diff --git a/sdk/SdkArchitecture.md b/sdk/SdkArchitecture.md
index ad7fb3e8f..9b28ae2c5 100644
--- a/sdk/SdkArchitecture.md
+++ b/sdk/SdkArchitecture.md
@@ -126,23 +126,26 @@ it, and `leave()` also sends the leave.
 
 ## Vocabulary
 
-| Word              | Meaning here                                                                                                                                                                                          | Elsewhere                                                         |
-| ----------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------- |
-| client            | a `MatrixRTCClient`: one room, one slot, one local member                                                                                                                                             | `MatrixClient` is always written out                              |
-| membership        | one device's MatrixRTC state event, seen by hosts as `RTCMembership`, the js-sdk `CallMembership` behind it                                                                                           |                                                                   |
-| member            | an `RTCMember`: a membership plus what the SDK derives from it (display name, transport, media)                                                                                                       | Element Call's "matrixLivekitMember"                              |
-| media             | what a member sends, once it has arrived on its transport: `tracks$` on the member, `null` while waiting for media, and `encryptionError$` beside it. The backend supplies the same two fields        | Element Call's `MediaViewModel` is a view of it                   |
-| track             | a `MediaTrack`: one publication of a member, audio or video, with `source`, `kind`, `id` and the behaviors on it. `LocalAudioMediaTrack` and `LocalVideoMediaTrack` add the controls over our own     | LiveKit `TrackPublication` and `Track`, which never leave the SDK |
-| source            | a `MediaSource`: what a track is captured from, `microphone`, `camera`, `screenShare`, `screenShareAudio` or `unknown`. Picked out of `tracks$` with `trackBySource$`                                 | LiveKit `Track.Source`                                            |
-| publish request   | a `PublishRequest`: a source, where to capture it from and how to encode it. Given at the join in `options.publish`, or later to `publish` on the local member                                        |                                                                   |
-| active            | `isActive$` on an audio track: the track carries sound, as the backend measures it. "Speaking" is a call's reading of it on the microphone track                                                      | LiveKit "speaking", `isSpeaking`                                  |
-| attach, detach    | handing a `<video>` or `<audio>` element to a track and taking it back. The SDK owns the stream and the observers on the element                                                                      | LiveKit `Track.attach`                                            |
-| transport         | where media is exchanged: a LiveKit service url today, described by `TransportMetadata`                                                                                                               | MSC4143 "focus"                                                   |
-| media backend     | a `MediaBackend`: what carries the media, behind `sdk/src/media-backend/api.ts`. One per client, chosen with `options.backend`; LiveKit under `sdk/src/media-backend/livekit/` is the one that exists | "SFU", a LiveKit room, a peer connection                          |
-| application, slot | what the session is for (`m.call`) and which one of them in the room (`ROOM`)                                                                                                                         | MSC4143                                                           |
+The words the SDK is written in. They name the shape after the slot and
+participation split in
+[`client-into-slot-session-split-plan.md`](./client-into-slot-session-split-plan.md);
+the sections below still describe `MatrixRTCClient` until that lands.
 
-Names use `RTC` in capitals, as the js-sdk does: `MatrixRTCClient`, `RTCMember`,
-`matrixRTCMode`.
+| Word                        | Meaning here                                                                                                                                                                                                                                   | Elsewhere                                                                                                   |
+| --------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------- |
+| slot                        | an `RTCSlot`: the namespace in a room that memberships join into, `application` plus `id`. It exists while the room does, with nobody in it too. Holds `members$`, its open or closed `status$`, and `join()`                  | the js-sdk `MatrixRTCSession` is one per slot; the MSC4143 slot event                                       |
+| participation               | an `RTCParticipation`: the span between our `join()` of a slot and our `leave()`. Holds the connection and its `status$`, `localMember$`, `remoteMembers$`, the connected transports and the data channel. Made by `slot.join()`, never reused | "session" is not used here, because in the js-sdk it means the slot                                         |
+| member                      | an `RTCMember`: one membership in a slot with what the SDK derives from it, `rtcBackendIdentity`, `userId`, `deviceId`, `memberId`, `displayName$`, `avatarUrl$`, `transport$` and `applicationData$`. No media. What the slot hands out in `members$`         | the js-sdk `CallMembership` behind it, which never leaves the SDK; Element Call's old "matrixLivekitMember" |
+| local member, remote member | `LocalRTCMember` and `RemoteRTCMember`: an `RTCMember` plus its `MemberMedia`, which only a participation can carry. The local one's tracks carry the controls. What the participation hands out in `localMember$` and `remoteMembers$`        | LiveKit `LocalParticipant` and `RemoteParticipant`, which never leave the SDK                               |
+| media                       | a `MemberMedia`: what a member sends once it has arrived on its transport, `tracks$`, null while waiting, and `encryptionError$` beside it. The media backend supplies exactly these two fields per member                                     | Element Call's `MediaViewModel` is a view of it                                                             |
+| track                       | a `MediaTrack`: one publication of a member, audio or video, with `source`, `kind`, `id` and the behaviors on it. `LocalAudioMediaTrack` and `LocalVideoMediaTrack` add the controls over our own                                              | LiveKit `TrackPublication` and `Track`, which never leave the SDK                                           |
+| publish request             | a `PublishRequest`: a source, where to capture it from and how to encode it. Given at the join in `publish`, or later to `publish` on the participation                                                                                        |                                                                                                             |
+| attach, detach              | handing a `<video>` or `<audio>` element to a track and taking it back. The SDK owns the stream and the observers on the element                                                                                                               | LiveKit `Track.attach`                                                                                      |
+| transport                   | an `RTCTransport`: where media is exchanged, a LiveKit service url today. Carries the raw js-sdk `Transport` from the membership and, in `resolved$`, what the backend fetched to connect to it                                                | MSC4143 "focus"                                                                                             |
+| media backend               | a `MediaBackend`: what carries the media, behind `sdk/src/media-backend/api.ts`. One per participation, chosen with `backend` in the join options; LiveKit under `sdk/src/media-backend/livekit/` is the one that exists                       | "SFU", a LiveKit room, a peer connection                                                                    |
+
+Names use `RTC` in capitals, as the js-sdk does: `RTCSlot`, `RTCParticipation`,
+`RTCMember`, `matrixRTCMode`.
 
 ## Reactive primitives
 
@@ -181,7 +184,7 @@ context, it is in the view model.
 
 ### Creating one
 
-````ts
+```ts
 import { type MatrixClient, type Room } from "matrix-js-sdk";
 import { type RTCNotificationType } from "matrix-js-sdk/lib/matrixrtc";
 
diff --git a/sdk/dev/main.ts b/sdk/dev/main.ts
index d3b65c6ae..00ad59219 100644
--- a/sdk/dev/main.ts
+++ b/sdk/dev/main.ts
@@ -15,12 +15,13 @@ import { logger } from "matrix-js-sdk/lib/logger";
 import { combineLatest, type Observable, of, switchMap } from "rxjs";
 import {
   type AudioMediaTrack,
-  createMatrixRTCClient,
+  createRTCSlot,
   E2eeType,
+  type LocalRTCMember,
   MatrixRTCMode,
   ObservableScope,
-  type RTCMember,
-  type MatrixRTCClient,
+  type RemoteRTCMember,
+  type RTCParticipation,
   trackBySource$,
   type VideoMediaTrack,
 } from "@element-hq/matrixrtc-sdk";
@@ -69,33 +70,34 @@ async function start(
     const room = await joinRoom(client, roomIdOrAlias);
 
     const scope = new ObservableScope();
-    const rtcClient = createMatrixRTCClient(scope, client, room, {
+    const slot = createRTCSlot(scope, client, room, {
       encryptionSystem: { kind: E2eeType.PER_PARTICIPANT },
       matrixRTCMode: MatrixRTCMode.Compatibility,
+    });
+    const rtcParticipation = slot.join({
       publish: [{ source: "microphone" }, { source: "camera" }],
     });
-    rtcClient.status$.pipe(scope.bind()).subscribe((s) => {
+    rtcParticipation.status$.pipe(scope.bind()).subscribe((s) => {
       status.textContent = s;
     });
-    rtcClient.fatalError$.pipe(scope.bind()).subscribe((error) => {
+    rtcParticipation.fatalError$.pipe(scope.bind()).subscribe((error) => {
       if (error !== null) status.textContent = `Error: ${error.message}`;
     });
-    showMembers(scope, rtcClient);
-    showMessages(scope, rtcClient);
-    rtcClient.join();
+    showMembers(scope, rtcParticipation);
+    showMessages(scope, rtcParticipation);
 
     toggle(buttons.microphone, async (enabled) =>
-      setPublished(rtcClient, "microphone", enabled),
+      setPublished(rtcParticipation, "microphone", enabled),
     );
     toggle(buttons.camera, async (enabled) =>
-      setPublished(rtcClient, "camera", enabled),
+      setPublished(rtcParticipation, "camera", enabled),
     );
     toggle(buttons.screenShare, async (enabled) =>
-      setPublished(rtcClient, "screenShare", enabled),
+      setPublished(rtcParticipation, "screenShare", enabled),
     );
     buttons.leave.hidden = false;
     buttons.leave.onclick = (): void => {
-      rtcClient.leave();
+      rtcParticipation.leave();
       scope.end();
       members.replaceChildren();
       messages.replaceChildren();
@@ -115,20 +117,20 @@ async function start(
  * published and unpublished.
  */
 async function setPublished(
-  rtcClient: MatrixRTCClient,
+  rtcParticipation: RTCParticipation,
   source: "microphone" | "camera" | "screenShare",
   enabled: boolean,
 ): Promise<boolean> {
-  const member = rtcClient.localMember$.value;
-  if (member === null) return !enabled;
-  const track = member.tracks$.value?.find((t) => t.source === source);
+  const track = rtcParticipation.localMember$.value?.tracks$.value?.find(
+    (t) => t.source === source,
+  );
   if (track && source === "screenShare") {
-    if (!enabled) await member.unpublish(track.id);
+    if (!enabled) await rtcParticipation.unpublish(source);
     return enabled;
   }
   if (track) return track.setEnabled(enabled);
   if (!enabled) return false;
-  await member.publish({ source });
+  await rtcParticipation.publish({ source });
   return true;
 }
 
