diff --git a/.github/workflows/manual-deploy.yml b/.github/workflows/manual-deploy.yml
index 01bac788ad..ac312f3317 100644
--- a/.github/workflows/manual-deploy.yml
+++ b/.github/workflows/manual-deploy.yml
@@ -335,6 +335,35 @@ jobs:
             exit 1
           fi
 
+  recycle-prerender:
+    name: Recycle prerender after host is live
+    # The prerender fleet deploys before the realm server (the manager,
+    # worker, and realm server all depend on it being up for boot indexing),
+    # so its tabs warm against the OLD host shell the realm server was still
+    # serving at that point. Once the realm server is up serving the new
+    # shell, re-deploy the prerender service so its tabs re-warm against it.
+    # The reusable deploy passes force-new-deployment, so this recycles fresh
+    # tasks even though the image is unchanged.
+    #
+    # Gate on `deploy-realm-server` (wait-for-service-stability: true → the
+    # new realm server is up and serving the new shell), NOT on
+    # `post-deploy-realm-server`: that job is a separate post-deploy hook
+    # (it POSTs the realm server's `/_post-deployment` endpoint) and can fail
+    # independently of the recycle's only real precondition — the new shell
+    # being served. Coupling to it would skip this recycle on an unrelated
+    # hook failure.
+    needs: [build-prerender, deploy-realm-server]
+    uses: cardstack/gh-actions/.github/workflows/ecs-deploy.yml@main
+    secrets: inherit
+    with:
+      container-name: "boxel-prerender-server"
+      environment: ${{ inputs.environment }}
+      cluster: ${{ inputs.environment }}
+      service-name: "boxel-prerender-server-${{ inputs.environment }}"
+      image: ${{ needs.build-prerender.outputs.image }}
+      timeout-minutes: 10
+      wait-for-service-stability: true
+
   apply-observability:
     # Push the observability/ package's dashboards/folders/data sources/alerts
     # into the production self-host Grafana as part of the deploy. The
@@ -375,6 +404,7 @@ jobs:
         post-deploy-worker,
         deploy-realm-server,
         post-deploy-realm-server,
+        recycle-prerender,
         apply-observability,
       ]
     if: github.event_name == 'workflow_dispatch' && always()
diff --git a/packages/realm-server/handlers/handle-post-deployment.ts b/packages/realm-server/handlers/handle-post-deployment.ts
index 7be38ee876..a139307f69 100644
--- a/packages/realm-server/handlers/handle-post-deployment.ts
+++ b/packages/realm-server/handlers/handle-post-deployment.ts
@@ -20,6 +20,7 @@ export default function handlePostDeployment({
   definitionLookup,
   queue,
   realmServerSecretSeed,
+  reportHostShell,
 }: CreateRoutesArgs): (ctxt: Koa.Context, next: Koa.Next) => Promise<void> {
   return async function (ctxt: Koa.Context, _next: Koa.Next) {
     if (ctxt.request.headers.authorization !== realmServerSecretSeed) {
@@ -27,6 +28,14 @@ export default function handlePostDeployment({
       return;
     }
 
+    // This hook fires after the deploy reports the service stable, so the new
+    // host shell is live and load-balancer-routable. Re-report the host-shell
+    // token to the prerender manager from here so the fleet's recycle signal
+    // reflects the now-serving shell, closing the rolling-deploy window where
+    // the boot-time report could precede the new task receiving traffic.
+    // Fire-and-forget — best-effort, must not affect the hook's response.
+    void reportHostShell?.();
+
     await definitionLookup.clearAllDefinitions();
 
     let boxelUiChangeCheckerResult =
diff --git a/packages/realm-server/main.ts b/packages/realm-server/main.ts
index b8db76c556..0a95099fde 100644
--- a/packages/realm-server/main.ts
+++ b/packages/realm-server/main.ts
@@ -363,6 +363,45 @@ const smokeTestHostApp = async () => {
   throw lastError ?? new Error('host app smoke test timed out');
 };
 
