Add m.livekit with two backends

Run the media backend inside a combination of backends
The participation now builds its default backend by combining every backend the SDK has, publishing on the type of the transport the host names (LiveKit without one) and handing each remote member to the backend of its transport. LiveKit is the only member of the combination for now. The LiveKit backend can run without a local transport, so it subscribes only while another backend publishes, and probes the homeserver for delayed leave delegation only when it first publishes. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-11-10 01:02:27 +00:00 · 2026-10-29 16:58:28 +01:00 · 2026-10-29 10:34:03 +01:00
11 changed files with 400 additions and 109 deletions
@@ -107,7 +107,7 @@
    "livekit-client": "^2.18.1",
    "lodash-es": "^4.17.21",
    "loglevel": "^1.9.1",
-    "matrix-js-sdk": "github:matrix-org/matrix-js-sdk#3f05dc4d104a3dfc753c5e3549a18c74fab4b076",
+    "matrix-js-sdk": "github:matrix-org/matrix-js-sdk#b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2",
    "matrix-widget-api": "^1.18.0",
    "node-stdlib-browser": "^1.3.1",
    "normalize.css": "^8.0.1",
@@ -66,7 +66,7 @@ importers:
        version: 10.1.2(react-dom@19.2.8(react@19.2.8))(react@19.2.8)
      '@rolldown/plugin-babel':
        specifier: ^0.2.3
-        version: 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.0)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
+        version: 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.8)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
      '@sentry/react':
        specifier: ^8.0.0
        version: 8.55.2(react@19.2.8)
@@ -138,7 +138,7 @@ importers:
        version: 10.4.0(@fontsource/inconsolata@5.3.0)(@fontsource/inter@5.3.0)(@types/react-dom@19.2.5(@types/react@19.2.18))(@types/react@19.2.18)(@vector-im/compound-design-tokens@11.3.1(@types/react@19.2.18)(react@19.2.8))(react-dom@19.2.8(react@19.2.8))(react@19.2.8)
      '@vitejs/plugin-react':
        specifier: ^6.0.2
-        version: 6.1.1(@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.0)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)))(babel-plugin-react-compiler@1.0.0)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
+        version: 6.1.1(@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.8)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)))(babel-plugin-react-compiler@1.0.0)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
      '@vitest/browser-playwright':
        specifier: ^4.1.5
        version: 4.1.11(playwright@1.62.1)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))(vitest@4.1.11)
@@ -191,8 +191,8 @@ importers:
        specifier: ^1.9.1
        version: 1.9.2
      matrix-js-sdk:
-        specifier: github:matrix-org/matrix-js-sdk#3f05dc4d104a3dfc753c5e3549a18c74fab4b076
-        version: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076
+        specifier: github:matrix-org/matrix-js-sdk#b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2
+        version: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2
      matrix-widget-api:
        specifier: ^1.18.0
        version: 1.20.0
@@ -397,8 +397,8 @@ packages:
    resolution: {integrity: sha512-Nq8OhGWiZIZGV6hLHoyAKLLcJihP/xFeBMGJoUrxTX2psI8dCifzLhZISFb+VWS3wFMRDmCGw5R+dOySCqPLhw==}
    engines: {node: '>=6.9.0'}

-  '@babel/runtime@8.0.0':
-    resolution: {integrity: sha512-sL6cvO2IfkSu/iU+zs2S/w01B7A8V7suXSIKEN4hPFFdZoiPGxrj5pAG0lCaqLWiEIrjKzdznIWuaLcxPR53qw==}
+  '@babel/runtime@8.0.8':
+    resolution: {integrity: sha512-dEu8nGlRqjsJsvviUUoSMfcRHpwomCiRlXsc2mPfJ79+ffTpDm+JtfUtTMzUrqCApHQnCsVPD3gT1ZT8RgBRCQ==}

  '@babel/template@7.29.7':
    resolution: {integrity: sha512-puq+Gf35oI24FeN11LkoUQFqv9uwNeWpxXZi/Ji3rRIoKAzKnxRaZ+Gkj0vKS9ZCiTESfng1N9LyOyXvo+m+Gg==}
@@ -408,10 +408,6 @@ packages:
    resolution: {integrity: sha512-B3lYAs2nkKskbgQ1nfy8Ma9ao9Q1rntTAQHrzzyDuj+TSO+X3le6PfWwqDvodsb+OumE10eMFfuMCTQSQ5iuIQ==}
    engines: {node: '>=6.9.0'}