@@ -148,49 +150,57 @@ function toggle(
 /** Sends what the form holds on the "chat" topic, and lists what arrives. */
 function showMessages(
   scope: ObservableScope,
-  rtcClient: MatrixRTCClient,
+  rtcParticipation: RTCParticipation,
 ): void {
   dataForm.hidden = false;
   dataForm.onsubmit = (event): void => {
     event.preventDefault();
     const text = new FormData(dataForm).get("text") as string;
-    rtcClient.sendData("chat", text).then(
+    rtcParticipation.sendData("chat", text).then(
       () => dataForm.reset(),
       (e: unknown) => {
         status.textContent = `Error: ${e}`;
       },
     );
   };
-  rtcClient.data$.pipe(scope.bind()).subscribe(({ member, topic, text }) => {
-    const line = messages.appendChild(document.createElement("li"));
-    line.dataset.testid = "message";
-    line.dataset.topic = topic;
-    line.dataset.userId = member.userId;
-    line.textContent = `${member.displayName$.value}: ${text}`;
-  });
+  rtcParticipation.data$
+    .pipe(scope.bind())
+    .subscribe(({ member, topic, text }) => {
+      const line = messages.appendChild(document.createElement("li"));
+      line.dataset.testid = "message";
+      line.dataset.topic = topic;
+      line.dataset.userId = member.userId;
+      line.textContent = `${member.displayName$.value}: ${text}`;
+    });
 }
 
-function showMembers(scope: ObservableScope, rtcClient: MatrixRTCClient): void {
+function showMembers(
+  scope: ObservableScope,
+  rtcParticipation: RTCParticipation,
+): void {
   const tiles = new Map<string, HTMLElement>();
-  combineLatest([rtcClient.localMember$, rtcClient.remoteMembers$])
+  combineLatest([rtcParticipation.localMember$, rtcParticipation.remoteMembers$])
     .pipe(scope.bind())
     .subscribe(([local, remote]) => {
       const current = local === null ? remote : [local, ...remote];
       for (const [id, tile] of tiles)
-        if (!current.some((m) => m.id === id)) {
+        if (!current.some((m) => m.rtcBackendIdentity === id)) {
           tile.remove();
           tiles.delete(id);
         }
       for (const member of current)
-        if (!tiles.has(member.id)) {
+        if (!tiles.has(member.rtcBackendIdentity)) {
           const tile = memberTile(scope, member);
-          tiles.set(member.id, tile);
+          tiles.set(member.rtcBackendIdentity, tile);
           members.append(tile);
         }
     });
 }
 
-function memberTile(scope: ObservableScope, member: RTCMember): HTMLElement {
+function memberTile(
+  scope: ObservableScope,
+  member: LocalRTCMember | RemoteRTCMember,
+): HTMLElement {
   const tile = document.createElement("section");
   tile.dataset.testid = "member";
   tile.dataset.userId = member.userId;
diff --git a/sdk/index.ts b/sdk/index.ts
index 1784d3b23..31719fe84 100644
--- a/sdk/index.ts
+++ b/sdk/index.ts
@@ -56,7 +56,7 @@ export * from "./src/api";
 export * from "./src/media-api";
 export * from "./src/media-backend/api";
 export { trackBySource$, type TrackOfSource } from "./src/utils/tracks";
-export { createMatrixRTCClient } from "./src/MatrixRTCClient";
+export { createRTCSlot } from "./src/RTCSlot";
 export {
   createLivekitBackend,
   type LivekitBackendOptions,
diff --git a/sdk/src/MatrixRTCClient.ts b/sdk/src/RTCParticipation.ts
similarity index 53%
rename from sdk/src/MatrixRTCClient.ts
rename to sdk/src/RTCParticipation.ts
index 47a5507fe..029a8b32a 100644
--- a/sdk/src/MatrixRTCClient.ts
+++ b/sdk/src/RTCParticipation.ts
@@ -6,29 +6,40 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type MatrixClient, type Room } from "matrix-js-sdk";
-import { type Logger, logger as rootLogger } from "matrix-js-sdk/lib/logger";
+import { type Logger } from "matrix-js-sdk/lib/logger";
 import {
   type MatrixRTCSession as JsSdkRTCSession,
   MatrixRTCSessionEvent,
-  MatrixRTCSessionManager,
   MembershipManagerEvent,
 } from "matrix-js-sdk/lib/matrixrtc";
 import { type IMembershipManager } from "matrix-js-sdk/lib/matrixrtc/IMembershipManager";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
 import { v4 as uuidv4 } from "uuid";
-import { combineLatest, filter, from, fromEvent, map, Observable } from "rxjs";
+import {
+  BehaviorSubject,
+  combineLatest,
+  filter,
+  from,
+  fromEvent,
+  map,
+  Observable,
+} from "rxjs";
 
 import { defaultSessionTimings, MatrixRTCMode } from "./config";
 import { E2eeType } from "./encryption";
+import { type Behavior } from "./reactive/Behavior";
 import { type ObservableScope } from "./reactive/ObservableScope";
-import { filterBehavior, generateItems } from "./reactive/observable";
+import { generateItems } from "./reactive/observable";
 import { mapScoped } from "./utils/mapScoped";
 import {
   type DataMessage,
   type LocalRTCMember,
-  type MatrixRTCClient,
-  type MatrixRTCClientOptions,
   type RemoteRTCMember,
+  type RTCMember,
+  type RTCParticipation,
+  type RTCParticipationOptions,
+  type RTCSlot,
+  type RTCSlotOptions,
 } from "./api";
 import {
   type MediaBackend,
@@ -36,7 +47,6 @@ import {
   type MediaKey,
 } from "./media-backend/api";
 import { createLivekitBackend } from "./media-backend/livekit/LivekitBackend";
-import { MatrixRTCError } from "./errors";
 import { createHomeserverConnected$ } from "./matrixrtc/HomeserverConnected";
 import { joinJsSdkSession } from "./matrixrtc/joinJsSdkSession";
 import {
@@ -44,49 +54,55 @@ import {
   type PreparedTransport,
 } from "./matrixrtc/LocalMember";
 import { discoverLocalTransport } from "./matrixrtc/LocalTransport";
-import {
-  createMatrixMemberMetadata$,
-  createRoomMembers$,
-} from "./matrixrtc/MatrixMemberMetadata";
 import {
   createLocalRTCMember,
   createRemoteRTCMember,
-  membershipKeys,
 } from "./matrixrtc/Members";
-import {
-  createKeyRotationSuppressed$,
-  createMemberships$,
-} from "./matrixrtc/JsRtcSessionBehaviors";
-import { fatalError, sessionStatus } from "./matrixrtc/status";
-import { createTransportRegistry } from "./matrixrtc/Transports";
+import { createKeyRotationSuppressed$ } from "./matrixrtc/JsRtcSessionBehaviors";
+import { fatalError, participationStatus } from "./matrixrtc/status";
+import { type TransportRegistry } from "./matrixrtc/Transports";
+
+/** What the slot lends a participation. */
+export interface ParticipationDeps {
+  client: MatrixClient;
+  room: Room;
+  slot: RTCSlot;
+  jsSdkSession: JsSdkRTCSession;
+  members$: Behavior<RTCMember[]>;
+  transports: TransportRegistry;
+  slotOptions: Pick<RTCSlotOptions, "matrixRTCMode" | "encryptionSystem">;
+  logger: Logger;
+}
+
+/** The participation, plus what the slot reads from it without exposing it. */
+export interface ParticipationInternals {
+  participation: RTCParticipation;
+  connections$: MediaBackend["connections$"];
+}
 
