tracks ?? []),
     ),
@@ -534,10 +529,10 @@ export function createCallViewModel$(
         return desired;
       }
       if (!enabled) return false;
-      const member = rtcClient.localMember$.value;
-      if (member === null) return false;
       try {
-        await member.publish(publishRequest(source, mediaDevices, capture));
+        await rtcParticipation.publish(
+          publishRequest(source, mediaDevices, capture),
+        );
         return true;
       } catch (e) {
         logger.error(`Failed to publish the ${source}`, e);
@@ -587,7 +582,7 @@ export function createCallViewModel$(
       .pipe(scope.bind())
       .subscribe((selected) => {
         if (selected === undefined) return;
-        rtcClient.setAudioOutputDeviceId(selected.id).catch((e) => {
+        rtcParticipation.setAudioOutputDeviceId(selected.id).catch((e) => {
           logger.error("Failed to switch the audio output device", e);
         });
       });
@@ -604,7 +599,7 @@ export function createCallViewModel$(
 
   const members$ = scope.behavior<(LocalRTCMember | RemoteRTCMember)[]>(
     combineLatest(
-      [rtcClient.localMember$, rtcClient.remoteMembers$],
+      [rtcParticipation.localMember$, rtcParticipation.remoteMembers$],
       (local, remote) => (local === null ? remote : [local, ...remote]),
     ),
   );
@@ -633,7 +628,7 @@ export function createCallViewModel$(
   });
 
   const handsRaised$ = scope.behavior(
-    handsRaisedSubject$.pipe(pauseWhen(rtcClient.reconnecting$)),
+    handsRaisedSubject$.pipe(pauseWhen(rtcParticipation.reconnecting$)),
   );
 
   const reactions$ = scope.behavior(
@@ -646,7 +641,7 @@ export function createCallViewModel$(
           ]),
         ),
       ),
-      pauseWhen(rtcClient.reconnecting$),
+      pauseWhen(rtcParticipation.reconnecting$),
     ),
   );
 
@@ -663,7 +658,10 @@ export function createCallViewModel$(
           for (const member of members) {
             const mediaId = `${member.userId}:${member.deviceId}`;
             for (let dup = 0; dup < 1 + duplicateTiles; dup++) {
-              yield { keys: [dup, mediaId, member.id], data: member };
+              yield {
+                keys: [dup, mediaId, member.rtcBackendIdentity],
+                data: member,
+              };
             }
           }
         },
@@ -672,14 +670,14 @@ export function createCallViewModel$(
           return createWrappedUserMedia(scope, {
             id: `${mediaId}:${dup}`,
             userId: member.userId,
-            rtcBackendIdentity: member.id,
+            rtcBackendIdentity: member.rtcBackendIdentity,
             member,
             encryptionSystem: options.encryptionSystem,
             focusUrl$: scope.behavior(
               member.transport$.pipe(map((transport) => transport?.id)),
             ),
             mediaDevices,
-            pretendToBeDisconnected$: rtcClient.reconnecting$,
+            pretendToBeDisconnected$: rtcParticipation.reconnecting$,
             displayName$: member.displayName$,
             mxcAvatarUrl$: member.avatarUrl$,
             handRaised$: scope.behavior(
@@ -761,9 +759,12 @@ export function createCallViewModel$(
    *  - There can be multiple participants for one Matrix user if they join from
    *    multiple devices.
    */
-  const participantCount$ = rtcClient.memberCount$;
+  // One per membership, so one user may count several times
+  const participantCount$ = scope.behavior(
+    rtcParticipation.slot.members$.pipe(map((members) => members.length)),
+  );
 
-  const keyRotationSuppressed$ = rtcClient.keyRotationSuppressed$;
+  const keyRotationSuppressed$ = rtcParticipation.keyRotationSuppressed$;
 
   const leaveSoundEffect$ = userMedia$.pipe(
     pairwise(),
@@ -1473,15 +1474,13 @@ export function createCallViewModel$(
     (hideScreensharing || !("getDisplayMedia" in (navigator.mediaDevices ?? {}))
       ? null
       : (): void => {
-          const member = rtcClient.localMember$.value;
-          if (member === null) return;
           const track = screenShare$.value;
           (track === undefined
-            ? member.publish({
+            ? rtcParticipation.publish({
                 source: "screenShare",
                 capture: capture.screenShare,
               })
-            : member.unpublish(track.id)
+            : rtcParticipation.unpublish("screenShare")
           ).catch((e: unknown) => {
             logger.error(
               `Screen share ${track === undefined ? "start" : "stop"} failed`,
@@ -1495,24 +1494,22 @@ export function createCallViewModel$(
           });
         });
 