-  '@babel/traverse@7.29.8':
-    resolution: {integrity: sha512-I5z7H3bf/41ktsNVLtpN0wAa336HkqIHQ5BuPLEhTkt1jVSyZpeNKIzTgEWmlxjdg81R0IgUCcaE+Ok3NvrfZg==}
-    engines: {node: '>=6.9.0'}
-
  '@babel/types@7.29.11':
    resolution: {integrity: sha512-WCsemW40Cpc6gouC1SrY6PBaZ+oLRlEVY9XBvZBI7il/HyUrb2nJ505wE1MHEFyDZNnrWDEuy4jL50WVHfTTSQ==}
    engines: {node: '>=6.9.0'}
@@ -3421,11 +3417,6 @@ packages:
    engines: {node: '>=0.4.0'}
    hasBin: true

-  acorn@8.18.0:
-    resolution: {integrity: sha512-lGq+9yr1/GuAWaVYIHRjvvySG5/4VfKIvC8EWxStPdcDh/Ka7FG3twP6v4d5BkravUilhIAsG4Qj83t02LWUPQ==}
-    engines: {node: '>=0.4.0'}
-    hasBin: true
-
  acorn@8.19.0:
    resolution: {integrity: sha512-oJlA3XiRm7Cyk6qFD2Jn8ak9B7jSy0qy00ADO3+8dpT0LSjFihQYv4C02LFCSYJV3Q37xYwwRy5m+IpIiUzqWw==}
    engines: {node: '>=0.4.0'}
@@ -4759,9 +4750,9 @@ packages:
  matrix-events-sdk@0.0.1:
    resolution: {integrity: sha512-1QEOsXO+bhyCroIe2/A5OwaxHvBm7EsSQ46DEDn8RBIfQwN5HWBpFvyWWR4QY0KHPPnnJdI99wgRiAl7Ad5qaA==}

-  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076:
-    resolution: {gitHosted: true, integrity: sha512-Ly8eS7fNNKDo9KmPvTkE49KvD8iMUmrVWJMNERNuxc8UksMLMG36NLz3fs8HytdwrB7AKPfyVo392VZsxILDIg==, tarball: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076}
-    version: 44.0.0
+  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2:
+    resolution: {gitHosted: true, integrity: sha512-AXD5ClohyPLBY30QfmkD99fAGlAMsQ+2flR8rWqTy8aNpLHnTUisNhAClDgkAaDTnnFbOX/vZ8hwVzmYSefqTA==, tarball: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2}
+    version: 45.0.0
    engines: {node: '>=22.0.0'}

  matrix-widget-api@1.20.0:
@@ -6357,10 +6348,10 @@ snapshots:
      '@babel/helper-compilation-targets': 7.29.7
      '@babel/helper-module-transforms': 7.29.7(@babel/core@7.29.7(supports-color@7.2.0))(supports-color@7.2.0)
      '@babel/helpers': 7.29.7
-      '@babel/parser': 7.29.8
+      '@babel/parser': 7.29.11
      '@babel/template': 7.29.7
-      '@babel/traverse': 7.29.8(supports-color@7.2.0)
-      '@babel/types': 7.29.8
+      '@babel/traverse': 7.29.10(supports-color@7.2.0)
+      '@babel/types': 7.29.11
      '@jridgewell/remapping': 2.3.5
      convert-source-map: 2.0.0
      debug: 4.4.3(supports-color@7.2.0)
@@ -6372,8 +6363,8 @@ snapshots:

  '@babel/generator@7.29.8':
    dependencies:
-      '@babel/parser': 7.29.8
-      '@babel/types': 7.29.8
+      '@babel/parser': 7.29.11
+      '@babel/types': 7.29.11
      '@jridgewell/gen-mapping': 0.3.13
      '@jridgewell/trace-mapping': 0.3.31
      jsesc: 3.1.0
@@ -6390,8 +6381,8 @@ snapshots:

  '@babel/helper-module-imports@7.29.7(supports-color@7.2.0)':
    dependencies:
-      '@babel/traverse': 7.29.8(supports-color@7.2.0)
-      '@babel/types': 7.29.8
+      '@babel/traverse': 7.29.10(supports-color@7.2.0)
+      '@babel/types': 7.29.11
    transitivePeerDependencies:
      - supports-color

@@ -6400,7 +6391,7 @@ snapshots:
      '@babel/core': 7.29.7(supports-color@7.2.0)
      '@babel/helper-module-imports': 7.29.7(supports-color@7.2.0)
      '@babel/helper-validator-identifier': 7.29.7
-      '@babel/traverse': 7.29.8(supports-color@7.2.0)
+      '@babel/traverse': 7.29.10(supports-color@7.2.0)
    transitivePeerDependencies:
      - supports-color

@@ -6417,7 +6408,7 @@ snapshots:
  '@babel/helpers@7.29.7':
    dependencies:
      '@babel/template': 7.29.7
-      '@babel/types': 7.29.8
+      '@babel/types': 7.29.11

  '@babel/parser@7.29.11':
    dependencies:
@@ -6433,13 +6424,13 @@ snapshots:

  '@babel/runtime@7.29.7': {}

-  '@babel/runtime@8.0.0': {}
+  '@babel/runtime@8.0.8': {}

  '@babel/template@7.29.7':
    dependencies:
      '@babel/code-frame': 7.29.7
-      '@babel/parser': 7.29.8
-      '@babel/types': 7.29.8
+      '@babel/parser': 7.29.11
+      '@babel/types': 7.29.11

  '@babel/traverse@7.29.10(supports-color@7.2.0)':
    dependencies:
@@ -6448,19 +6439,7 @@ snapshots:
      '@babel/helper-globals': 7.29.7
      '@babel/parser': 7.29.11
      '@babel/template': 7.29.7
-      '@babel/types': 7.29.8
-      debug: 4.4.3(supports-color@7.2.0)
-    transitivePeerDependencies:
-      - supports-color
-
-  '@babel/traverse@7.29.8(supports-color@7.2.0)':
-    dependencies:
-      '@babel/code-frame': 7.29.7
-      '@babel/generator': 7.29.8
-      '@babel/helper-globals': 7.29.7
-      '@babel/parser': 7.29.8
-      '@babel/template': 7.29.7
-      '@babel/types': 7.29.8
+      '@babel/types': 7.29.11
      debug: 4.4.3(supports-color@7.2.0)
    transitivePeerDependencies:
      - supports-color
@@ -8167,13 +8146,13 @@ snapshots:
  '@rolldown/binding-win32-x64-msvc@1.2.15':
    optional: true

-  '@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.0)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))':
+  '@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.8)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))':
    dependencies:
      '@babel/core': 7.29.7(supports-color@7.2.0)
      picomatch: 4.0.4
      rolldown: 1.2.15
    optionalDependencies:
-      '@babel/runtime': 8.0.0
+      '@babel/runtime': 8.0.8
      vite: 8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)

  '@rolldown/pluginutils@1.0.1': {}
@@ -8925,12 +8904,12 @@ snapshots:
      - '@types/react-dom'
      - react-dom

-  '@vitejs/plugin-react@6.1.1(@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.0)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)))(babel-plugin-react-compiler@1.0.0)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))':
+  '@vitejs/plugin-react@6.1.1(@rolldown/plugin-babel@0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.8)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)))(babel-plugin-react-compiler@1.0.0)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))':
    dependencies:
      '@rolldown/pluginutils': 1.0.1
      vite: 8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0)
    optionalDependencies:
-      '@rolldown/plugin-babel': 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.0)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
+      '@rolldown/plugin-babel': 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(@babel/runtime@8.0.8)(rolldown@1.2.15)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))
      babel-plugin-react-compiler: 1.0.0

  '@vitest/browser-playwright@4.1.11(playwright@1.62.1)(vite@8.2.4(@types/node@24.13.3)(esbuild@0.28.2)(jiti@2.7.0)(sass@1.103.1)(terser@5.46.1)(yaml@2.9.0))(vitest@4.1.11)':
@@ -9055,14 +9034,12 @@ snapshots:

  '@webcontainer/env@1.1.1': {}

-  acorn-jsx@5.3.2(acorn@8.18.0):
+  acorn-jsx@5.3.2(acorn@8.19.0):
    dependencies:
-      acorn: 8.18.0
+      acorn: 8.19.0

  acorn@8.16.0: {}

-  acorn@8.18.0: {}
-
  acorn@8.19.0: {}

  agent-base@6.0.2(supports-color@7.2.0):
@@ -9733,8 +9710,8 @@ snapshots:

  espree@9.6.1:
    dependencies:
-      acorn: 8.18.0
-      acorn-jsx: 5.3.2(acorn@8.18.0)
+      acorn: 8.19.0
+      acorn-jsx: 5.3.2(acorn@8.19.0)
      eslint-visitor-keys: 3.4.3

  esprima@4.0.1: {}
@@ -10468,9 +10445,9 @@ snapshots:

  matrix-events-sdk@0.0.1: {}

-  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076:
+  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/b3f17e3ee8ca53f5625f7d8c482155d5a82ef4a2:
    dependencies:
-      '@babel/runtime': 8.0.0
+      '@babel/runtime': 8.0.8
      '@matrix-org/matrix-sdk-crypto-wasm': 18.9.0
      another-json: 0.2.0
      bs58: 6.0.0
@@ -11701,7 +11678,7 @@ snapshots:
  terser@5.46.1:
    dependencies:
      '@jridgewell/source-map': 0.3.11
-      acorn: 8.18.0
+      acorn: 8.19.0
      commander: 2.20.3
      source-map-support: 0.5.21