+// Report the host-shell token this realm server is serving to the prerender
+// manager. The manager echoes it on heartbeat responses so prerender servers
+// recycle their browsers when it changes — i.e. when a deploy ships a new
+// host bundle. Runs at boot (after the smoke test confirmed the shell is
+// reachable): a deploy restarts this process, so a new bundle is reported
+// here and picked up by the prerender fleet. Best-effort — a missing or
+// unreachable manager must never block realm-server boot.
+const reportHostShellToManager = async () => {
+  try {
+    let html = await getIndexHTML();
+    let { createHash } = await import('crypto');
+    let hash = createHash('md5').update(html).digest('hex').slice(0, 8);
+    // Report to the manager URL the realm server already uses (prerendererUrl);
+    // PRERENDER_MANAGER_URL is only set on the prerender-server tasks.
+    let managerURL = prerendererUrl.replace(/\/$/, '');
+    let response = await fetch(`${managerURL}/host-shell`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/vnd.api+json',
+        Accept: 'application/vnd.api+json',
+      },
+      body: JSON.stringify({ data: { attributes: { hash } } }),
+    });
+    if (response.ok) {
+      console.log(
+        `Reported host shell token ${hash} to prerender manager at ${managerURL}`,
+      );
+    } else {
+      console.warn(
+        `Prerender manager rejected host shell report: ${response.status}`,
+      );
+    }
+  } catch (e: any) {
+    console.warn(
+      `Failed to report host shell token to prerender manager: ${e?.message ?? e}`,
+    );
+  }
+};
+
 (async () => {
   try {
     await smokeTestHostApp();
@@ -594,6 +633,7 @@ const smokeTestHostApp = async () => {
       ? getRegistrationSecret
       : undefined,
     prerenderer,
+    reportHostShell: reportHostShellToManager,
   });
 
   let httpServer = server.listen(port);
@@ -735,6 +775,16 @@ const smokeTestHostApp = async () => {
   // wait for first-request mount via reconciler.lookupOrMount().
   await server.start();
 
+  // Now that the HTTP listener is accepting traffic and serving the new host
+  // shell, tell the prerender manager which shell we're serving so the fleet
+  // recycles after a host redeploy. Reporting earlier (before the listener is
+  // live) races a rolling deploy: the manager could echo the new token while
+  // the load balancer still routes to the old task, so a prerender would
+  // recycle against the old shell, record the new token, and stop retrying.
+  // The post-deployment hook reports again once the service is fully stable.
+  // Fire-and-forget — a missing/unreachable manager must never affect serving.
+  void reportHostShellToManager();
+
   // Begin the reconciler's background poll loop (LISTEN realm_registry +
   // 30s safety poll). It picks up changes from peer instances (publish,
   // unpublish, delete) and reconciles them into local mounted state.
diff --git a/packages/realm-server/prerender/manager-app.ts b/packages/realm-server/prerender/manager-app.ts
index e3c2f41dcc..313b0f7ab4 100644
--- a/packages/realm-server/prerender/manager-app.ts
+++ b/packages/realm-server/prerender/manager-app.ts
@@ -8,6 +8,7 @@ import {
 import { format } from 'date-fns';
 import {
   PRERENDER_JOB_ID_HEADER,
+  PRERENDER_HOST_SHELL_HASH_HEADER,
   PRERENDER_REQUEST_ID_HEADER,
   PRERENDER_SERVER_DRAINING_STATUS_CODE,
   PRERENDER_SERVER_STATUS_DRAINING,
@@ -64,6 +65,11 @@ type Registry = {
   servers: Map<string, ServerInfo>; // key: serverUrl
   affinities: Map<string, string[]>; // affinityKey (<type>:<value>) -> assigned serverUrls (deque semantics)
   lastAccessByAffinity: Map<string, number>;
+  // Latest host-shell token reported by a realm server (POST /host-shell).
+  // Echoed to prerender servers on every heartbeat response so they recycle
+  // when it changes (host redeployed). Undefined until first reported; reset
+  // on manager restart, re-learned from the next realm-server boot report.
+  hostShellHash?: string;
 };
 
 const log = logger('prerender-manager');
@@ -507,6 +513,11 @@ export function buildPrerenderManagerApp(options?: {
         warmedAffinities,
         affinityVacancy,
       });
+      // Echo the current host-shell token so the server can recycle its
+      // browser when the host is redeployed (see PRERENDER_HOST_SHELL_HASH_HEADER).
+      if (registry.hostShellHash) {
+        ctxt.set(PRERENDER_HOST_SHELL_HASH_HEADER, registry.hostShellHash);
+      }
       ctxt.status = 204;
       ctxt.set('X-Prerender-Server-Id', url);
     } catch (e) {
@@ -516,6 +527,56 @@ export function buildPrerenderManagerApp(options?: {
     }
   });
 