 /**
- * Takes the whole `MatrixClient` rather than a slice of it, and finds the
- * MatrixRTC session itself. The js-sdk is the MatrixRTC implementation today;
- * when the rust-rtc crate replaces it, the SDK has to bridge the crate to the
- * js-sdk client, and only the SDK knows what that bridge needs. Holding the
- * client keeps that change inside the SDK.
+ * Our span in a slot: the backend, the local transport, the join state
+ * machine and the members with the media it carries for them. Starts joining
+ * at once and ends with `leave()`, which also ends `scope`.
  */
-export function createMatrixRTCClient(
+export function createRTCParticipation(
   scope: ObservableScope,
-  client: MatrixClient,
-  room: Room,
-  options: MatrixRTCClientOptions,
-): MatrixRTCClient {
-  const logger = rootLogger.getChild("[MatrixRTCClient]");
-  const userId = client.getUserId();
-  const deviceId = client.getDeviceId();
-  if (!(userId && deviceId))
-    throw new MatrixRTCError("The client has to be logged in");
-  const { encryptionSystem, matrixRTCMode } = options;
-  const timings = { ...defaultSessionTimings, ...options.timings };
-
-  const jsSdkSession = sessionManager(
-    scope,
+  {
     client,
-    options,
-    logger,
-  ).getRoomSession(room);
+    room,
+    slot,
+    jsSdkSession,
+    members$,
+    transports,
+    slotOptions: { matrixRTCMode, encryptionSystem },
+    logger: parentLogger,
+  }: ParticipationDeps,
+  options: RTCParticipationOptions,
+): ParticipationInternals {
+  const logger = parentLogger.getChild("[RTCParticipation]");
+  const userId = client.getUserId()!;
+  const deviceId = client.getDeviceId()!;
+  const timings = { ...defaultSessionTimings, ...options.timings };
 
   const ownMembershipIdentity: CallMembershipIdentityParts = {
     userId,
@@ -100,7 +116,7 @@ export function createMatrixRTCClient(
         : `${userId}:${deviceId}`,
   };
 
-  const backend = createBackend(scope, client, options, {
+  const backend = createBackend(scope, client, options, matrixRTCMode, {
     roomId: room.roomId,
     ownMembershipIdentity,
     publish: options.publish,
@@ -108,10 +124,7 @@ export function createMatrixRTCClient(
     mediaKeys$: mediaKeys$(jsSdkSession),
     timings,
     logger,
-  });
-
-  const memberships$ = createMemberships$(scope, jsSdkSession);
-  const transports = createTransportRegistry(scope, backend.connections$);
+    });
 
   const preparedTransport$ = from(
     discoverLocalTransport(client, backend.transportType, options, logger).then(
@@ -162,56 +175,40 @@ export function createMatrixRTCClient(
     logger,
   });
 
-  const context = {
-    metadata: createMatrixMemberMetadata$(
-      scope,
-      scope.behavior(memberships$.pipe(map(({ value }) => value))),
-      createRoomMembers$(scope, room),
-    ),
-    transports,
-  };
-
   const remoteMembers$ = scope.behavior<RemoteRTCMember[]>(
-    memberships$.pipe(
-      map(({ value }) => value),
+    members$.pipe(
       generateItems(
-        "MatrixRTCClient remoteMembers",
-        function* (memberships) {
-          for (const membership of memberships) {
-            if (
-              membership.userId === userId &&
-              membership.deviceId === deviceId
-            )
-              continue;
-            yield { keys: membershipKeys(membership), data: membership };
-          }
+        "RTCParticipation remoteMembers",
+        function* (members) {
+          for (const member of members)
+            if (!member.local) yield { keys: [member] as const, data: member };
         },
-        (memberScope, membership$) =>
+        (memberScope, member$) =>
           createRemoteRTCMember(
-            memberScope,
-            membership$,
-            backend.mediaFor$(memberScope, membership$),
-            context,
+            member$.value,
+            backend.mediaFor(memberScope, member$.value),
           ),
       ),
     ),
   );
 
-  const localMembership$ = scope.behavior(
-    memberships$.pipe(
+  // Matched by the identity we joined with: a stale membership of ours from
+  // an earlier participation is `local` in the slot but is not this one
+  const ownMember$ = scope.behavior(
+    members$.pipe(
       map(
-        ({ value }) =>
-          value.find(
-            (membership) =>
-              membership.userId === userId && membership.deviceId === deviceId,
+        (members) =>
+          members.find(
+            (member) =>
+              member.local &&
+              member.memberId === ownMembershipIdentity.memberId,
           ) ?? null,
       ),
-      filterBehavior((membership) => membership !== null),
     ),
   );
   const localMember$ = scope.behavior<LocalRTCMember | null>(
-    mapScoped(scope, localMembership$, (memberScope, membership$) =>
-      createLocalRTCMember(memberScope, membership$, backend.local, context),
+    mapScoped(scope, ownMember$, (_memberScope, member) =>
+      createLocalRTCMember(member, backend.local),
     ).pipe(map((member) => member ?? null)),
   );
 
@@ -219,7 +216,9 @@ export function createMatrixRTCClient(
   // ever hears from attested members
   const data$ = backend.data$.pipe(
     map(({ senderId, topic, text }): DataMessage | null => {
-      const member = remoteMembers$.value.find((m) => m.id === senderId);
+      const member = remoteMembers$.value.find(
+        (m) => m.rtcBackendIdentity === senderId,
+      );
       if (member === undefined) {
         logger.warn(`Dropping data from ${senderId}: not a member`);
         return null;
@@ -229,48 +228,56 @@ export function createMatrixRTCClient(
     filter((message) => message !== null),
   );
 
+  const left$ = new BehaviorSubject(false);
   const status$ = scope.behavior(
     combineLatest(
       [
         localMembership.state$,
-        localMembership.joinRequested$,
         localMembership.connected$,
         localMembership.reconnecting$,
+        left$,
       ],
-      sessionStatus,
+      participationStatus,
     ),
   );
 
+  const leave = (): void => {
+    if (left$.value) return;
+    left$.next(true);
+    localMembership.leave();
+    scope.end();
+  };
+
   return {
-    join: localMembership.requestJoinAndPublish,
-    leave: localMembership.requestDisconnect,
-    status$,
-    connected$: localMembership.connected$,
-    reconnecting$: localMembership.reconnecting$,
-    disconnectReason$: localMembership.disconnectReason$,
-    fatalError$: scope.behavior(localMembership.state$.pipe(map(fatalError))),
-    localMember$,
-    remoteMembers$,
-    memberCount$: scope.behavior(
-      combineLatest(
-        [localMember$, remoteMembers$],
-        (local, remote) => remote.length + (local === null ? 0 : 1),
-      ),
-    ),
-    keyRotationSuppressed$: createKeyRotationSuppressed$(scope, jsSdkSession),
-    connectedTransports$: transports.connected$,
-    setAudioOutputDeviceId: async (deviceId) =>
-      backend.setAudioOutputDeviceId(deviceId),
-    sendData: async (topic, text) => backend.sendData(topic, text),
-    data$,
+    connections$: backend.connections$,
+    participation: {
+      slot,
+      leave,
+      status$,
+      connected$: localMembership.connected$,
+      reconnecting$: localMembership.reconnecting$,
+      disconnectReason$: localMembership.disconnectReason$,
+      fatalError$: scope.behavior(localMembership.state$.pipe(map(fatalError))),
+      localMember$,
+      remoteMembers$,
+      publish: async (request) => backend.local.publish(request),
+      unpublish: async (source) => backend.local.unpublish(source),
+      keyRotationSuppressed$: createKeyRotationSuppressed$(scope, jsSdkSession),
+      connectedTransports$: transports.connected$,
+      setAudioOutputDeviceId: async (deviceId) =>
+        backend.setAudioOutputDeviceId(deviceId),
+      sendData: async (topic, text) => backend.sendData(topic, text),
+      data$,
+    },
   };
 }
 
-/** The host's backend, or LiveKit configured from the client options. */
+/** The host's backend, or LiveKit configured from the participation options. */
 function createBackend(
   scope: ObservableScope,
   client: MatrixClient,
-  { backend, mediaQuality, matrixRTCMode }: MatrixRTCClientOptions,
+  { backend, mediaQuality }: RTCParticipationOptions,
+  matrixRTCMode: MatrixRTCMode,
   context: MediaBackendContext,
 ): MediaBackend {
   if (backend) return backend(scope, context);
@@ -282,7 +289,7 @@ function createBackend(
   });
 }
 