@@ -66,9 +66,15 @@ media backend: whatever carries the media, behind the `MediaBackend` interface i
 `sdk/src/media-backend/api.ts`. The client is the join of the two: a member exists
 once its membership does, and gets its media once the backend has something
 for that membership on its transport; `state$` is the membership state combined
-with the local media connection. LiveKit is the one backend today; a
-cascading SFU or full mesh is another folder under `sdk/src/media-backend/`, handed
-in through `options.backend`, and nothing in the left box changes.
+with the local media connection. LiveKit (`livekit/`) is the one backend today,
+and the client runs it inside a combination (`combineBackends.ts`). It publishes
+on the backend of the type of `options.transport`, LiveKit without one, and
+hands every remote member to the backend of that member's transport. A backend
+nobody is on connects nowhere. LiveKit joins it twice (`createLivekitBackends`),
+once for the members on `livekit` and once for those on `m.livekit`, sharing
+its connections, so members of either type on one SFU share one. A cascading
+SFU or full mesh is another folder under `sdk/src/media-backend/`, added to the combination, and nothing in the
+left box changes; `options.backend` replaces the combination altogether.

 ### How Element Call uses it

@@ -149,7 +155,7 @@ the sections below still describe `MatrixRTCClient` until that lands.
 | 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                                                                    |
+| media backend               | a `MediaBackend`: what carries the media, behind `sdk/src/media-backend/api.ts`. One per participation: the SDK's backends combined, publishing on the type of `transport` in the join options, unless `backend` replaces them                         | "SFU", a LiveKit room, a peer connection                                                                    |

 Names use `RTC` in capitals, as the js-sdk does: `RTCSlot`, `RTCParticipation`,
 `RTCMember`, `matrixRTCMode`.
@@ -50,7 +50,8 @@ import {
  type MediaBackendContext,
  type MediaKey,
 } from "./media-backend/api";
-import { createLivekitBackend } from "./media-backend/livekit/LivekitBackend";
+import { combineBackends } from "./media-backend/combineBackends";
+import { createLivekitBackends } from "./media-backend/livekit/LivekitBackend";
 import { findRemotePublishLivekitTransport } from "./media-backend/livekit/RemotePublishTransport";
 import { createHomeserverConnected$ } from "./matrixrtc/HomeserverConnected";
 import { joinJsSdkSession } from "./matrixrtc/joinJsSdkSession";
@@ -136,7 +137,7 @@ export function createRTCParticipation(
  // fallback: another member's SFU may let us publish (MSC4195). LiveKit only,
  // so it stays out of the backend interface
  const transportFromBackends = async (): Promise<Transport | null> =>
-    backend.transportType === "livekit"
+    backend.transportType === "livekit" || backend.transportType === "m.livekit"
      ? findRemotePublishLivekitTransport({
          client,
          // A snapshot: if nobody with a usable SFU is here yet, the user has
@@ -310,15 +311,23 @@ export function createRTCParticipation(
  };
 }

-/** The host's backend, or LiveKit configured from the participation options. */
+/**
+ * The host's backend, or every backend the SDK has, combined: we publish on
+ * the type of the transport the host named (LiveKit without one) and receive
+ * from members on any of them.
+ */
 function createBackend(
  scope: ObservableScope,
  client: MatrixClient,
-  { backend, mediaQuality }: RTCParticipationOptions,
+  { backend, mediaQuality, transport }: RTCParticipationOptions,
  context: MediaBackendContext,
 ): MediaBackend {
  if (backend) return backend(scope, context);
-  return createLivekitBackend(scope, context, { client, mediaQuality });
+  return combineBackends(
+    scope,
+    createLivekitBackends(scope, context, { client, mediaQuality }),
+    transport?.type ?? "livekit",
+  );
 }