+  // The realm server reports the host-shell token it is currently serving
+  // (POST at boot, after it has fetched the new shell). A change means the
+  // host was redeployed; prerender servers pick it up on their next heartbeat
+  // and recycle. Storing the latest token (rather than counting) keeps this
+  // robust across the manager's own restart in the deploy train — the next
+  // realm-server boot re-reports the current token.
+  router.post('/host-shell', async (ctxt) => {
+    try {
+      let req = await fetchRequestFromContext(ctxt);
+      let raw = await req.text();
+      let requestBody: any = {};
+      if (raw) {
+        try {
+          requestBody = JSON.parse(raw);
+        } catch (e) {
+          log.debug('Invalid JSON body on /host-shell; treating as empty:', e);
+        }
+      }
+      let hash = requestBody?.data?.attributes?.hash;
+      if (typeof hash !== 'string' || hash.trim().length === 0) {
+        ctxt.status = 400;
+        ctxt.body = { errors: [{ status: 400, message: 'hash is required' }] };
+        return;
+      }
+      // Normalize and bound the token before storing it: it is echoed into a
+      // response header on every heartbeat, so a stray-whitespace variant would
+      // spuriously read as a change, and an oversized value would bloat every
+      // heartbeat response. The real token is a short hex digest, so anything
+      // longer is malformed — reject rather than silently truncate (a truncated
+      // token would never match and would recycle forever).
+      hash = hash.trim();
+      if (hash.length > 64) {
+        ctxt.status = 400;
+        ctxt.body = { errors: [{ status: 400, message: 'hash too long' }] };
+        return;
+      }
+      if (registry.hostShellHash !== hash) {
+        log.info(
+          `host shell token changed (${registry.hostShellHash ?? 'none'} -> ${hash}); prerender servers will recycle on next heartbeat`,
+        );
+        registry.hostShellHash = hash;
+      }
+      ctxt.status = 204;
+    } catch (e) {
+      log.error('Error in /host-shell:', e);
+      ctxt.status = 500;
+      ctxt.body = { errors: [{ status: 500, message: 'host-shell error' }] };
+    }
+  });
+
   // maintenance: clear affinity assignments and capacity tracking
   router.post('/prerender-maintenance/reset', async (ctxt) => {
     for (let [, info] of registry.servers) {
diff --git a/packages/realm-server/prerender/prerender-app.ts b/packages/realm-server/prerender/prerender-app.ts
index 2445d268e1..ec85d5492a 100644
--- a/packages/realm-server/prerender/prerender-app.ts
+++ b/packages/realm-server/prerender/prerender-app.ts
@@ -22,6 +22,7 @@ import { Prerenderer } from './index.ts';
 import type { Timings } from './render-runner.ts';
 import { resolvePrerenderManagerURL } from './config.ts';
 import {
+  PRERENDER_HOST_SHELL_HASH_HEADER,
   PRERENDER_JOB_ID_HEADER,
   PRERENDER_REQUEST_ID_HEADER,
   PRERENDER_SERVER_DRAINING_STATUS_CODE,
@@ -60,6 +61,30 @@ export function decorateRenderErrorDiagnostics(
   };
 }
 