-/** The session's per-participant keys: the ones it already holds on subscribe, then each new one. */
+/** The slot's per-participant keys: the ones it already holds on subscribe, then each new one. */
 function mediaKeys$(session: JsSdkRTCSession): Observable<MediaKey> {
   return new Observable((subscriber) => {
     const onKey = (
@@ -296,24 +303,3 @@ function mediaKeys$(session: JsSdkRTCSession): Observable<MediaKey> {
     return () => session.off(MatrixRTCSessionEvent.EncryptionKeyChanged, onKey);
   });
 }
-
-/**
- * The js-sdk keeps one session manager per slot. The client comes with the
- * manager for the default slot; any other slot gets a manager of its own,
- * which lives as long as the scope.
- */
-function sessionManager(
-  scope: ObservableScope,
-  client: MatrixClient,
-  { application = "m.call", slot = "ROOM" }: MatrixRTCClientOptions,
-  logger: Logger,
-): MatrixRTCSessionManager {
-  if (application === "m.call" && slot === "ROOM") return client.matrixRTC;
-  const manager = new MatrixRTCSessionManager(logger, client, {
-    application,
-    id: slot,
-  });
-  manager.start();
-  scope.onEnd(() => manager.stop());
-  return manager;
-}
diff --git a/sdk/src/RTCSlot.ts b/sdk/src/RTCSlot.ts
new file mode 100644
index 000000000..734596ea5
--- /dev/null
+++ b/sdk/src/RTCSlot.ts
@@ -0,0 +1,185 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type MatrixClient, type Room } from "matrix-js-sdk";
+import { EventType } from "matrix-js-sdk/lib/@types/event";
+import { type Logger, logger as rootLogger } from "matrix-js-sdk/lib/logger";
+import {
+  type MatrixRTCSession as JsSdkRTCSession,
+  MatrixRTCSessionManager,
+  type SlotDescription,
+} from "matrix-js-sdk/lib/matrixrtc";
+import { type MatrixEvent } from "matrix-js-sdk/lib/models/event";
+import { RoomStateEvent } from "matrix-js-sdk/lib/models/room-state";
+import { BehaviorSubject, filter, fromEvent, map, of, switchMap } from "rxjs";
+
+import { ObservableScope } from "./reactive/ObservableScope";
+import { generateItems } from "./reactive/observable";
+import {
+  type RTCMember,
+  type RTCParticipation,
+  type RTCParticipationOptions,
+  type RTCSlot,
+  type RTCSlotOptions,
+} from "./api";
+import { MatrixRTCError } from "./errors";
+import {
+  createRTCParticipation,
+  type ParticipationInternals,
+} from "./RTCParticipation";
+import { createMemberships$ } from "./matrixrtc/JsRtcSessionBehaviors";
+import {
+  createMatrixMemberMetadata$,
+  createRoomMembers$,
+} from "./matrixrtc/MatrixMemberMetadata";
+import {
+  createRTCMember,
+  type MemberContext,
+  membershipKeys,
+} from "./matrixrtc/Members";
+import { createTransportRegistry } from "./matrixrtc/Transports";
+
+/**
+ * Takes the whole `MatrixClient` rather than a slice of it, and finds the
+ * MatrixRTC session itself. The js-sdk is the MatrixRTC implementation today;
+ * when the rust-rtc crate replaces it, the SDK has to bridge the crate to the
+ * js-sdk client, and only the SDK knows what that bridge needs. Holding the
+ * client keeps that change inside the SDK.
+ */
+export function createRTCSlot(
+  scope: ObservableScope,
+  client: MatrixClient,
+  room: Room,
+  options: RTCSlotOptions,
+): RTCSlot {
+  const logger = rootLogger.getChild("[RTCSlot]");
+  const userId = client.getUserId();
+  const deviceId = client.getDeviceId();
+  if (!(userId && deviceId))
+    throw new MatrixRTCError("The client has to be logged in");
+  const { application = "m.call", id = "ROOM" } = options;
+
+  const jsSdkSession = sessionManager(
+    scope,
+    client,
+    { application, id },
+    logger,
+  ).getRoomSession(room);
+  const memberships$ = createMemberships$(scope, jsSdkSession);
+
+  const active$ = new BehaviorSubject<ParticipationInternals | null>(null);
+  const participationScopes = new Set<ObservableScope>();
+  scope.onEnd(() => {
+    for (const participationScope of participationScopes)
+      participationScope.end();
+  });
+  const transports = createTransportRegistry(
+    scope,
+    scope.behavior(
+      active$.pipe(switchMap((active) => active?.connections$ ?? of([]))),
+    ),
+  );
+  const context: MemberContext = {
+    metadata: createMatrixMemberMetadata$(
+      scope,
+      scope.behavior(memberships$.pipe(map(({ value }) => value))),
+      createRoomMembers$(scope, room),
+    ),
+    transports,
+    own: { userId, deviceId },
+  };
+
+  const members$ = scope.behavior<RTCMember[]>(
+    memberships$.pipe(
+      map(({ value }) => value),
+      generateItems(
+        "RTCSlot members",
+        function* (memberships) {
+          for (const membership of memberships)
+            yield { keys: membershipKeys(membership), data: membership };
+        },
+        (memberScope, membership$) =>
+          createRTCMember(memberScope, membership$, context),
+      ),
+    ),
+  );
+
+  const status$ = scope.behavior(
+    fromEvent(room, RoomStateEvent.Events, (event: MatrixEvent) => event).pipe(
+      filter((event) => event.getType() === EventType.RTCSlot),
+      map(() => slotStatus(jsSdkSession)),
+    ),
+    slotStatus(jsSdkSession),
+  );
+
+  const slot: RTCSlot = {
+    roomId: room.roomId,
+    application,
+    id,
+    status$,
+    members$,
+    participation$: scope.behavior(
+      active$.pipe(map((active) => active?.participation ?? null)),
+    ),
+    join: (participationOptions: RTCParticipationOptions): RTCParticipation => {
+      if (active$.value !== null)
+        throw new MatrixRTCError(
+          "Already in this slot: leave the participation before joining again",
+        );
+      const participationScope = new ObservableScope();
+      participationScopes.add(participationScope);
+      const internals = createRTCParticipation(
+        participationScope,
+        {
+          client,
+          room,
+          slot,
+          jsSdkSession,
+          members$,
+          transports,
+          slotOptions: options,
+          logger,
+        },
+        participationOptions,
+      );
+      participationScope.onEnd(() => {
+        participationScopes.delete(participationScope);
+        if (active$.value === internals) active$.next(null);
+      });
+      active$.next(internals);
+      return internals.participation;
+    },
+  };
+  return slot;
+}
+
+function slotStatus(
+  session: Pick<JsSdkRTCSession, "getRtcSlot">,
+): "open" | "closed" | undefined {
+  return session.getRtcSlot()?.status;
+}
+
+/**
+ * The js-sdk keeps one session manager per slot. The client comes with the
+ * manager for the default slot; any other slot gets a manager of its own,
+ * which lives as long as the scope.
+ */
+function sessionManager(
+  scope: ObservableScope,
+  client: MatrixClient,
+  { application, id }: SlotDescription,
+  logger: Logger,
+): MatrixRTCSessionManager {
+  if (application === "m.call" && id === "ROOM") return client.matrixRTC;
+  const manager = new MatrixRTCSessionManager(logger, client, {
+    application,
+    id,
+  });
+  manager.start();
+  scope.onEnd(() => manager.stop());
+  return manager;
+}
diff --git a/sdk/src/api.ts b/sdk/src/api.ts
index b3ffa78f0..80df023d8 100644
--- a/sdk/src/api.ts
+++ b/sdk/src/api.ts
@@ -6,13 +6,12 @@ Please see LICENSE in the repository root for full details.
 */
 
 /**
- * The public types of the SDK: the client, its members and transports. What a
- * member sends is in `media-api.ts`. `createMatrixRTCClient` in
- * `MatrixRTCClient.ts` is the only way to obtain an implementation of them.
+ * The public types of the SDK: the slot, the participation, their members and
+ * transports. What a member sends is in `media-api.ts`. `createRTCSlot` in
+ * `RTCSlot.ts` is the only way to obtain an implementation of them.
  */
 
 import {
-  type CallMembership,
   type RTCNotificationType,
   type Transport,
 } from "matrix-js-sdk/lib/matrixrtc";
@@ -20,12 +19,12 @@ import { type Observable } from "rxjs";
 
 import { type Behavior } from "./reactive/Behavior";
 import {
-  type AudioMediaTrack,
-  type EncryptionError,
   type LocalAudioMediaTrack,
+  type LocalMemberMedia,
   type LocalVideoMediaTrack,
+  type MediaSource,
+  type MemberMedia,
   type PublishRequest,
-  type VideoMediaTrack,
 } from "./media-api";
 import { type MediaBackendFactory } from "./media-backend/api";
 import { type EncryptionSystem } from "./encryption";
@@ -34,26 +33,72 @@ import {
   type MediaQuality,
   type SessionTimings,
 } from "./config";
+import { type MatrixRTCError } from "./errors";
+
+export { MatrixRTCError } from "./errors";
 
 // ---------------------------------------------------------------------------
-// MatrixRTCClient
+// Slot
 