-  // Tell the host and the analytics about the user's joins and leaves
-  const join = (): void => {
-    PosthogAnalytics.instance.eventCallEnded.cacheStartCall(new Date());
-    PosthogAnalytics.instance.eventCallStarted.track(matrixRoom.roomId);
-    rtcClient.join();
-    hostBridge.notifyJoined().catch((e) => {
-      logger.error("Failed to notify the host that we joined", e);
-    });
-  };
+  // Tell the host and the analytics about the user's joins and leaves. The
+  // participation is joined by the time the view model is built on it
+  PosthogAnalytics.instance.eventCallEnded.cacheStartCall(new Date());
+  PosthogAnalytics.instance.eventCallStarted.track(matrixRoom.roomId);
+  hostBridge.notifyJoined().catch((e) => {
+    logger.error("Failed to notify the host that we joined", e);
+  });
   const leave = (): void => {
-    rtcClient.leave();
+    rtcParticipation.leave();
     hostBridge.notifyHungUp().catch((e) => {
       logger.error("Failed to notify the host that we hung up", e);
     });
   };
 
   let reconnectStart: { time: number; reason: DisconnectReason } | null = null;
-  rtcClient.disconnectReason$
+  rtcParticipation.disconnectReason$
     .pipe(distinctUntilChanged(), pairwise(), scope.bind())
     .subscribe(([prev, reason]) => {
       if (reason !== null) {
@@ -1538,7 +1535,6 @@ export function createCallViewModel$(
     ringingStatusLocation: header === HeaderStyle.AppBar ? "app_bar" : "tile",
     leave$: leave$,
     hangup: (): void => userHangup$.next(),
-    join,
     leave,
     toggleScreenSharing: toggleScreenSharing,
     sharingScreen$: sharingScreen$,
@@ -1549,15 +1545,15 @@ export function createCallViewModel$(
     unhoverScreen: (): void => screenUnhover$.next(),
 
     fatalError$: scope.behavior(
-      rtcClient.fatalError$.pipe(
+      rtcParticipation.fatalError$.pipe(
         map((error) => error && fromMatrixRTCError(error)),
       ),
     ),
     participantCount$: participantCount$,
     keyRotationSuppressed$: keyRotationSuppressed$,
-    localMember$: rtcClient.localMember$,
-    remoteMembers$: rtcClient.remoteMembers$,
-    connectedTransports$: rtcClient.connectedTransports$,
+    localMember$: rtcParticipation.localMember$,
+    remoteMembers$: rtcParticipation.remoteMembers$,
+    connectedTransports$: rtcParticipation.connectedTransports$,
     handsRaised$: handsRaised$,
     reactions$: reactions$,
     joinSoundEffect$: joinSoundEffect$,
@@ -1586,8 +1582,8 @@ export function createCallViewModel$(
     overflowing$,
     earpieceMode$: earpieceMode$,
     audioOutputSwitcher$: audioOutputSwitcher$,
-    reconnecting$: rtcClient.reconnecting$,
-    connected$: rtcClient.connected$,
+    reconnecting$: rtcParticipation.reconnecting$,
+    connected$: rtcParticipation.connected$,
     screenShareError$,
     dismissScreenShareError: (): void => screenShareError$.next(null),
   };
diff --git a/src/state/CallViewModel/CallViewModelTestUtils.ts b/src/state/CallViewModel/CallViewModelTestUtils.ts
index 73032a779..d41119514 100644
--- a/src/state/CallViewModel/CallViewModelTestUtils.ts
+++ b/src/state/CallViewModel/CallViewModelTestUtils.ts
@@ -11,7 +11,7 @@ import {
   constant,
   E2eeType,
   type LocalRTCMember,
-  type MatrixRTCClient,
+  type RTCParticipation,
   type MatrixRTCMode,
   type ObservableScope,
   type RemoteRTCMember,
@@ -33,7 +33,7 @@ import {
   mockConfig,
   mockMatrixRoom,
   mockMatrixRoomMember,
-  mockMatrixRTCClient,
+  mockRTCParticipation,
   mockAudioTrack,
   mockMediaDevices,
   mockVideoTrack,
@@ -112,8 +112,8 @@ export function withCallViewModel(mode: MatrixRTCMode) {
       rtcSession: MockRTCSession,
       subjects: {
         raisedHands$: BehaviorSubject<Record<string, RaisedHandInfo>>;
-        /** The client the view model was built on, with spies for its calls. */
-        rtcClient: MatrixRTCClient;
+        /** The participation the view model was built on, with spies for its calls. */
+        rtcParticipation: RTCParticipation;
       },
       setSyncState: (value: SyncState) => void,
     ) => void,
@@ -241,7 +241,7 @@ export function withCallViewModel(mode: MatrixRTCMode) {
         ),
       ),
     );
-    const rtcClient = mockMatrixRTCClient(scope, {
+    const rtcParticipation = mockRTCParticipation(scope, {
       localMember$,
       remoteMembers$,
       connected$,
@@ -260,7 +260,7 @@ export function withCallViewModel(mode: MatrixRTCMode) {
 