 /** The slot's per-participant keys: the ones it already holds on subscribe, then each new one. */
@@ -230,7 +230,7 @@ export interface DataMessage {
 * backend-specific developer panel and for connection diagnostics.
 */
 export interface TransportMetadata {
-  /** `"livekit"` today. */
+  /** `"m.livekit"` or `"livekit"` today. */
  type: string;
  /** Stable key, unique per transport in the slot: the raw transport, serialised with sorted keys. */
  id: string;
@@ -0,0 +1,91 @@
+/*
+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 { describe, expect, it, vi } from "vitest";
+import { BehaviorSubject, NEVER } from "rxjs";
+
+import { type RTCMember, type TransportMetadata } from "../api";
+import { type MemberMedia } from "../media-api";
+import { constant } from "../reactive/Behavior";
+import { ObservableScope } from "../reactive/ObservableScope";
+import { type MediaBackend } from "./api";
+import { combineBackends } from "./combineBackends";
+
+function fakeBackend(type: string): MediaBackend & { ended: string[] } {
+  const ended: string[] = [];
+  return {
+    ended,
+    transportType: type,
+    prepareLocalTransport: vi.fn(async () =>
+      Promise.resolve({ canDelegateDelayedLeave: false }),
+    ),
+    delegateDelayedLeave: vi.fn(),
+    local: {} as MediaBackend["local"],
+    mediaFor: (scope, member): MemberMedia => {
+      scope.onEnd(() => ended.push(member.memberId));
+      return {
+        tracks$: constant([{ id: `${type}:${member.memberId}` }] as never),
+        encryptionError$: NEVER,
+      };
+    },
+    connections$: constant([
+      { transport: { type }, state: "connected" as never, resolved: undefined },
+    ]),
+    setAudioOutputDeviceId: vi.fn(),
+    sendData: vi.fn(),
+    data$: NEVER,
+  };
+}
+
+function transport(type: string): TransportMetadata {
+  return { type, id: type, raw: { type }, resolved$: constant(undefined) };
+}
+
+describe("combineBackends", () => {
+  it("routes each member to the backend of its transport type", () => {
+    const scope = new ObservableScope();
+    const livekit = fakeBackend("livekit");
+    const other = fakeBackend("other");
+    const combined = combineBackends(scope, [livekit, other], "livekit");
+    expect(combined.transportType).toBe("livekit");
+    expect(combined.connections$.value).toHaveLength(2);
+
+    const transport$ = new BehaviorSubject<TransportMetadata | undefined>(
+      transport("other"),
+    );
+    const memberScope = new ObservableScope();
+    const media = combined.mediaFor(memberScope, {
+      memberId: "m1",
+      transport$,
+    } as unknown as RTCMember);
+    expect(media.tracks$.value).toEqual([{ id: "other:m1" }]);
+
+    transport$.next(transport("livekit"));
+    expect(other.ended).toEqual(["m1"]);
+    expect(media.tracks$.value).toEqual([{ id: "livekit:m1" }]);
+
+    transport$.next(transport("unknown"));
+    expect(media.tracks$.value).toBeNull();
+    // The publishing backend keeps every member for as long as it is one
+    expect(livekit.ended).toEqual([]);
+    memberScope.end();
+    expect(livekit.ended).toEqual(["m1"]);
+    scope.end();
+  });
+
+  it("publishes on the backend of the publish type", () => {
+    const scope = new ObservableScope();
+    const livekit = fakeBackend("livekit");
+    const other = fakeBackend("other");
+    const combined = combineBackends(scope, [livekit, other], "other");
+    expect(combined.transportType).toBe("other");
+    expect(() => combineBackends(scope, [livekit, other], "unknown")).toThrow(
+      "No media backend for transport unknown",
+    );
+    scope.end();
+  });
+});
@@ -0,0 +1,80 @@
+/*
+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 { combineLatest, map, merge, NEVER, of, switchMap } from "rxjs";
+
+import { type RTCMember } from "../api";
+import { MatrixRTCError } from "../errors";
+import { type MemberMedia } from "../media-api";
+import { type ObservableScope } from "../reactive/ObservableScope";
+import { mapScoped } from "../utils/mapScoped";
+import { type MediaBackend } from "./api";
+
+/**
+ * Several backends as one: we publish on the backend of `publishType`, and
+ * receive from every member on a transport any of the backends serves. A
+ * member is handed to the backend of its current transport type. A backend
+ * nobody is on costs nothing: it connects only for the members handed to it.
+ */
+export function combineBackends(
+  scope: ObservableScope,
+  all: MediaBackend[],
+  publishType: string,
+): MediaBackend {
+  const byType = new Map(all.map((b) => [b.transportType, b]));
+  const publish = byType.get(publishType);
+  if (publish === undefined)
+    throw new MatrixRTCError(`No media backend for transport ${publishType}`);
+
+  const mediaFor = (
+    memberScope: ObservableScope,
+    member: RTCMember,
+  ): MemberMedia => {
+    // The publishing backend sees every member, whatever its transport, so
+    // that it can tell members from strangers when a backend is asked for our
+    // media by members on other transports. It connects nowhere for them
+    const publishMedia = publish.mediaFor(memberScope, member);
+    const type$ = memberScope.behavior(
+      member.transport$.pipe(map((transport) => transport?.type)),
+    );
+    const media$ = mapScoped(memberScope, type$, (s, type) =>
+      type === publish.transportType
+        ? publishMedia
+        : byType.get(type)?.mediaFor(s, member),
+    );
+    return {
+      tracks$: memberScope.behavior(
+        media$.pipe(switchMap((media) => media?.tracks$ ?? of(null))),
+      ),
+      encryptionError$: media$.pipe(
+        switchMap((media) => media?.encryptionError$ ?? NEVER),
+      ),
+    };
+  };
+
+  return {
+    transportType: publish.transportType,
+    prepareLocalTransport: async (transport) =>
+      publish.prepareLocalTransport(transport),
+    delegateDelayedLeave: async (delayId) =>
+      publish.delegateDelayedLeave(delayId),
+    local: publish.local,
+    mediaFor,
+    connections$: scope.behavior(
+      combineLatest(all.map((b) => b.connections$)).pipe(
+        map((lists) => lists.flat()),
+      ),
+    ),
+    setAudioOutputDeviceId: async (deviceId) => {
+      await Promise.all(
+        all.map(async (b) => b.setAudioOutputDeviceId(deviceId)),
+      );
+    },
+    sendData: async (topic, text) => publish.sendData(topic, text),
+    data$: merge(...all.map((b) => b.data$)),
+  };
+}
@@ -101,8 +101,11 @@ export class ConnectionMap {
 interface Props {
  scope: ObservableScope;
  connectionFactory: ConnectionFactory;
-  /** The transport we publish on; the one connection with the publisher role. */
-  localTransport: TransportLocator;
+  /**
+   * The transport we publish on; the one connection with the publisher role.
+   * Null for a backend that only subscribes, while another backend publishes.
+   */
+  localTransport: TransportLocator | null;
  /** Every transport some member is on, deduplicated against the local one. */
  remoteTransports$: Behavior<Epoch<TransportLocator[]>>;
  logger: Logger;
@@ -128,7 +131,11 @@ export function createConnectionMap$({
    scope.behavior(
      remoteTransports$.pipe(
        mapEpoch((remoteTransports) =>
-          removeDuplicateTransports([localTransport, ...remoteTransports]),
+          removeDuplicateTransports(
+            localTransport
+              ? [localTransport, ...remoteTransports]
+              : remoteTransports,
+          ),
        ),
        tap((transports) =>
          logger.debug("localAndRemoteTransports$ = ", transports),
@@ -136,7 +143,7 @@ export function createConnectionMap$({
      ),
    );