-export interface MatrixRTCClientOptions {
-  encryptionSystem: EncryptionSystem;
-  /** Resolved by the host; the SDK reads neither config.json nor settings. */
+export interface RTCSlotOptions {
+  /** The application the slot belongs to, as named in the membership. Default `m.call`. */
+  application?: string;
+  /** The application's slot in the room. Default `ROOM`. */
+  id?: string;
+  /** How memberships and keys in this slot are written. Resolved by the host. */
   matrixRTCMode: MatrixRTCMode;
-  /** Published at the join; `publish` on the local member adds to it from then on. */
+  encryptionSystem: EncryptionSystem;
+}
+
+/**
+ * The namespace in a room that memberships join into. It exists while the
+ * room does, with nobody in it too. The room has to be the one the client's
+ * sync loop maintains (`client.getRoom(roomId)`, once the join has synced):
+ * the detached copy `joinRoom` returns for a room joined just now never
+ * receives the state the members are read from.
+ */
+export interface RTCSlot {
+  readonly roomId: string;
+  readonly application: string;
+  readonly id: string;
+  /** From the slot state event; undefined while the room has none. */
+  status$: Behavior<"open" | "closed" | undefined>;
+  /**
+   * Everyone in the slot, one object per membership, kept for as long as the
+   * membership is. Our own membership is in here too, with `local: true`,
+   * once the room has seen it. No media: that is the participation's.
+   */
+  members$: Behavior<RTCMember[]>;
+  /** The participation we are in, null between `leave()` and the next `join()`. */
+  participation$: Behavior<RTCParticipation | null>;
+  /**
+   * Joins the slot and returns the participation that lasts until its
+   * `leave()`. Throws while `participation$` is not null: a slot has one local
+   * member.
+   */
+  join(options: RTCParticipationOptions): RTCParticipation; // Maybe MediaSession,
+}
+
+// ---------------------------------------------------------------------------
+// Participation
+
+export interface RTCParticipationOptions {
+  /**
+   * Published at the join; `publish` on the participation adds to it from then
+   * on. Kept as an option, rather than only the method, because the LiveKit
+   * backend reads the initial requests to build the room's default capture
+   * options before the first connection exists.
+   */
   publish: PublishRequest[];
   /**
+   * LEGACY
+   *
+   * Conside already removing it. just skips code in js-sdk. But rust sdk will not have this feature.
    * MSC4075 notification sent with the join. A parameter of the MatrixRTC
    * join itself, so it is here even though it is named after calls; reacting
    * to a notification (ringing, timeouts, declines) is the application's job.
    */
   sendNotificationType?: RTCNotificationType;
-  /** The application the session belongs to, as named in the membership. Default `m.call`. */
-  application?: string;
-  /** The application's slot in the room. Default `ROOM`. */
-  slot?: string;
   /**
    * Whatever the application wants to say about itself in the membership,
    * under namespaced keys. Opaque to the SDK, apart from `m.call.intent`
@@ -61,7 +106,7 @@ export interface MatrixRTCClientOptions {
    * field of its own.
    */
   applicationData?: Record<string, unknown>;
-  /** Session timings the host has configured; the defaults otherwise. */
+  /** Timings the host has configured; the defaults otherwise. */
   timings?: Partial<SessionTimings>;
   /** Limits on what is published; LiveKit's defaults otherwise. */
   mediaQuality?: MediaQuality;
@@ -76,51 +121,58 @@ export interface MatrixRTCClientOptions {
   backend?: MediaBackendFactory;
 }
 
-export type ConnectionStatus =
+export type ParticipationStatus =
   | "waitingForTransport"
   | "connecting"
   | "connected"
   | "reconnecting"
-  | "disconnected";
+  | "disconnected"
+  /** `leave()` was called; the participation is over and stays so. */
+  | "left";
 
-/** Why the client is not connected: the first failing of its three links. */
+/** Why the participation is not connected: the first failing of its three links. */
 export type DisconnectReason = "sync" | "membership" | "probablyLeft" | "media";
 
-import { type MatrixRTCError } from "./errors";
-
-export { MatrixRTCError } from "./errors";
-
-/**
- * A session in one room. The room has to be the one the client's sync loop
- * maintains (`client.getRoom(roomId)`, once the join has synced): the
- * detached copy `joinRoom` returns for a room joined just now never receives
- * the state the members are read from.
- */
-export interface MatrixRTCClient {
-  join(): void;
+/** The span between our `join()` of a slot and our `leave()`. */
+export interface RTCParticipation {
+  readonly slot: RTCSlot;
+  /** Sends the leave, tears the connections down and ends the participation. */
   leave(): void;
   /** Collapsed view of the local member's state machine. */
-  status$: Behavior<ConnectionStatus>;
+  status$: Behavior<ParticipationStatus>;
   connected$: Behavior<boolean>;
   reconnecting$: Behavior<boolean>;
   /** Null while connected, the first failing link otherwise. */
   disconnectReason$: Behavior<DisconnectReason | null>;
-
-  /** A transport, Matrix or connection error that stops the session. */
+  /** A transport, Matrix or connection error that stops the participation. */
   fatalError$: Behavior<MatrixRTCError | null>;
 
+  /** Null until our own membership has been seen in the room. */
   localMember$: Behavior<LocalRTCMember | null>;
+  /**
+   * The slot's other members, each with the media this participation carries
+   * for it. A call view needs nothing but this and `localMember$` to render.
+   */
   remoteMembers$: Behavior<RemoteRTCMember[]>;
-  /** `remoteMembers.length`, plus one for the local member once it exists. */
-  memberCount$: Behavior<number>;
 
   /**
-   * Whether the session has grown large enough that MatrixRTC has stopped
+   * Publishes a source and resolves with its track, which also shows up in
+   * `localMember$.tracks$`. Works from the moment `join()` returns: before the
+   * transport is connected.
+   */
+  publish(
+    request: PublishRequest,
+  ): Promise<LocalAudioMediaTrack | LocalVideoMediaTrack>;
+  /** Removes one of our publications; a screen share takes its audio with it. */
+  unpublish(source: MediaSource): Promise<void>;
+
+  /**
+   * Whether the slot has grown large enough that MatrixRTC has stopped
    * rotating the media encryption key.
    */
   keyRotationSuppressed$: Behavior<boolean>;
 
-  /** Transports the session currently holds a live connection to. */
+  /** Transports the participation currently holds a live connection to. */
   connectedTransports$: Behavior<TransportMetadata[]>;
 
   /**
@@ -139,9 +191,9 @@ export interface MatrixRTCClient {
    */
   sendData(topic: string, text: string): Promise<void>;
   /**
-   * What remote members sent with `sendData`, on every transport the client
-   * is connected to. A message from an identity that is not a member is
-   * dropped, so a host only ever hears from attested members.
+   * What remote members sent with `sendData`, on every transport the
+   * participation is connected to. A message from an identity that is not a
+   * member is dropped, so a host only ever hears from attested members.
    */
   data$: Observable<DataMessage>;
 }
@@ -164,7 +216,7 @@ export interface DataMessage {
 export interface TransportMetadata {
   /** `"livekit"` today. */
   type: string;
-  /** Stable key, unique per transport in the session: the raw transport, serialised with sorted keys. */
+  /** Stable key, unique per transport in the slot: the raw transport, serialised with sorted keys. */
   id: string;
   /** The transport object as it appears in the membership. */
   raw: Transport;
@@ -190,63 +242,39 @@ export type ResolvedTransport =
 // ---------------------------------------------------------------------------
 // Members
 
-/**
- * What a membership says, as hosts read it. The js-sdk `CallMembership` is
- * behind it; anything beyond this is the js-sdk's API, not the SDK's.
- */
-export type RTCMembership = Pick<
-  CallMembership,
-  | "userId"
-  | "deviceId"
-  | "memberId"
-  | "rtcBackendIdentity"
-  | "application"
-  | "applicationData"
-  | "getTransport"
-  | "transports"
-  | "createdTs"
-  | "getAbsoluteExpiry"
->;
-
+/** A membership in the slot, with what the SDK derives from it. No media. */
 export interface RTCMember {
   local: boolean;
-  /** The identity the media backend knows this member by. */
-  id: string;
+  /**
+   * The identity this member has on the media backend's transport: the
+   * participant identity the SFU sees, and the sender of its data packets. The
+   * SDK uses it to match what arrives on the transport back to the member; a
+   * host needs it only as a stable key per membership or as a debug label.
+   *
+   * Today it is read off the membership, where the js-sdk computes it:
+   * `${userId}:${deviceId}` before sticky events, a hash of `memberId` in
+   * Matrix 2.0 mode. Eventually the backend will derive it itself, so treat it
+   * as opaque.
+   */
+  rtcBackendIdentity: string;
   userId: string;
   deviceId: string;
-  membership$: Behavior<RTCMembership>;
-  displayName$: Behavior<string>;
-  avatarUrl$: Behavior<string | undefined>;
+  /** The membership's own id; what the Matrix 2.0 identity is derived from. */
+  memberId: string;
   /** Which transport this member is on; undefined when the membership has none. */
   transport$: Behavior<TransportMetadata | undefined>;
-  /**
-   * The member's tracks, in publication order, once it has shown up on its
-   * transport; null until then ("waiting for media"). An entry stays the same
-   * object for as long as the same publication is behind it. Which track is
-   * which is in its `source`; `trackBySource$` picks one out.
-   */
-  tracks$: Behavior<(AudioMediaTrack | VideoMediaTrack)[] | null>;
-  /** Emits when the SFU reports a key problem for this member. */
-  encryptionError$: Observable<EncryptionError>;
+  /** What the member's application says about itself, e.g. `m.call.intent`. */
+  applicationData$: Behavior<Record<string, unknown>>;
+
+  displayName$: Behavior<string>;
+  avatarUrl$: Behavior<string | undefined>;
 }
 
-export interface RemoteRTCMember extends RTCMember {
+export interface RemoteRTCMember extends RTCMember, MemberMedia {
   local: false;
 }
 
