fix(call): serve RTC transports to Element Call over MSC4515 — calls work again

Since the upstream v0.25.0 merge, Element Call in widget mode no longer
reads .well-known for its LiveKit transport; it asks the HOST via the
widget API (org.matrix.msc4515.get_rtc_transports, capability
org.matrix.msc4515.rtc_transports). cinny never granted the capability
nor implemented WidgetDriver.getRtcTransports(), so discovery returned
nothing and every join failed with "Call is not supported"
(MISSING_MATRIX_RTC_TRANSPORT) — the [LocalMembership] Multiple
Transport Errors line in the browser console.

- matrix-widget-api 1.17.0 -> 1.18.0 (adds MSC4515; also changes the
  sendDelayedEvent driver signature, adapted — parent delay ids were
  removed from the draft).
- Grant MSC4515RtcTransports in getCallCapabilities.
- CallWidgetDriver.getRtcTransports(): homeserver /rtc/transports
  (MSC4143) first, then the .well-known org.matrix.msc4143.rtc_foci list
  (what matrix.lotusguild.org advertises today). Unit-tested.

Server side needs no change: the well-known already carries the livekit
focus and the JWT service answers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

package-lock.json

@@ -54,7 +54,7 @@
         "linkify-react": "4.3.3",
         "linkifyjs": "4.3.3",
         "matrix-js-sdk": "41.7.0",
-        "matrix-widget-api": "1.17.0",
+        "matrix-widget-api": "1.18.0",
         "millify": "6.1.0",
         "pdfjs-dist": "6.3.289",
         "prismjs": "1.30.0",
@@ -10045,9 +10045,9 @@
       }
     },
     "node_modules/matrix-widget-api": {
-      "version": "1.17.0",
-      "resolved": "https://registry.npmjs.org/matrix-widget-api/-/matrix-widget-api-1.17.0.tgz",
-      "integrity": "sha512-5FHoo3iEP3Bdlv5jsYPWOqj+pGdFQNLWnJLiB0V7Ygne7bb+Gsj3ibyFyHWC6BVw+Z+tSW4ljHpO17I9TwStwQ==",
+      "version": "1.18.0",
+      "resolved": "https://registry.npmjs.org/matrix-widget-api/-/matrix-widget-api-1.18.0.tgz",
+      "integrity": "sha512-4T2f2koWmx05p1BLcT/9YGGGPSXpPT+PA4Oap/5fjhXsWPxMGJiGL97YpANMYLKsnE1sScYFeuyxIgdc1Qo+Ew==",
       "license": "Apache-2.0",
       "dependencies": {
         "@types/events": "^3.0.0",

package.json

@@ -1,7 +1,7 @@
 {
   "name": "lotus-chat",
   "version": "4.12.3-lotus",
-  "description": "Lotus Chat \u2014 Matrix client for Lotus Guild",
+  "description": "Lotus Chat — Matrix client for Lotus Guild",
   "main": "index.js",
   "type": "module",
   "engines": {
@@ -81,7 +81,7 @@
     "linkify-react": "4.3.3",
     "linkifyjs": "4.3.3",
     "matrix-js-sdk": "41.7.0",
-    "matrix-widget-api": "1.17.0",
+    "matrix-widget-api": "1.18.0",
     "millify": "6.1.0",
     "pdfjs-dist": "6.3.289",
     "prismjs": "1.30.0",

src/app/plugins/call/CallWidgetDriver.rtcTransports.test.ts

@@ -0,0 +1,58 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import type { MatrixClient } from 'matrix-js-sdk';
+import { CallWidgetDriver } from './CallWidgetDriver';
+
+// MSC4515: Element Call (widget mode) asks the host for RTC transports. Without a
+// working answer every call fails with MISSING_MATRIX_RTC_TRANSPORT.
+function fakeClient(opts: {
+  serverTransports?: unknown[] | Error;
+  wellKnown?: Record<string, unknown>;
+}): MatrixClient {
+  return {
+    getDeviceId: () => 'DEV',
+    getSafeUserId: () => '@me:example.org',
+    _unstable_getRTCTransports: async () => {
+      if (opts.serverTransports instanceof Error) throw opts.serverTransports;
+      return opts.serverTransports ?? [];
+    },
+    getClientWellKnown: () => opts.wellKnown,
+  } as unknown as MatrixClient;
+}
+
+const focus = { type: 'livekit', livekit_service_url: 'https://matrix.example.org' };
+
+test('getRtcTransports prefers the homeserver MSC4143 endpoint', async () => {
+  const driver = new CallWidgetDriver(fakeClient({ serverTransports: [focus] }), '!r:x');
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, [focus]);
+});
+
+test('getRtcTransports falls back to .well-known rtc_foci when the endpoint 404s', async () => {
+  const driver = new CallWidgetDriver(
+    fakeClient({
+      serverTransports: new Error('404'),
+      wellKnown: { 'org.matrix.msc4143.rtc_foci': [focus, { junk: true }, null] },
+    }),
+    '!r:x',
+  );
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, [focus]);
+});
+
+test('getRtcTransports returns an empty list when nothing is advertised', async () => {
+  const driver = new CallWidgetDriver(
+    fakeClient({ serverTransports: [], wellKnown: { 'm.homeserver': {} } }),
+    '!r:x',
+  );
+  const r = await driver.getRtcTransports();
+  assert.deepEqual(r.rtc_transports, []);
+});
+
+test('the MSC4515 capability is granted to the call widget', async () => {
+  const driver = new CallWidgetDriver(fakeClient({}), '!r:x');
+  const allowed = await driver.validateCapabilities(
+    new Set(['org.matrix.msc4515.rtc_transports', 'org.example.not_allowed']),
+  );
+  assert.deepEqual(Array.from(allowed), ['org.matrix.msc4515.rtc_transports']);
+});