     const vm = createCallViewModel$(
       scope,
-      rtcClient,
+      rtcParticipation,
       room,
       mediaDevices,
       muteStates,
@@ -279,6 +279,6 @@ export function withCallViewModel(mode: MatrixRTCMode) {
     );
     void mode;
 
-    continuation(vm, rtcSession, { raisedHands$, rtcClient }, setSyncState);
+    continuation(vm, rtcSession, { raisedHands$, rtcParticipation }, setSyncState);
   };
 }
diff --git a/src/state/media/MemberMediaViewModel.ts b/src/state/media/MemberMediaViewModel.ts
index 5ba969104..bdff672d5 100644
--- a/src/state/media/MemberMediaViewModel.ts
+++ b/src/state/media/MemberMediaViewModel.ts
@@ -13,6 +13,7 @@ import {
   type EncryptionSystem,
   type MediaTrack,
   type ObservableScope,
+  type MemberMedia,
   type RTCMember,
   trackBySource$,
   type VideoMediaTrack,
@@ -64,10 +65,7 @@ export interface BaseMemberMediaViewModel extends BaseMediaViewModel {
 }
 
 /** The member whose media this is, as far as the media view models read it. */
-export type MediaMember = Pick<
-  RTCMember,
-  "local" | "tracks$" | "encryptionError$"
->;
+export type MediaMember = Pick<RTCMember, "local"> & MemberMedia;
 
 export interface MemberMediaInputs extends BaseMediaViewModel {
   member: MediaMember;
diff --git a/src/tracks/MemberAudioRenderer.tsx b/src/tracks/MemberAudioRenderer.tsx
index fe1b687df..dd9beb14b 100644
--- a/src/tracks/MemberAudioRenderer.tsx
+++ b/src/tracks/MemberAudioRenderer.tsx
@@ -100,7 +100,7 @@ export const MemberAudioRenderer: FC<MemberAudioRendererProps> = ({
     <div style={{ display: "none" }}>
       {members.map((member) => (
         <MemberAudio
-          key={member.id}
+          key={member.rtcBackendIdentity}
           tracks$={member.tracks$}
           muted={muted}
           plugins={plugins}
diff --git a/src/utils/test-viewmodel.ts b/src/utils/test-viewmodel.ts
index 7972a207a..c30d08cda 100644
--- a/src/utils/test-viewmodel.ts
+++ b/src/utils/test-viewmodel.ts
@@ -26,7 +26,7 @@ import {
 import {
   mockConfig,
   mockMatrixRoom,
-  mockMatrixRTCClient,
+  mockRTCParticipation,
   mockMediaDevices,
   mockMuteStates,
   MockRTCSession,
@@ -160,7 +160,7 @@ export function getBasicCallViewModelEnvironment(
   const muteStates = mockMuteStates();
   const mediaDevices = mediaDevicesOverride ?? mockMediaDevices({});
   // Every membership is a member whose media has arrived
-  const rtcClient = mockMatrixRTCClient(scope, {
+  const rtcParticipation = mockRTCParticipation(scope, {
     localMember$: scope.behavior(
       rtcMemberships$.pipe(
         map((memberships) =>
@@ -196,7 +196,7 @@ export function getBasicCallViewModelEnvironment(
   });
   const vm = createCallViewModel$(
     scope,
-    rtcClient,
+    rtcParticipation,
     matrixRoom,
     mediaDevices,
     muteStates,
diff --git a/src/utils/test.ts b/src/utils/test.ts
index eff7919f2..2592351e5 100644
--- a/src/utils/test.ts
+++ b/src/utils/test.ts
@@ -5,6 +5,7 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 import {
+  BehaviorSubject,
   combineLatest,
   map,
   NEVER,
@@ -67,7 +68,9 @@ import {
   type LocalAudioMediaTrack,
   type LocalRTCMember,
   type LocalVideoMediaTrack,
-  type MatrixRTCClient,
+  type RTCMember,
+  type RTCParticipation,
+  type RTCSlot,
   type MatrixRTCError,
   type PublishRequest,
   type RemoteRTCMember,
@@ -467,13 +470,14 @@ export function mockRTCMember(
           resolved$: constant(undefined),
         };
   const base = {
-    id: membership.rtcBackendIdentity,
+    rtcBackendIdentity: membership.rtcBackendIdentity,
     userId: membership.userId,
     deviceId: membership.deviceId,
-    membership$: constant(membership),
+    memberId: membership.memberId,
     displayName$: constant(member.rawDisplayName ?? membership.userId),
     avatarUrl$: constant(member.getMxcAvatarUrl()),
     transport$: constant(transport),
+    applicationData$: constant(membership.applicationData),
     encryptionError$,
   };
   if (!local) return { ...base, local: false, tracks$ };
@@ -484,18 +488,10 @@ export function mockRTCMember(
     tracks$: tracks$ as Behavior<
       (LocalAudioMediaTrack | LocalVideoMediaTrack)[] | null
     >,
-    publish: vi.fn(async (request: PublishRequest) =>
-      Promise.resolve(
-        request.source === "microphone"
-          ? mockAudioTrack()
-          : mockVideoTrack({ source: request.source }),
-      ),
-    ),
-    unpublish: vi.fn(async () => Promise.resolve()),
   };
 }
 
