diff --git a/backend/app-service-othersite.yaml b/backend/app-service-othersite.yaml
new file mode 100644
index 000000000..ef2a7ada7
--- /dev/null
+++ b/backend/app-service-othersite.yaml
@@ -0,0 +1,17 @@
+id: "lk-jwt-service-othersite"
+as_token: "ec_dev_as_token_othersite_0123456789ab"
+hs_token: "ec_dev_hs_token_othersite_0123456789ab"
+sender_localpart: "_lk_jwt_service"
+namespaces:
+  users:
+    - exclusive: false
+      regex: "@.*:homeserver-1" # Cover all local users
+# Required for Synapse to call POST /_matrix/app/v1/ping on this service.
+url: "http://auth-server-1:16080"
+# Enable membership look-ups via /is_joined.
+io.element.msc4502.scopes:
+  ["urn:matrix:client:io.element.msc4502:rooms:is_joined"]
+# Route /_matrix/client/(v*|unstable/*)/rtc/livekit/* C-S and S-S requests to this
+# service.
+io.element.msc4512.proxy_prefix: "rtc/livekit"
+io.element.msc4512.proxy_url: "http://auth-server-1:16080"
diff --git a/backend/app-service.yaml b/backend/app-service.yaml
new file mode 100644
index 000000000..b480df64e
--- /dev/null
+++ b/backend/app-service.yaml
@@ -0,0 +1,17 @@
+id: "lk-jwt-service"
+as_token: "ec_dev_as_token_main_0123456789ab"
+hs_token: "ec_dev_hs_token_main_0123456789ab"
+sender_localpart: "_lk_jwt_service"
+namespaces:
+  users:
+    - exclusive: false
+      regex: "@.*:homeserver" # Cover all local users
+# Required for Synapse to call POST /_matrix/app/v1/ping on this service.
+url: "http://auth-server:6080"
+# Enable membership look-ups via /is_joined.
+io.element.msc4502.scopes:
+  ["urn:matrix:client:io.element.msc4502:rooms:is_joined"]
+# Route /_matrix/client/(v*|unstable/*)/rtc/livekit/* C-S and S-S requests to this
+# service.
+io.element.msc4512.proxy_prefix: "rtc/livekit"
+io.element.msc4512.proxy_url: "http://auth-server:6080"
diff --git a/backend/dev_homeserver-othersite.yaml b/backend/dev_homeserver-othersite.yaml
index 3f4c18413..18f986505 100644
--- a/backend/dev_homeserver-othersite.yaml
+++ b/backend/dev_homeserver-othersite.yaml
@@ -42,6 +42,13 @@ experimental_features:
   msc4354_enabled: true
   # MatrixRTC
   msc4143_enabled: true
+  # Enable membership look-up via /is_joined.
+  msc4502_enabled: true
+  # Enable C-S & S-S request proxying for application services.
+  msc4512_enabled: true
+
+app_service_config_files:
+  - /data/cfg/app-service.yaml
 
 # The maximum allowed duration by which sent events can be delayed, as
 # per MSC4140. Must be a positive value if set.  Defaults to no
diff --git a/backend/dev_homeserver.yaml b/backend/dev_homeserver.yaml
index e38e91e5f..e99a50934 100644
--- a/backend/dev_homeserver.yaml
+++ b/backend/dev_homeserver.yaml
@@ -42,6 +42,13 @@ experimental_features:
   msc4354_enabled: true
   # MatrixRTC
   msc4143_enabled: true
+  # Enable membership look-up via /is_joined.
+  msc4502_enabled: true
+  # Enable C-S & S-S request proxying for application services.
+  msc4512_enabled: true
+
+app_service_config_files:
+  - /data/cfg/app-service.yaml
 