+// Pure decision for the host-shell recycle reconcile. Given the token the
+// manager last reported (`reported`, null when it doesn't know one yet) and
+// the token this server warmed against (`warmed`, undefined before the first
+// report), decide whether to recycle and what the baseline token becomes:
+//   - no report          → keep the current baseline, don't recycle
+//   - first report seen   → adopt it as the baseline, don't recycle (we just
+//                           warmed against whatever shell is current)
+//   - same as baseline    → no-op
+//   - differs from baseline → recycle and advance the baseline
+// Exported for unit testing; the live caller layers the draining / in-flight
+// guards and the async recycle on top.
+export function decideHostShellRecycle(
+  reported: string | null,
+  warmed: string | undefined,
+): { recycle: boolean; nextWarmed: string | undefined } {
+  if (!reported) {
+    return { recycle: false, nextWarmed: warmed };
+  }
+  if (warmed === undefined || warmed === reported) {
+    return { recycle: false, nextWarmed: reported };
+  }
+  return { recycle: true, nextWarmed: reported };
+}
+
 export function buildPrerenderApp(options: {
   serverURL: string;
   maxPages?: number;
@@ -1021,6 +1046,13 @@ export function createPrerenderHttpServer(options?: {
   let drainingResolved = false;
   let drainingDeferred = new Deferred<void>();
   let heartbeatTimer: NodeJS.Timeout | undefined;
+  // Host-shell token the standbys were last warmed against, learned from the
+  // manager's heartbeat responses (PRERENDER_HOST_SHELL_HASH_HEADER). When the
+  // manager reports a different token — the host was redeployed and the realm
+  // server is now serving a new shell — the browser is recycled so pages
+  // reload it. Undefined until the first heartbeat that carries a token.
+  let warmedHostShellHash: string | undefined;
+  let recyclingForHostChange = false;
   let isClosing = false;
   let fatalExitOnUncaught = options?.fatalExitOnUncaught ?? true;
   let serverURL = resolvePrerenderServerURL(options?.port);
@@ -1079,7 +1111,7 @@ export function createPrerenderHttpServer(options?: {
       log.debug(
         `POST heartbeat to ${managerURL}/prerender-servers with body:\n${JSON.stringify(body, null, 2)}`,
       );
-      await fetch(`${managerURL}/prerender-servers`, {
+      let response = await fetch(`${managerURL}/prerender-servers`, {
         method: 'POST',
         headers: {
           'Content-Type': 'application/vnd.api+json',
@@ -1088,13 +1120,54 @@ export function createPrerenderHttpServer(options?: {
         body: JSON.stringify(body),
       }).catch((e) => {
         log.debug('Prerender manager heartbeat request failed:', e);
+        return undefined;
       });
+      if (response) {
+        reconcileHostShell(
+          response.headers.get(PRERENDER_HOST_SHELL_HASH_HEADER),
+        );
+      }
     } catch (e) {
       // best-effort, but log for visibility
       log.debug('Error while attempting heartbeat with prerender manager:', e);
     }
   }
 
+  // Compare the manager's current host-shell token against the one we warmed
+  // against. A change means the host was redeployed, so recycle the browser
+  // (fire-and-forget; the heartbeat itself must not block on the restart).
+  function reconcileHostShell(hash: string | null) {
+    if (draining || recyclingForHostChange) {
+      return;
+    }
+    let { recycle, nextWarmed } = decideHostShellRecycle(
+      hash,
+      warmedHostShellHash,
+    );
+    if (!recycle) {
+      // Either nothing reported, or we adopted a baseline / matched — record
+      // the (possibly newly-adopted) token and we're done.
+      warmedHostShellHash = nextWarmed;
+      return;
+    }
+    recyclingForHostChange = true;
+    log.info(
+      `host shell changed (${warmedHostShellHash} -> ${hash}); recycling prerender browser`,
+    );
+    void prerenderer
+      .recycle()
+      .then(() => {
+        warmedHostShellHash = nextWarmed;
+      })
+      .catch((e) => {
+        // Leave warmedHostShellHash unchanged so the next heartbeat retries.
+        log.error('Failed to recycle prerender browser on host change:', e);
+      })
+      .finally(() => {
+        recyclingForHostChange = false;
+      });
+  }
+
   function startHeartbeatLoop() {
     if (heartbeatTimer) return;
     void sendHeartbeat();
diff --git a/packages/realm-server/prerender/prerender-constants.ts b/packages/realm-server/prerender/prerender-constants.ts
index 7ec8005b19..31afbd97e9 100644
--- a/packages/realm-server/prerender/prerender-constants.ts
+++ b/packages/realm-server/prerender/prerender-constants.ts
@@ -2,6 +2,16 @@ export const PRERENDER_SERVER_STATUS_HEADER = 'X-Boxel-Prerender-Server-Status';
 export const PRERENDER_SERVER_STATUS_DRAINING = 'draining';
 export const PRERENDER_SERVER_DRAINING_STATUS_CODE = 410;
 