-export interface MockClientInputs {
+export interface MockParticipationInputs {
   localMember$?: Behavior<LocalRTCMember | null>;
   remoteMembers$?: Behavior<RemoteRTCMember[]>;
   connected$?: Behavior<boolean>;
@@ -506,8 +502,11 @@ export interface MockClientInputs {
   connectedTransports$?: Behavior<TransportMetadata[]>;
 }
 
-/** A client that does nothing but hold the state a test hands it. */
-export function mockMatrixRTCClient(
+/**
+ * A participation that does nothing but hold the state a test hands it, in a
+ * slot whose members are the same ones without their media.
+ */
+export function mockRTCParticipation(
   scope: ObservableScope,
   {
     localMember$ = constant(null),
@@ -518,10 +517,25 @@ export function mockMatrixRTCClient(
     fatalError$ = constant(null),
     keyRotationSuppressed$ = constant(false),
     connectedTransports$ = constant([]),
-  }: MockClientInputs = {},
-): MatrixRTCClient {
-  return {
-    join: vi.fn(),
+  }: MockParticipationInputs = {},
+): RTCParticipation {
+  const members$ = scope.behavior<RTCMember[]>(
+    combineLatest([localMember$, remoteMembers$], (local, remote) =>
+      local === null ? remote : [local, ...remote],
+    ),
+  );
+  const participation$ = new BehaviorSubject<RTCParticipation | null>(null);
+  const slot: RTCSlot = {
+    roomId: "!room:example.org",
+    application: "m.call",
+    id: "ROOM",
+    status$: constant("open"),
+    members$,
+    participation$,
+    join: vi.fn(() => participation$.value!),
+  };
+  const participation: RTCParticipation = {
+    slot,
     leave: vi.fn(),
     status$: scope.behavior(
       connected$.pipe(map((c) => (c ? "connected" : "connecting"))),
@@ -532,18 +546,30 @@ export function mockMatrixRTCClient(
     fatalError$,
     localMember$,
     remoteMembers$,
-    memberCount$: scope.behavior(
-      combineLatest(
-        [localMember$, remoteMembers$],
-        (local, remote) => remote.length + (local === null ? 0 : 1),
+    publish: vi.fn(async (request: PublishRequest) =>
+      Promise.resolve(
+        request.source === "microphone"
+          ? mockAudioTrack()
+          : mockVideoTrack({ source: request.source }),
       ),
     ),
+    unpublish: vi.fn(async () => Promise.resolve()),
     keyRotationSuppressed$,
     connectedTransports$,
     setAudioOutputDeviceId: vi.fn(async () => Promise.resolve()),
     sendData: vi.fn(async () => Promise.resolve()),
     data$: NEVER,
   };
+  participation$.next(participation);
+  return participation;
+}
+
+/** The slot of a `mockRTCParticipation`, whose `join()` returns that participation. */
+export function mockRTCSlot(
+  scope: ObservableScope,
+  inputs: MockParticipationInputs = {},
+): RTCSlot {
+  return mockRTCParticipation(scope, inputs).slot;
 }
 
 export function mockConfig(