 # The maximum allowed duration by which sent events can be delayed, as
 # per MSC4140. Must be a positive value if set.  Defaults to no
diff --git a/backend/playwright_homeserver-othersite.yaml b/backend/playwright_homeserver-othersite.yaml
index 5bce4c1d5..a647b74a0 100644
--- a/backend/playwright_homeserver-othersite.yaml
+++ b/backend/playwright_homeserver-othersite.yaml
@@ -42,6 +42,13 @@ experimental_features:
   msc4354_enabled: true
   # MatrixRTC
   msc4143_enabled: true
+  # Enable membership look-up via /is_joined.
+  msc4502_enabled: true
+  # Enable C-S & S-S request proxying for application services.
+  msc4512_enabled: true
+
+app_service_config_files:
+  - /data/cfg/app-service.yaml
 
 # The maximum allowed duration by which sent events can be delayed, as
 # per MSC4140. Must be a positive value if set.  Defaults to no
diff --git a/backend/playwright_homeserver.yaml b/backend/playwright_homeserver.yaml
index d0439e42c..8497c017d 100644
--- a/backend/playwright_homeserver.yaml
+++ b/backend/playwright_homeserver.yaml
@@ -42,6 +42,13 @@ experimental_features:
   msc4354_enabled: true
   # MatrixRTC
   msc4143_enabled: true
+  # Enable membership look-up via /is_joined.
+  msc4502_enabled: true
+  # Enable C-S & S-S request proxying for application services.
+  msc4512_enabled: true
+
+app_service_config_files:
+  - /data/cfg/app-service.yaml
 
 # The maximum allowed duration by which sent events can be delayed, as
 # per MSC4140. Must be a positive value if set.  Defaults to no
diff --git a/docker-compose-dev.yml b/docker-compose-dev.yml
index 3dc86f2f1..3de89ec38 100644
--- a/docker-compose-dev.yml
+++ b/docker-compose-dev.yml
@@ -3,7 +3,7 @@ networks:
 
 services:
   auth-service:
-    image: ghcr.io/element-hq/lk-jwt-service:0.4.4
+    image: ghcr.io/element-hq/lk-jwt-service:0.7.0
     pull_policy: always
     hostname: auth-server
     environment:
@@ -15,6 +15,14 @@ services:
       # a self-signed certificate
       - LIVEKIT_INSECURE_SKIP_VERIFY_TLS=YES_I_KNOW_WHAT_I_AM_DOING
       - LIVEKIT_FULL_ACCESS_HOMESERVERS=*
+      # Registers this instance as an application service on the main homeserver
+      - LIVEKIT_AS_REGISTRATION_FILE=/etc/lk-jwt-service/app-service.yaml
+      - LIVEKIT_HS_SERVER_NAME=synapse.m.localhost
+      # Neither homeserver serves /.well-known/matrix/client, so the C-S API
+      # location has to be given explicitly.
+      - LIVEKIT_CS_API_URL_OVERRIDES=synapse.m.localhost=http://homeserver:8008,synapse.othersite.m.localhost=http://homeserver-1:18008
+    volumes:
+      - ./backend/app-service.yaml:/etc/lk-jwt-service/app-service.yaml:Z
     deploy:
       restart_policy:
         condition: on-failure
@@ -25,7 +33,7 @@ services:
       - ecbackend
 
   auth-service-1:
-    image: ghcr.io/element-hq/lk-jwt-service:0.4.4
+    image: ghcr.io/element-hq/lk-jwt-service:0.7.0
     pull_policy: always
     hostname: auth-server-1
     environment:
@@ -37,6 +45,14 @@ services:
       # a self-signed certificate
       - LIVEKIT_INSECURE_SKIP_VERIFY_TLS=YES_I_KNOW_WHAT_I_AM_DOING
       - LIVEKIT_FULL_ACCESS_HOMESERVERS=*
+      # Registers this instance as an application service on the other homeserver.
+      - LIVEKIT_AS_REGISTRATION_FILE=/etc/lk-jwt-service/app-service.yaml
+      - LIVEKIT_HS_SERVER_NAME=synapse.othersite.m.localhost
+      # Neither homeserver serves /.well-known/matrix/client, so the C-S API
+      # location has to be given explicitly.
+      - LIVEKIT_CS_API_URL_OVERRIDES=synapse.m.localhost=http://homeserver:8008,synapse.othersite.m.localhost=http://homeserver-1:18008
+    volumes:
+      - ./backend/app-service-othersite.yaml:/etc/lk-jwt-service/app-service.yaml:Z
     deploy:
       restart_policy:
         condition: on-failure
@@ -94,7 +110,10 @@ services:
 