+// Opaque token for the current host shell (the realm server's rewritten
+// index.html). The realm server reports it to the manager at boot
+// (POST /host-shell); the manager echoes the latest value on every
+// heartbeat response via this header, and a prerender server recycles its
+// browser when the value differs from the shell it last warmed against —
+// i.e. the host was redeployed. The token only has to change when the host
+// bundle changes; prerender servers treat it opaquely.
+export const PRERENDER_HOST_SHELL_HASH_HEADER =
+  'X-Boxel-Prerender-Host-Shell-Hash';
+
 // CS-10872: correlates one client-initiated prerender call across
 // remote-prerenderer → manager → prerender-server. The client assigns
 // the ID on the first request; the manager and prerender-server echo
diff --git a/packages/realm-server/prerender/prerenderer.ts b/packages/realm-server/prerender/prerenderer.ts
index 65059b09d0..2481b6974a 100644
--- a/packages/realm-server/prerender/prerenderer.ts
+++ b/packages/realm-server/prerender/prerenderer.ts
@@ -209,6 +209,17 @@ export class Prerenderer {
     await this.#pagePool.warmStandbys();
   }
 
+  // Recycle the whole browser to pick up a redeployed host. Reuses the
+  // failure-recovery restart path (closeAll → restart Chrome → re-warm
+  // standbys); the full browser restart also clears Chrome's HTTP cache, so
+  // re-warmed pages reload the current host shell rather than a cached stale
+  // bundle. Coalesced via the same in-flight guard as #restartBrowser, so
+  // overlapping recycle signals collapse to one restart.
+  async recycle(): Promise<void> {
+    log.info('Recycling prerender browser to pick up a redeployed host');
+    await this.#restartBrowser();
+  }
+
   // Emit the `render cancelled` log line (format from CS-10872)
   // and, on a `rendering`-state cancel, tear down the affinity so
   // the next request gets a fresh tab rather than one whose
diff --git a/packages/realm-server/routes.ts b/packages/realm-server/routes.ts
index e319ee8f19..6b313e7637 100644
--- a/packages/realm-server/routes.ts
+++ b/packages/realm-server/routes.ts
@@ -110,6 +110,10 @@ export type CreateRoutesArgs = {
   };
   assetsURL: URL;
   prerenderer?: Prerenderer;
+  // Reports the current host-shell token to the prerender manager. The
+  // post-deployment hook calls it so the fleet's recycle signal is refreshed
+  // once the new code is live and the service is stable.
+  reportHostShell?: () => Promise<void>;
   searchCache: JobScopedSearchCache;
 };
 
diff --git a/packages/realm-server/server.ts b/packages/realm-server/server.ts
index 787d9164d7..9dd4c551b3 100644
--- a/packages/realm-server/server.ts
+++ b/packages/realm-server/server.ts
@@ -894,6 +894,7 @@ export class RealmServer {
       }
     | undefined;
   private prerenderer: Prerenderer | undefined;
+  private reportHostShell: (() => Promise<void>) | undefined;
   private reconciler: RealmRegistryReconciler;
   private searchCache: JobScopedSearchCache;
   private cachedApp: ReturnType<RealmServer['buildApp']> | undefined;
@@ -919,6 +920,7 @@ export class RealmServer {
     getRegistrationSecret,
     domainsForPublishedRealms,
     prerenderer,
+    reportHostShell,
     searchCache,
   }: {
     serverURL: URL;
@@ -945,6 +947,10 @@ export class RealmServer {
       boxelSite?: string;
     };
     prerenderer?: Prerenderer;
+    // Reports the current host-shell token to the prerender manager. main.ts
+    // wires this so the post-deployment hook can re-report once the service is
+    // stable (the boot-time report fires as soon as this server starts serving).
+    reportHostShell?: () => Promise<void>;
     // Optional so test harnesses that construct a RealmServer directly get a
     // private cache for free. main.ts passes a shared instance so the
     // JobsFinishedListener can evict the same cache the handlers populate.
@@ -989,6 +995,7 @@ export class RealmServer {
     this.realms = realms;
     this.reconciler = reconciler;
     this.prerenderer = prerenderer;
+    this.reportHostShell = reportHostShell;
     this.searchCache = searchCache ?? new JobScopedSearchCache(dbAdapter);
   }
 