src/app/plugins/call/CallWidgetDriver.ts

@@ -8,6 +8,8 @@ import {
   type IWidgetApiErrorResponseDataDetails,
   type ISearchUserDirectoryResult,
   type IGetMediaConfigResult,
+  type IRtcTransportsResult,
+  type IRtcTransport,
   OpenIDRequestState,
   SimpleObservable,
   IOpenIDUpdate,
@@ -79,29 +81,18 @@ export class CallWidgetDriver extends WidgetDriver {
     return { roomId, eventId: r.event_id };
   }
 
+  // matrix-widget-api >= 1.18 dropped the `parentDelayId` argument (MSC4140
+  // parent delays were removed from the spec draft); the signature is now
+  // (delay, eventType, content, stateKey?, roomId?).
   public async sendDelayedEvent(
-    delay: number | null,
-    parentDelayId: string | null,
+    delay: number,
     eventType: string,
-    content: IContent,
+    content: unknown,
     stateKey: string | null = null,
     targetRoomId: string | null = null,
   ): Promise<ISendDelayedEventDetails> {
     const roomId = targetRoomId || this.inRoomId;
-
-    let delayOpts;
-    if (delay !== null) {
-      delayOpts = {
-        delay,
-        ...(parentDelayId !== null && { parent_delay_id: parentDelayId }),
-      };
-    } else if (parentDelayId !== null) {
-      delayOpts = {
-        parent_delay_id: parentDelayId,
-      };
-    } else {
-      throw new Error('Must provide at least one of delay or parentDelayId');
-    }
+    const delayOpts = { delay };
 
     let r: SendDelayedEventResponse | null;
     if (stateKey !== null) {
@@ -295,6 +286,36 @@ export class CallWidgetDriver extends WidgetDriver {
     };
   }
 
+  /**
+   * MSC4515: the embedded Element Call asks us for the RTC transports because
+   * a widget can't make authenticated homeserver calls itself. Prefer the
+   * homeserver's MSC4143 `/rtc/transports` endpoint; fall back to the
+   * `org.matrix.msc4143.rtc_foci` list in the client .well-known (what
+   * Synapse deployments without MSC4143 advertise today, incl. ours).
+   * Returning an empty list makes EC report "Call is not supported".
+   */
+  public async getRtcTransports(): Promise<IRtcTransportsResult> {
+    try {
+      const fromServer = await this.mx._unstable_getRTCTransports();
+      if (Array.isArray(fromServer) && fromServer.length > 0) {
+        return { rtc_transports: fromServer as unknown as IRtcTransport[] };
+      }
+    } catch {
+      // 404 / M_UNRECOGNIZED on homeservers without MSC4143 — fall through.
+    }
+    const wellKnown = this.mx.getClientWellKnown() as
+      | { 'org.matrix.msc4143.rtc_foci'?: unknown }
+      | undefined;
+    const foci = wellKnown?.['org.matrix.msc4143.rtc_foci'];
+    const transports = Array.isArray(foci)
+      ? (foci.filter(
+          (f): f is IRtcTransport =>
+            !!f && typeof f === 'object' && typeof (f as { type?: unknown }).type === 'string',
+        ) as IRtcTransport[])
+      : [];
+    return { rtc_transports: transports };
+  }
+
   public async getMediaConfig(): Promise<IGetMediaConfigResult> {
     return this.mx.getMediaConfig();
   }

src/app/plugins/call/utils.ts

@@ -20,6 +20,10 @@ export function getCallCapabilities(
   capabilities.add(MatrixCapabilities.MSC3846TurnServers);
   capabilities.add(MatrixCapabilities.MSC4157SendDelayedEvent);
   capabilities.add(MatrixCapabilities.MSC4157UpdateDelayedEvent);
+  // MSC4515: Element Call >= 0.22 in widget mode discovers its LiveKit
+  // transport by asking the host (it no longer reads .well-known itself), so
+  // without this capability every call fails with MISSING_MATRIX_RTC_TRANSPORT.
+  capabilities.add(MatrixCapabilities.MSC4515RtcTransports);
   capabilities.add(`org.matrix.msc2762.timeline:${roomId}`);
   capabilities.add(`org.matrix.msc2762.state:${roomId}`);