   synapse:
     hostname: homeserver
-    image: ghcr.io/element-hq/synapse:latest
+    # develop (not latest) is required for application service C-S/S-S proxying
+    # (MSC4512) and membership look-ups (MSC4502), which are not in a stable
+    # release yet.
+    image: ghcr.io/element-hq/synapse:develop
     pull_policy: always
     environment:
       - SYNAPSE_CONFIG_PATH=/data/cfg/homeserver.yaml
@@ -107,12 +126,16 @@ services:
     volumes:
       - ./backend/synapse_tmp:/data:Z
       - ./backend/dev_homeserver.yaml:/data/cfg/homeserver.yaml:Z
+      - ./backend/app-service.yaml:/data/cfg/app-service.yaml:Z
     networks:
       - ecbackend
 
   synapse-1:
     hostname: homeserver-1
-    image: ghcr.io/element-hq/synapse:latest
+    # develop (not latest) is required for application service C-S/S-S proxying
+    # (MSC4512) and membership look-ups (MSC4502), which are not in a stable
+    # release yet.
+    image: ghcr.io/element-hq/synapse:develop
     pull_policy: always
     environment:
       - SYNAPSE_CONFIG_PATH=/data/cfg/homeserver.yaml
@@ -125,6 +148,7 @@ services:
     volumes:
       - ./backend/synapse_tmp_othersite:/data:Z
       - ./backend/dev_homeserver-othersite.yaml:/data/cfg/homeserver.yaml:Z
+      - ./backend/app-service-othersite.yaml:/data/cfg/app-service.yaml:Z
     networks:
       - ecbackend
 
diff --git a/docker-compose-playwright.yml b/docker-compose-playwright.yml
index 0c7f5a89c..424f1c650 100644
--- a/docker-compose-playwright.yml
+++ b/docker-compose-playwright.yml
@@ -3,12 +3,18 @@
 services:
   synapse:
     # Pin to a SHA so that upstream cannot break our tests. Renovate handles regular updates.
-    image: ghcr.io/element-hq/synapse:latest@sha256:da325af40104051923899e5f5a2f1d537e6e3ccf2f0f38285689ae9bbfdb190a
+    # develop (not latest) is required for application service C-S/S-S proxying
+    # (MSC4512) and membership look-ups (MSC4502), which are not in a stable
+    # release yet.
+    image: ghcr.io/element-hq/synapse:develop@sha256:337921fd22be310d453344b2265f6c9942315446595df3cf81866f71ca2055a7
     volumes:
       - ./backend/playwright_homeserver.yaml:/data/cfg/homeserver.yaml:Z
   synapse-1:
     # Pin to a SHA so that upstream cannot break our tests. Renovate handles regular updates.
-    image: ghcr.io/element-hq/synapse:latest@sha256:da325af40104051923899e5f5a2f1d537e6e3ccf2f0f38285689ae9bbfdb190a
+    # develop (not latest) is required for application service C-S/S-S proxying
+    # (MSC4512) and membership look-ups (MSC4502), which are not in a stable
+    # release yet.
+    image: ghcr.io/element-hq/synapse:develop@sha256:337921fd22be310d453344b2265f6c9942315446595df3cf81866f71ca2055a7
     volumes:
       - ./backend/playwright_homeserver-othersite.yaml:/data/cfg/homeserver.yaml:Z
   element-web:
diff --git a/playwright/reconnect.spec.ts b/playwright/reconnect.spec.ts
index bd4dd1996..81e70b9cc 100644
--- a/playwright/reconnect.spec.ts
+++ b/playwright/reconnect.spec.ts
@@ -7,6 +7,8 @@ Please see LICENSE in the repository root for full details.
 