-  const localTransportKey = keyFor(localTransport);
+  const localTransportKey = localTransport && keyFor(localTransport);

  const connections$ = scope.behavior(
    localAndRemoteTransports$.pipe(
@@ -28,7 +28,7 @@ import {
  ownMemberMock,
  testScope,
 } from "../../utils/test";
-import { MediaConnectionState } from "../api";
+import { MediaConnectionState, type MediaBackendContext } from "../api";
 import * as auth from "./auth";
 import { type TransportLocator } from "./auth";
 import { type Connection, type ConnectionState } from "./Connection";
@@ -36,6 +36,7 @@ import * as connectionFactory from "./ConnectionFactory";
 import { toMediaConnectionState } from "./connectionState";
 import {
  createLivekitBackend,
+  createLivekitBackends,
  createTransportRegistry,
  supportsDelegation,
 } from "./LivekitBackend";
@@ -214,21 +215,19 @@ describe("createLivekitBackend", () => {
    fetchMock.reset();
  });

+  const context: MediaBackendContext = {
+    roomId: "!room:example.org",
+    slotId: "m.call#ROOM",
+    ownMembershipIdentity: ownMemberMock,
+    publish: [],
+    encryptionSystem: { kind: E2eeType.NONE },
+    mediaKeys$: NEVER,
+    timings: defaultSessionTimings,
+    logger,
+  };
+
  function backend(): ReturnType<typeof createLivekitBackend> {
-    return createLivekitBackend(
-      testScope(),
-      {
-        roomId: "!room:example.org",
-        slotId: "m.call#ROOM",
-        ownMembershipIdentity: ownMemberMock,
-        publish: [],
-        encryptionSystem: { kind: E2eeType.NONE },
-        mediaKeys$: NEVER,
-        timings: defaultSessionTimings,
-        logger,
-      },
-      { client },
-    );
+    return createLivekitBackend(testScope(), context, { client });
  }

  // Delegation goes through whichever service authenticates us as a
@@ -475,6 +474,70 @@ describe("createLivekitBackend", () => {
      ]);
    });