-export interface LocalRTCMember extends RTCMember {
+/** Our member: the same as a remote one, with tracks that carry the controls. */
+export interface LocalRTCMember extends RTCMember, LocalMemberMedia {
   local: true;
-  tracks$: Behavior<(LocalAudioMediaTrack | LocalVideoMediaTrack)[] | null>;
-  /**
-   * Publishes a source and resolves with its track once it is in `tracks$`.
-   * Before the transport is connected the request is remembered and applied
-   * once it is. Rejects where the device could not be used, including the
-   * user closing the picker. One publication per source: publishing a source
-   * again unmutes it.
-   */
-  publish(
-    request: PublishRequest,
-  ): Promise<LocalAudioMediaTrack | LocalVideoMediaTrack>;
-  /** Removes one of our tracks; a screen share takes its audio with it. */
-  unpublish(id: string): Promise<void>;
 }
diff --git a/sdk/src/matrixrtc/LocalMember.test.ts b/sdk/src/matrixrtc/LocalMember.test.ts
index d37d15a9e..4cc9372d6 100644
--- a/sdk/src/matrixrtc/LocalMember.test.ts
+++ b/sdk/src/matrixrtc/LocalMember.test.ts
@@ -46,25 +46,22 @@ describe("createLocalMembership$", () => {
     });
 
     local.connectionState$.next(MediaConnectionState.Connected);
-    expect(membership.state$.value).toEqual({
-      media: PublishState.WaitingForUser,
-      matrix: Status.Connected,
-    });
-
-    membership.requestJoinAndPublish();
     await flushPromises();
     expect(membership.state$.value).toEqual({
       media: PublishState.Publishing,
       matrix: Status.Connected,
     });
     expect(membership.connected$.value).toBe(true);
+
+    membership.leave();
+    expect(membership.state$.value).toEqual({
+      media: PublishState.WaitingForUser,
+      matrix: Status.Connected,
+    });
   });
 
   it("publishes only while joined and the homeserver is reachable", async () => {
     const { membership, local, homeserver } = setup();
-    expect(local.setPublishing).toHaveBeenLastCalledWith(false);
-
-    membership.requestJoinAndPublish();
     expect(local.setPublishing).toHaveBeenLastCalledWith(true);
 
     homeserver.combined$.next([false, "sync"]);
@@ -72,7 +69,7 @@ describe("createLocalMembership$", () => {
     homeserver.combined$.next([true, null]);
     expect(local.setPublishing).toHaveBeenLastCalledWith(true);
 
-    membership.requestDisconnect();
+    membership.leave();
     expect(local.setPublishing).toHaveBeenLastCalledWith(false);
     // A sync outage after leaving must not bring the media back
     homeserver.combined$.next([false, "sync"]);
@@ -82,10 +79,8 @@ describe("createLocalMembership$", () => {
   });
 
   it("joins with the delegated timings and hands over each delay id where the backend can", async () => {
-    const { membership, prepared$, delayId$, joinMatrixRTC, delegate } =
-      setup();
+    const { prepared$, delayId$, joinMatrixRTC, delegate } = setup();
     prepared$.next({ transport, canDelegateDelayedLeave: true });
-    membership.requestJoinAndPublish();
     await flushPromises();
     expect(joinMatrixRTC).toHaveBeenCalledWith(
       transport,
@@ -101,10 +96,8 @@ describe("createLocalMembership$", () => {
   });
 
   it("never hands the leave over where the backend cannot take it", async () => {
-    const { membership, prepared$, delayId$, joinMatrixRTC, delegate } =
-      setup();
+    const { prepared$, delayId$, joinMatrixRTC, delegate } = setup();
     prepared$.next({ transport, canDelegateDelayedLeave: false });
-    membership.requestJoinAndPublish();
     delayId$.next("delay-1");
     await flushPromises();
     expect(joinMatrixRTC).toHaveBeenCalledWith(
@@ -124,7 +117,6 @@ describe("createLocalMembership$", () => {
     const { membership, local, prepared$ } = setup();
     prepared$.next({ transport, canDelegateDelayedLeave: false });
     local.connectionState$.next(MediaConnectionState.Connected);
-    membership.requestJoinAndPublish();
 
     local.connectionState$.next(MediaConnectionState.Reconnecting);
     expect(membership.state$.value).toEqual({
diff --git a/sdk/src/matrixrtc/LocalMember.ts b/sdk/src/matrixrtc/LocalMember.ts
index 79972acb6..31b099d36 100644
--- a/sdk/src/matrixrtc/LocalMember.ts
+++ b/sdk/src/matrixrtc/LocalMember.ts
@@ -89,9 +89,8 @@ interface Props {
 }
 
 export interface LocalMembership {
-  requestJoinAndPublish: () => void;
-  requestDisconnect: () => void;
-  joinRequested$: Behavior<boolean>;
+  /** Sends the leave and stops publishing. Final: there is no joining again. */
+  leave: () => void;
   state$: Behavior<LocalMemberState>;
   /** Fully connected: to the homeserver, the session and the transport. */
   connected$: Behavior<boolean>;
@@ -101,8 +100,8 @@ export interface LocalMembership {
 }
 
 /**
- * The local member's state machine: waits for the transport, publishes once
- * asked to join, and enters and leaves the MatrixRTC session in step.
+ * The local member's state machine: waits for the transport, then publishes
+ * and enters the MatrixRTC session, and leaves both in step on `leave()`.
  */
 export function createLocalMembership$({
   scope,
@@ -131,22 +130,22 @@ export function createLocalMembership$({
     null,
   );
 
-  const joinRequested$ = new BehaviorSubject(false);
+  const joined$ = new BehaviorSubject(true);
   const matrixError$ = new BehaviorSubject<MatrixRTCError | null>(null);
 
   // Nothing leaves this device while it may already have been dropped from
   // the session: the member would show as away while still being heard
   combineLatest(
-    [joinRequested$, homeserverConnected.combined$],
-    (join, [connected]) => join && connected,
+    [joined$, homeserverConnected.combined$],
+    (joined, [connected]) => joined && connected,
   )
     .pipe(distinctUntilChanged(), scope.bind())
     .subscribe((publish) => local.setPublishing(publish));
 
   scope.reconcile(
-    scope.behavior(combineLatest([transport$, joinRequested$])),
-    async ([prepared, shouldJoin]) => {
-      if (prepared === null || !shouldJoin) return;
+    scope.behavior(combineLatest([transport$, joined$])),
+    async ([prepared, joined]) => {
+      if (prepared === null || !joined) return;
       try {
         joinMatrixRTC(
           prepared.transport,
@@ -191,8 +190,8 @@ export function createLocalMembership$({
   });
 
   const mediaState$ = scope.behavior<LocalMemberMediaState>(
-    combineLatest([local.connectionState$, joinRequested$]).pipe(
-      map(([connectionState, shouldPublish]) => {
+    combineLatest([local.connectionState$, joined$]).pipe(
+      map(([connectionState, joined]) => {
         if (connectionState !== MediaConnectionState.Connected)
           return {
             connection:
@@ -200,9 +199,7 @@ export function createLocalMembership$({
                 ? toMatrixRTCError(connectionState)
                 : connectionState,
           };
-        return shouldPublish
-          ? PublishState.Publishing
-          : PublishState.WaitingForUser;
+        return joined ? PublishState.Publishing : PublishState.WaitingForUser;
       }),
       distinctUntilChanged(deepCompare),
     ),
@@ -260,9 +257,7 @@ export function createLocalMembership$({
   );
 
   return {
-    requestJoinAndPublish: () => joinRequested$.next(true),
-    requestDisconnect: () => joinRequested$.next(false),
-    joinRequested$,
+    leave: () => joined$.next(false),
     state$,
     connected$,
     reconnecting$,
diff --git a/sdk/src/matrixrtc/LocalTransport.ts b/sdk/src/matrixrtc/LocalTransport.ts
index e08f6f023..29c5190a1 100644
--- a/sdk/src/matrixrtc/LocalTransport.ts
+++ b/sdk/src/matrixrtc/LocalTransport.ts
@@ -9,7 +9,7 @@ import { type MatrixClient } from "matrix-js-sdk";
 import { type Logger } from "matrix-js-sdk/lib/logger";
 import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
 
-import { type MatrixRTCClientOptions } from "../api";
+import { type RTCParticipationOptions } from "../api";
 import { MatrixRTCTransportMissingError } from "../errors";
 import { TransportAutoDiscovery } from "./TransportAutoDiscovery";
 
@@ -27,7 +27,7 @@ export async function discoverLocalTransport(
   {
     transportUrl,
     fallbackTransportUrl,
-  }: Pick<MatrixRTCClientOptions, "transportUrl" | "fallbackTransportUrl">,
+  }: Pick<RTCParticipationOptions, "transportUrl" | "fallbackTransportUrl">,
   logger: Logger,
 ): Promise<Transport> {
   const transport = transportUrl
diff --git a/sdk/src/matrixrtc/Members.ts b/sdk/src/matrixrtc/Members.ts
index 2b2046b8c..ae47aae98 100644
--- a/sdk/src/matrixrtc/Members.ts
+++ b/sdk/src/matrixrtc/Members.ts
@@ -6,6 +6,7 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type CallMembership } from "matrix-js-sdk/lib/matrixrtc";
+import { deepCompare } from "matrix-js-sdk/lib/utils";
 import { distinctUntilChanged, map } from "rxjs";
 
 import { type Behavior } from "../reactive/Behavior";
@@ -16,16 +17,15 @@ import {
   type RemoteRTCMember,
   type RTCMember,
 } from "../api";
-import {
-  type LocalMediaBackend,
-  type MemberMediaFields,
-} from "../media-backend/api";
+import { type LocalMemberMedia, type MemberMedia } from "../media-api";
 import { type TransportRegistry } from "./Transports";
 