 import { expect, test } from "@playwright/test";
 
+import { SpaHelpers } from "./spa-helpers";
+
 // Skip test for Firefox, due to page.keyboard.press("Tab") not reliable on headless mode
 test.skip(
   ({ browserName }) => browserName === "firefox",
@@ -17,6 +19,12 @@ test("can only interact with header and footer while reconnecting", async ({
   page,
 }) => {
   await page.goto("/");
+
+  // The reconnecting state is entered via the probablyLeft timer, which
+  // mirrors the delayed leave event's timeout. With delegation that timeout
+  // is one hour, putting it out of reach of the clock fast-forward below.
+  // Keep the leave client-managed so its short timeout applies.
+  await SpaHelpers.disableLeaveDelegation(page);
   await page.getByTestId("home_callName").click();
   await page.getByTestId("home_callName").fill("Test call");
   await page.getByTestId("home_displayName").click();
diff --git a/playwright/spa-call-sticky.spec.ts b/playwright/spa-call-sticky.spec.ts
index 1dbda735b..b45399da2 100644
--- a/playwright/spa-call-sticky.spec.ts
+++ b/playwright/spa-call-sticky.spec.ts
@@ -19,6 +19,7 @@ async function setupTwoUserSpaCall(
   browser: Browser,
   page: Page,
   browserName: string,
+  opts: { disableGuestLeaveDelegation?: boolean } = {},
 ): Promise<{ guestPage: Page }> {
   test.skip(
     browserName === "firefox",
@@ -52,6 +53,9 @@ async function setupTwoUserSpaCall(
 
   await guestPage.goto("/");
 
+  if (opts.disableGuestLeaveDelegation)
+    await SpaHelpers.disableLeaveDelegation(guestPage);
+
   let pevaraHasSentStickyEvent = false;
 
   const pevaraResolver = Promise.withResolvers<void>();
@@ -102,7 +106,13 @@ test("One to One rejoin after improper leave does not crash EC", async ({
   page,
   browserName,
 }) => {
-  const { guestPage } = await setupTwoUserSpaCall(browser, page, browserName);
+  // With delegation, the backend sends the guest's delayed leave event within
+  // moments of the improper leave, so the stale membership this test needs
+  // would be cleaned up before the rejoin. Keep the guest's leave
+  // client-managed so the stale membership lingers.
+  const { guestPage } = await setupTwoUserSpaCall(browser, page, browserName, {
+    disableGuestLeaveDelegation: true,
+  });
 
   await SpaHelpers.expectVideoTilesCount(page, 2);
   await SpaHelpers.expectVideoTilesCount(guestPage, 2);
diff --git a/playwright/spa-helpers.ts b/playwright/spa-helpers.ts
index 5f99e32d3..71150b8ba 100644
--- a/playwright/spa-helpers.ts
+++ b/playwright/spa-helpers.ts
@@ -111,6 +111,24 @@ async function setRtcModeFromSettings(
   await page.getByTestId("modal_close").click();
 }
 
+/**
+ * Makes the delayed-leave delegation support probes fail so that the client
+ * manages its delayed leave event itself instead of delegating it to the
+ * backend.
+ *
+ * Must be installed before the page joins a call.
+ */
+async function disableLeaveDelegation(page: Page): Promise<void> {
+  // Covers both the transport probe (<livekit_service_url>/delegate_delayed_leave)
+  // and the homeserver probe (MSC4195, .../rtc/livekit/delegate_delayed_leave).
+  await page.route("**/delegate_delayed_leave", async (route) =>
+    route.fulfill({
+      status: 404,
+      headers: { "Access-Control-Allow-Origin": "*" },
+    }),
+  );
+}
+
 /**
  * Expect a certain number of video tiles to be present and visible.
  */
@@ -133,5 +151,6 @@ export const SpaHelpers = {
   createCall,
   getCallInviteLink,
   joinCallFromInviteLink,
+  disableLeaveDelegation,
   expectVideoTilesCount,
 };