+    it("serves members on either type", async () => {
+      const b = await prepared();
+      const stableTransport: UnstableLivekitTransport = {
+        ...otherTransport,
+        type: "m.livekit",
+      };
+      const media = b.mediaFor(testScope(), bob(stableTransport, [legacyId]));
+      await flushPromises();
+      const remote = connections.find(
+        (c) => c.connection.role === "subscriber",
+      );
+      expect(remote?.connection.transport).toEqual(stableTransport);
+
+      remote!.participants$.next([
+        mockRemoteParticipant({
+          identity: legacyId,
+          trackPublications: new Map(),
+        }),
+      ]);
+      await flushPromises();
+      expect(media.tracks$.value).toEqual([]);
+    });
+
+    it("splits members by type across the two backends, on shared connections", async () => {
+      const [legacy, stable] = createLivekitBackends(testScope(), context, {
+        client,
+      });
+      expect([legacy.transportType, stable.transportType]).toEqual([
+        "livekit",
+        "m.livekit",
+      ]);
+      // Bob and Carl are on the same SFU, each under a type of his own
+      const carlId = "@carl:example.org:DEV000";
+      const bobOnLegacy = bob(otherTransport, [legacyId]);
+      const carlOnStable = {
+        ...bob({ ...otherTransport, type: "m.livekit" } as const, [carlId]),
+        rtcBackendIdentity: carlId,
+        memberId: carlId,
+      };
+      const media = {
+        bob: legacy.mediaFor(testScope(), bobOnLegacy),
+        carl: stable.mediaFor(testScope(), carlOnStable),
+        // Each backend leaves the members of the other type alone
+        carlOnLegacy: legacy.mediaFor(testScope(), carlOnStable),
+        bobOnStable: stable.mediaFor(testScope(), bobOnLegacy),
+      };
+      await flushPromises();
+      expect(connections).toHaveLength(1);
+
+      connections[0].participants$.next(
+        [legacyId, carlId].map((identity) =>
+          mockRemoteParticipant({ identity, trackPublications: new Map() }),
+        ),
+      );
+      await flushPromises();
+      expect(media.bob.tracks$.value).toEqual([]);
+      expect(media.carl.tracks$.value).toEqual([]);
+      expect(media.carlOnLegacy.tracks$.value).toBeNull();
+      expect(media.bobOnStable.tracks$.value).toBeNull();
+      // The shared connection is reported once, by the `livekit` backend
+      expect(legacy.connections$.value).toHaveLength(1);
+      expect(stable.connections$.value).toEqual([]);
+    });
+
    it("has no media for a member without a transport, and opens no connection", async () => {
      const b = await prepared();
      const media = b.mediaFor(testScope(), {
@@ -18,6 +18,7 @@ import {
  filter,
  firstValueFrom,
  map,
+  NEVER,
  type Observable,
  of,
  switchMap,
@@ -27,7 +28,7 @@ import { type ResolvedTransport, type RTCMember } from "../../api";
 import { type MediaQuality } from "../../config";
 import { MatrixRTCError } from "../../errors";
 import { type MemberMedia } from "../../media-api";
-import { type Behavior } from "../../reactive/Behavior";
+import { type Behavior, constant } from "../../reactive/Behavior";
 import {
  Epoch,
  type ObservableScope,
@@ -81,9 +82,44 @@ export interface LivekitBackendOptions {
 /**
 * MatrixRTC media over LiveKit SFUs: one room per transport, the local one
 * published on, every remote one subscribed to, each authenticated for its
- * role through the homeserver or the transport's JWT service.
+ * role through the homeserver or the transport's JWT service. Serves members
+ * on either LiveKit transport type, `livekit` and `m.livekit`.
 */
 export function createLivekitBackend(
+  scope: ObservableScope,
+  context: MediaBackendContext,
+  options: LivekitBackendOptions,
+): MediaBackend {
+  return livekitBackend(scope, context, options).backend;
+}
+
+/**
+ * The LiveKit backend once per transport type, for a combination of backends:
+ * `livekit` of the earlier versions of MSC4195 and `m.livekit` of the merged
+ * one. Each takes the members on its own type, and both share the connections,
+ * so members of either type on one SFU share a connection. The `livekit` one
+ * reports the shared connections and data.
+ */
+export function createLivekitBackends(
+  scope: ObservableScope,
+  context: MediaBackendContext,
+  options: LivekitBackendOptions,
+): MediaBackend[] {
+  const { backend, mediaForType } = livekitBackend(scope, context, options);
+  return [
+    { ...backend, mediaFor: mediaForType("livekit") },
+    {
+      ...backend,
+      transportType: "m.livekit",
+      mediaFor: mediaForType("m.livekit"),
+      connections$: constant([]),
+      setAudioOutputDeviceId: async () => Promise.resolve(),
+      data$: NEVER,
+    },
+  ];
+}
+
+function livekitBackend(
  scope: ObservableScope,
  {
    roomId,
@@ -96,7 +132,11 @@ export function createLivekitBackend(
    logger: parentLogger,
  }: MediaBackendContext,
  { client, mediaQuality }: LivekitBackendOptions,
-): MediaBackend {
+): {
+  backend: MediaBackend;
+  /** `mediaFor` for the members on transports of `type`, or of either type. */
+  mediaForType: (type?: string) => MediaBackend["mediaFor"];
+} {
  const logger = parentLogger.getChild("[LivekitBackend]");
  const keyProvider = createKeyProvider(encryptionSystem, mediaKeys$, logger);
  const transports = createTransportRegistry(scope);
@@ -115,9 +155,14 @@ export function createLivekitBackend(
  );