-/** What every member is built from, besides its own membership and media. */
+/** What every member is built from, besides its own membership. */
 export interface MemberContext {
   metadata: ReturnType<typeof createMatrixMemberMetadata$>;
   transports: TransportRegistry;
+  /** Whose memberships are `local`. */
+  own: { userId: string; deviceId: string };
 }
 
 /** Everything that tells one membership from another, for keying items. */
@@ -40,50 +40,21 @@ export function membershipKeys(
   ];
 }
 
-export function createRemoteRTCMember(
+/** The slot's view of a membership: identity, room state and transport, no media. */
+export function createRTCMember(
   scope: ObservableScope,
   membership$: Behavior<CallMembership>,
-  media: MemberMediaFields,
-  context: MemberContext,
-): RemoteRTCMember {
+  { metadata, transports, own }: MemberContext,
+): RTCMember {
+  const { userId, deviceId, memberId, rtcBackendIdentity } = membership$.value;
   return {
-    ...createRTCMember(scope, membership$, context),
-    local: false,
-    tracks$: media.tracks$,
-    encryptionError$: media.encryptionError$,
-  };
-}
-
-export function createLocalRTCMember(
-  scope: ObservableScope,
-  membership$: Behavior<CallMembership>,
-  local: Pick<
-    LocalMediaBackend,
-    "tracks$" | "encryptionError$" | "publish" | "unpublish"
-  >,
-  context: MemberContext,
-): LocalRTCMember {
-  return {
-    ...createRTCMember(scope, membership$, context),
-    local: true,
-    tracks$: local.tracks$,
-    encryptionError$: local.encryptionError$,
-    publish: async (request) => local.publish(request),
-    unpublish: async (id) => local.unpublish(id),
-  };
-}
-
-function createRTCMember(
-  scope: ObservableScope,
-  membership$: Behavior<CallMembership>,
-  { metadata, transports }: MemberContext,
-): Omit<RTCMember, "local" | "tracks$" | "encryptionError$"> {
-  const { userId, deviceId, rtcBackendIdentity } = membership$.value;
-  return {
-    id: rtcBackendIdentity,
+    local: userId === own.userId && deviceId === own.deviceId,
+    // Read off the membership for now; the backend will derive it once the
+    // identity scheme is its own and not the js-sdk's
+    rtcBackendIdentity,
     userId,
     deviceId,
-    membership$,
+    memberId,
     displayName$: scope.behavior(
       metadata
         .createDisplayNameBehavior$(scope, userId)
@@ -97,5 +68,26 @@ function createRTCMember(
         map((transport) => transport && transports.get(transport)),
       ),
     ),
+    applicationData$: scope.behavior(
+      membership$.pipe(
+        map((membership) => membership.applicationData),
+        distinctUntilChanged(deepCompare),
+      ),
+    ),
   };
 }
+
+/** The participation's view of a remote member: the slot's member plus its media. */
+export function createRemoteRTCMember(
+  member: RTCMember,
+  media: MemberMedia,
+): RemoteRTCMember {
+  return { ...member, local: false, ...media };
+}
+
+export function createLocalRTCMember(
+  member: RTCMember,
+  media: LocalMemberMedia,
+): LocalRTCMember {
+  return { ...member, local: true, ...media };
+}
diff --git a/sdk/src/matrixrtc/status.test.ts b/sdk/src/matrixrtc/status.test.ts
index d6a97ddaf..38aaea094 100644
--- a/sdk/src/matrixrtc/status.test.ts
+++ b/sdk/src/matrixrtc/status.test.ts
@@ -15,7 +15,7 @@ import {
   PublishState,
   TransportState,
 } from "./LocalMember";
-import { fatalError, sessionStatus } from "./status";
+import { fatalError, participationStatus } from "./status";
 
 const publishing: LocalMemberState = {
   media: PublishState.Publishing,
@@ -27,31 +27,39 @@ const connecting: LocalMemberState = {
 };
 const error = new MatrixRTCError("gone");
 
-describe("sessionStatus", () => {
+describe("participationStatus", () => {
   it.each<[string, LocalMemberState, boolean, boolean, boolean, string]>([
     [
       "no transport yet",
       TransportState.Waiting,
-      true,
+      false,
       false,
       false,
       "waitingForTransport",
     ],
-    ["not asked to join", connecting, false, false, false, "disconnected"],
-    ["joining", connecting, true, false, false, "connecting"],
-    ["connected", publishing, true, true, false, "connected"],
-    ["dropped after connecting", connecting, true, false, true, "reconnecting"],
-    ["a transport error", error, true, false, false, "disconnected"],
+    ["joining", connecting, false, false, false, "connecting"],
+    ["connected", publishing, true, false, false, "connected"],
+    [
+      "dropped after connecting",
+      connecting,
+      false,
+      true,
+      false,
+      "reconnecting",
+    ],
+    ["a transport error", error, false, false, false, "disconnected"],
     [
       "a matrix error",
       { ...publishing, matrix: error },
       true,
-      true,
+      false,
       false,
       "disconnected",
     ],
-  ])("%s", (_name, state, joinRequested, connected, reconnecting, expected) => {
-    expect(sessionStatus(state, joinRequested, connected, reconnecting)).toBe(
+    ["left", publishing, true, false, true, "left"],
+    ["left after an error", error, false, false, true, "left"],
+  ])("%s", (_name, state, connected, reconnecting, left, expected) => {
+    expect(participationStatus(state, connected, reconnecting, left)).toBe(
       expected,
     );
   });
diff --git a/sdk/src/matrixrtc/status.ts b/sdk/src/matrixrtc/status.ts
index b0b193bd6..af7e30af0 100644
--- a/sdk/src/matrixrtc/status.ts
+++ b/sdk/src/matrixrtc/status.ts
@@ -5,24 +5,25 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { type MatrixRTCError, type ConnectionStatus } from "../api";
+import { type MatrixRTCError, type ParticipationStatus } from "../api";
 import { type LocalMemberState, TransportState } from "./LocalMember";
 
 /** The local member's state, collapsed to what a host shows. */
-export function sessionStatus(
+export function participationStatus(
   state: LocalMemberState,
-  joinRequested: boolean,
   connected: boolean,
   reconnecting: boolean,
-): ConnectionStatus {
+  left: boolean,
+): ParticipationStatus {
+  if (left) return "left";
   if (fatalError(state) !== null) return "disconnected";
   if (state === TransportState.Waiting) return "waitingForTransport";
   if (connected) return "connected";
   if (reconnecting) return "reconnecting";
-  return joinRequested ? "connecting" : "disconnected";
+  return "connecting";
 }
 
-/** The error that stops the session, if the state holds one. */
+/** The error that stops the participation, if the state holds one. */
 export function fatalError(state: LocalMemberState): MatrixRTCError | null {
   if (state === TransportState.Waiting) return null;
   if (state instanceof Error) return state;
diff --git a/sdk/src/media-api.ts b/sdk/src/media-api.ts
index 05d33efc8..4ed5a1d0e 100644
--- a/sdk/src/media-api.ts
+++ b/sdk/src/media-api.ts
@@ -16,6 +16,8 @@ import {
   type TrackProcessor,
   type VideoCodec,
 } from "livekit-client";
+import { type Observable } from "rxjs";
+
 import { type Behavior } from "./reactive/Behavior";
 
 export interface AudioCaptureSettings {
@@ -42,6 +44,11 @@ export type PublishRequest =
       source: "camera";
       deviceId?: string;
       /** Background blur and the like. */
+      // REVIEW: this is ugly makes us need to import lk
+      // We need an interface:
+      //  - super high level: blur + background img
+      //  - lower level: rtc stream insertable stream (copy lk TrackProcessor concept)
+      //  - florian: should be independent of webCodec or WebRtc
       processor?: TrackProcessor<Track.Kind.Video>;
       capture?: VideoCaptureSettings;
     }
@@ -73,8 +80,14 @@ export interface MediaTrack {
   id: string;
   muted$: Behavior<boolean>;
   /** False when the SFU reports the track as unencrypted. */
+  // REVIEW: does it need to be a behavior
   encrypted$: Behavior<boolean>;
   /** Polled while subscribed. */
+  // REVIEW: we want the perf gain of not having it always run.
+  //
+  // proposals
+  //  - obs -> on subscribe start polling timer
+  //  - add public method the MediaTrack: for example: actiavteStatsPolling(ms)
   stats$: Behavior<MediaStreamStats>;
   /**
    * Rendering. The view hands its `<video>` or `<audio>` element over; the SDK
@@ -95,6 +108,7 @@ export interface AudioMediaTrack extends MediaTrack {
    * "speaking". LiveKit measures it per member, so every audio track of a
    * member reports the same value.
    */
+  // REVIEW: audio Level (could even super-seed isActive "audioLevel != 0")
   isActive$: Behavior<boolean>;
   /** Route playback through Web Audio, for earpiece pan and gain. Undefined resets. */
   setAudioContext(ctx: AudioContext | undefined, plugins?: AudioNode[]): void;
@@ -135,3 +149,24 @@ export interface LocalVideoMediaTrack extends VideoMediaTrack, LocalMediaTrack {
 }
 
 export type EncryptionError = "MissingKey" | "InvalidKey";
+
+/**
+ * What a member sends, once it has arrived on its transport: its tracks, null
+ * while nothing has arrived for it, and the key errors beside them. The media
+ * backend supplies exactly these two fields per member.
+ */
+export interface MemberMedia {
+  /**
+   * The member's tracks, in publication order, once it has shown up on its
+   * transport; null until then ("waiting for media"). An entry stays the same
+   * object for as long as the same publication is behind it. Which track is
+   * which is in its `source`; `trackBySource$` picks one out.
+   */
+  tracks$: Behavior<(AudioMediaTrack | VideoMediaTrack)[] | null>;
+  /** Emits when the SFU reports a key problem for this member. */
+  encryptionError$: Observable<EncryptionError>;
+}
+
+export interface LocalMemberMedia extends MemberMedia {
+  tracks$: Behavior<(LocalAudioMediaTrack | LocalVideoMediaTrack)[] | null>;
+}
diff --git a/sdk/src/media-backend/api.ts b/sdk/src/media-backend/api.ts
index 336db8f09..36252e795 100644
--- a/sdk/src/media-backend/api.ts
+++ b/sdk/src/media-backend/api.ts
@@ -17,15 +17,13 @@ import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
 import { type Observable } from "rxjs";
 