  // A new map, in a scope of its own, whenever the local transport changes:
-  // the publisher role sits on one connection, and which one is fixed per map
+  // the publisher role sits on one connection, and which one is fixed per map.
+  // Without a local transport the map only subscribes, for when another
+  // backend publishes; boxed so that `mapScoped` builds a map for null too
+  const boxedLocalTransport$ = scope.behavior(
+    localTransport$.pipe(map((localTransport) => ({ localTransport }))),
+  );
  const connectionMap$ = scope.behavior<Epoch<ConnectionMap>>(
-    mapScoped(scope, localTransport$, (mapScope, localTransport) =>
+    mapScoped(scope, boxedLocalTransport$, (mapScope, { localTransport }) =>
      createConnectionMap$({
        scope: mapScope,
        connectionFactory,
@@ -139,12 +184,15 @@ export function createLivekitBackend(
    logger,
  });

-  // Probed once: the answer cannot change within a session
-  const homeserverSupportsDelegation = supportsDelegation(
-    `${client.baseUrl}/_matrix/client/unstable/io.element.msc4195/rtc/livekit/delegate_delayed_leave`,
-    "homeserver",
-    logger,
-  );
+  // Probed once, when first needed: the answer cannot change within a
+  // session, and a backend that only subscribes never needs it
+  let homeserverProbe: Promise<boolean> | undefined;
+  const homeserverSupportsDelegation = async (): Promise<boolean> =>
+    (homeserverProbe ??= supportsDelegation(
+      `${client.baseUrl}/_matrix/client/unstable/io.element.msc4195/rtc/livekit/delegate_delayed_leave`,
+      "homeserver",
+      logger,
+    ));

  const ownServerName = serverNameOf(ownMembershipIdentity.userId);

@@ -176,7 +224,7 @@ export function createLivekitBackend(
              `transport ${flow.serviceUrl}`,
              logger,
            )
-          : homeserverSupportsDelegation;
+          : homeserverSupportsDelegation();
    }
    localTransport$.next(locator);
    // The membership is only sent once the publisher connection holds its
@@ -209,13 +257,15 @@ export function createLivekitBackend(
  const mediaFor = (
    memberScope: ObservableScope,
    member: RTCMember,
+    type?: string,
  ): MemberMedia => {
    const { backendIdentities } = member;
    const locator$ = memberScope.behavior(
      member.transport$.pipe(
        map((transport): TransportLocator | undefined => {
          const raw = transport?.raw;
-          return isUnstableLivekitTransport(raw)
+          return isUnstableLivekitTransport(raw) &&
+            (type === undefined || raw.type === type)
            ? {
                transport: raw,
                serverName: serverNameOfTransport(raw, member.userId),
@@ -253,6 +303,10 @@ export function createLivekitBackend(
    });
    return createLivekitMemberMedia(memberScope, source$, encryptionSystem);
  };
+  const mediaForType =
+    (type?: string): MediaBackend["mediaFor"] =>
+    (memberScope, member) =>
+      mediaFor(memberScope, member, type);

  const connections$ = scope.behavior<BackendConnection[]>(
    connectionMap$.pipe(
@@ -271,15 +325,18 @@ export function createLivekitBackend(
  });

  return {
-    transportType: "livekit",
-    prepareLocalTransport,
-    delegateDelayedLeave: delegateLeave,
-    local: localMedia_,
-    mediaFor,
-    connections$,
-    setAudioOutputDeviceId: localMedia_.setAudioOutputDeviceId,
-    sendData,
-    data$,
+    backend: {
+      transportType: "livekit",
+      prepareLocalTransport,
+      delegateDelayedLeave: delegateLeave,
+      local: localMedia_,
+      mediaFor: mediaForType(),
+      connections$,
+      setAudioOutputDeviceId: localMedia_.setAudioOutputDeviceId,
+      sendData,
+      data$,
+    },
+    mediaForType,
  };
 }

@@ -13,6 +13,7 @@ import {
 } from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";

+import { serverNameOfTransport } from "../../utils/serverName";
 import { isCsApiUnavailable } from "./auth";
 import * as csApi from "./auth/csApi";
 import { decodeSFUJWTPayload } from "./auth/types";
@@ -59,7 +60,7 @@ export async function findRemotePublishLivekitTransport({
      if (!isUnstableLivekitTransport(transport) || !("url" in transport))
        continue;
      const { url } = transport;
-      const serverName = membership.getTransportServerName(transport);
+      const serverName = serverNameOfTransport(transport, membership.userId);
      const key = JSON.stringify([url, serverName]);
      if (tried.has(key)) continue;
      tried.add(key);