-import {
-  type LocalRTCMember,
-  type ResolvedTransport,
-  type RTCMember,
-  type RTCMembership,
-} from "../api";
+import { type ResolvedTransport, type RTCMember } from "../api";
 import {
   type LocalAudioMediaTrack,
+  type LocalMemberMedia,
   type LocalVideoMediaTrack,
+  type MediaSource,
+  type MemberMedia,
   type PublishRequest,
 } from "../media-api";
 import { type SessionTimings } from "../config";
@@ -53,13 +51,6 @@ export interface MediaKey {
   key: Uint8Array<ArrayBuffer>;
 }
 
-/** The fields of a member the backend supplies, as the member exposes them. */
-export type MemberMediaFields = Pick<RTCMember, "tracks$" | "encryptionError$">;
-export type LocalMemberMediaFields = Pick<
-  LocalRTCMember,
-  "tracks$" | "encryptionError$"
->;
-
 export interface MediaBackendContext {
   roomId: string;
   ownMembershipIdentity: CallMembershipIdentityParts;
@@ -144,15 +135,12 @@ export interface MediaBackend {
   /**
    * The media of one remote member: its tracks, null while nothing has
    * arrived for it on its transport, and its key errors. Built in `scope`,
-   * which the caller ends with the member. This is also how the backend
-   * learns which remote transports exist: it follows the membership's
-   * transport for as long as `scope` lives, and connects to every transport
-   * some member is on.
+   * which the caller ends with the member or the participation, whichever
+   * goes first. This is also how the backend learns which remote transports
+   * exist: it follows the member's `transport$` for as long as `scope` lives,
+   * and connects to every transport some member is on.
    */
-  mediaFor$(
-    scope: ObservableScope,
-    membership$: Behavior<RTCMembership>,
-  ): MemberMediaFields;
+  mediaFor(scope: ObservableScope, member: RTCMember): MemberMedia;
 
   /** Every connection the backend holds. For debugging and devtool purposes.*/
   readonly connections$: Behavior<BackendConnection[]>;
@@ -175,7 +163,7 @@ export interface MediaBackend {
 }
 
 /** `tracks$` is null until the connection carries a local participant. */
-export interface LocalMediaBackend extends LocalMemberMediaFields {
+export interface LocalMediaBackend extends LocalMemberMedia {
   /**
    * State of the connection the local member publishes on. `Initialized`
    * while there is none; an `Error` is the connection's failure.
@@ -201,6 +189,6 @@ export interface LocalMediaBackend extends LocalMemberMediaFields {
   publish(
     request: PublishRequest,
   ): Promise<LocalAudioMediaTrack | LocalVideoMediaTrack>;
-  /** Removes one of our tracks by id; a screen share takes its audio with it. */
-  unpublish(id: string): Promise<void>;
+  /** Removes one of our publications; a screen share takes its audio with it. */
+  unpublish(source: MediaSource): Promise<void>;
 }
diff --git a/sdk/src/media-backend/livekit/LivekitBackend.ts b/sdk/src/media-backend/livekit/LivekitBackend.ts
index 96c805e74..0ba67bf37 100644
--- a/sdk/src/media-backend/livekit/LivekitBackend.ts
+++ b/sdk/src/media-backend/livekit/LivekitBackend.ts
@@ -23,11 +23,12 @@ import {
   switchMap,
 } from "rxjs";
 
-import { type ResolvedTransport, type RTCMembership } from "../../api";
+import { type ResolvedTransport, type RTCMember } from "../../api";
 
 import { type MediaQuality } from "../../config";
 import { MatrixRTCError } from "../../errors";
 import { type Behavior } from "../../reactive/Behavior";
+import { type MemberMedia } from "../../media-api";
 import {
   type Epoch,
   type ObservableScope,
@@ -37,7 +38,6 @@ import {
   type BackendConnection,
   type MediaBackend,
   type MediaBackendContext,
-  type MemberMediaFields,
   type TransportCapabilities,
 } from "../api";
 import { type Connection } from "./Connection";
@@ -147,14 +147,14 @@ export function createLivekitBackend(
     );
   };
 
-  const mediaFor$ = (
+  const mediaFor = (
     memberScope: ObservableScope,
-    membership$: Behavior<RTCMembership>,
-  ): MemberMediaFields => {
-    const identity = membership$.value.rtcBackendIdentity;
+    member: RTCMember,
+  ): MemberMedia => {
+    const identity = member.rtcBackendIdentity;
     const transport$ = memberScope.behavior(
-      membership$.pipe(
-        map((membership) => membership.getTransport()),
+      member.transport$.pipe(
+        map((transport) => transport?.raw),
         map((transport) =>
           isLivekitTransport(transport) ? transport : undefined,
         ),
@@ -224,7 +224,7 @@ export function createLivekitBackend(
     prepareLocalTransport,
     delegateDelayedLeave,
     local: localMedia_,
-    mediaFor$,
+    mediaFor,
     connections$,
     setAudioOutputDeviceId: localMedia_.setAudioOutputDeviceId,
     sendData,
diff --git a/sdk/src/media-backend/livekit/LivekitMemberMedia.ts b/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
index f7b191d7b..f4e19eb72 100644
--- a/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
+++ b/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
@@ -23,8 +23,12 @@ import { generateItems } from "../../reactive/observable";
 import { E2eeType, type EncryptionSystem } from "../../encryption";
 import { type Behavior } from "../../reactive/Behavior";
 import { mapScoped } from "../../utils/mapScoped";
-import { type EncryptionError, type MediaSource } from "../../media-api";
-import { type LocalMemberMediaFields, type MemberMediaFields } from "../api";
+import {
+  type EncryptionError,
+  type LocalMemberMedia,
+  type MediaSource,
+  type MemberMedia,
+} from "../../media-api";
 import {
   createLivekitMediaTrack,
   createLocalLivekitMediaTrack,
@@ -45,7 +49,7 @@ export function createLivekitMemberMedia(
   scope: ObservableScope,
   source$: Behavior<ParticipantSource>,
   encryptionSystem: EncryptionSystem,
-): MemberMediaFields {
+): MemberMedia {
   return {
     tracks$: memberTracks$(
       scope,
@@ -62,7 +66,7 @@ export function createLocalLivekitMemberMedia(
   source$: Behavior<ParticipantSource<LocalParticipant>>,
   encryptionSystem: EncryptionSystem,
   setEnabled: (source: MediaSource, enabled: boolean) => Promise<boolean>,
-): LocalMemberMediaFields {
+): LocalMemberMedia {
   return {
     tracks$: memberTracks$(
       scope,
diff --git a/sdk/src/media-backend/livekit/LocalMedia.ts b/sdk/src/media-backend/livekit/LocalMedia.ts
index ae176cce5..472cc4f54 100644
--- a/sdk/src/media-backend/livekit/LocalMedia.ts
+++ b/sdk/src/media-backend/livekit/LocalMedia.ts
@@ -191,14 +191,11 @@ export function createLivekitLocalMedia({
     }
   };
 
-  const unpublish = async (id: string): Promise<void> => {
-    const track = tracks$.value?.find((t) => t.id === id);
-    if (track === undefined) return;
-    // The screen share audio goes with its video
-    const source =
-      track.source === "screenShareAudio" ? "screenShare" : track.source;
-    desired.delete(source);
-    await publisher$.value?.unpublish(source);
+  const unpublish = async (source: MediaSource): Promise<void> => {
+    const published = publishedSource(source);
+    if (published === undefined) return;
+    desired.delete(published);
+    await publisher$.value?.unpublish(published);
   };
 
   const setAudioOutputDeviceId = async (deviceId: string): Promise<void> => {
@@ -268,3 +265,17 @@ function trackWithId$(
     filter((track) => track !== undefined),
   );
 }
+
+/** The publication a source belongs to: the screen share audio goes with its video. */
+function publishedSource(
+  source: MediaSource,
+): PublishRequest["source"] | undefined {
+  switch (source) {
+    case "screenShareAudio":
+      return "screenShare";
+    case "unknown":
+      return undefined;
+    default:
+      return source;
+  }
+}