@@ -1091,6 +1098,7 @@ export class RealmServer {
           matrixAdminPassword: this.matrixAdminPassword,
           domainsForPublishedRealms: this.domainsForPublishedRealms,
           prerenderer: this.prerenderer,
+          reportHostShell: this.reportHostShell,
           reconciler: this.reconciler,
           searchCache: this.searchCache,
         }),
diff --git a/packages/realm-server/tests/index.ts b/packages/realm-server/tests/index.ts
index 8f1f93aeb2..5dfa9da64b 100644
--- a/packages/realm-server/tests/index.ts
+++ b/packages/realm-server/tests/index.ts
@@ -191,6 +191,7 @@ const ALL_TEST_FILES: string[] = [
   './prerendering-test',
   './prerender-server-test',
   './prerender-manager-test',
+  './prerender-host-shell-recycle-test',
   './prerender-artifact-sink-test',
   './prerender-affinity-activity-test',
   './prerender-batch-ownership-test',
diff --git a/packages/realm-server/tests/prerender-host-shell-recycle-test.ts b/packages/realm-server/tests/prerender-host-shell-recycle-test.ts
new file mode 100644
index 0000000000..497fae4495
--- /dev/null
+++ b/packages/realm-server/tests/prerender-host-shell-recycle-test.ts
@@ -0,0 +1,43 @@
+import { module, test } from 'qunit';
+import { basename } from 'path';
+import { decideHostShellRecycle } from '../prerender/prerender-app.ts';
+
+// Unit tests for the host-shell recycle decision a prerender server makes on
+// every heartbeat: the manager echoes the current host-shell token, and the
+// server recycles its browser when that token differs from the one it warmed
+// against (the host was redeployed). See PRERENDER_HOST_SHELL_HASH_HEADER.
+module(basename(__filename), function () {
+  module('decideHostShellRecycle', function () {
+    test('no token reported yet → no recycle, baseline unchanged', function (assert) {
+      assert.deepEqual(decideHostShellRecycle(null, undefined), {
+        recycle: false,
+        nextWarmed: undefined,
+      });
+      assert.deepEqual(decideHostShellRecycle(null, 'aaa'), {
+        recycle: false,
+        nextWarmed: 'aaa',
+      });
+    });
+
+    test('first token seen → adopt as baseline, no recycle', function (assert) {
+      assert.deepEqual(decideHostShellRecycle('aaa', undefined), {
+        recycle: false,
+        nextWarmed: 'aaa',
+      });
+    });
+
+    test('token matches baseline → no-op', function (assert) {
+      assert.deepEqual(decideHostShellRecycle('aaa', 'aaa'), {
+        recycle: false,
+        nextWarmed: 'aaa',
+      });
+    });
+
+    test('token differs from baseline → recycle and advance baseline', function (assert) {
+      assert.deepEqual(decideHostShellRecycle('bbb', 'aaa'), {
+        recycle: true,
+        nextWarmed: 'bbb',
+      });
+    });
+  });
+});
diff --git a/packages/realm-server/tests/prerender-manager-test.ts b/packages/realm-server/tests/prerender-manager-test.ts
index abb6b5fb46..02a20af85f 100644
--- a/packages/realm-server/tests/prerender-manager-test.ts
+++ b/packages/realm-server/tests/prerender-manager-test.ts
@@ -8,6 +8,7 @@ import type { RealmHttpServer as Server } from '../server.ts';
 import http, { createServer } from 'http';
 import { buildPrerenderManagerApp } from '../prerender/manager-app.ts';
 import {
+  PRERENDER_HOST_SHELL_HASH_HEADER,
   PRERENDER_SERVER_DRAINING_STATUS_CODE,
   PRERENDER_SERVER_STATUS_DRAINING,
   PRERENDER_SERVER_STATUS_HEADER,
@@ -97,6 +98,59 @@ module(basename(__filename), function () {
       );
     });
 
+    test('reports the host shell token and echoes it on heartbeats', async function (assert) {
+      let { app } = buildPrerenderManagerApp();
+      let request: SuperTest<Test> = supertest(app.callback());
+      let headerKey = PRERENDER_HOST_SHELL_HASH_HEADER.toLowerCase();
+      let heartbeat = () =>
+        request.post('/prerender-servers').send({
+          data: {
+            type: 'prerender-server',
+            attributes: { capacity: 2, url: serverUrlA },
+          },
+        });
+
+      // No token reported yet → heartbeat carries no host-shell header.
+      let first = await heartbeat();
+      assert.strictEqual(first.status, 204, 'heartbeat accepted');
+      assert.strictEqual(
+        first.headers[headerKey],
+        undefined,
+        'no host-shell header before any report',
+      );
+
+      // Realm server reports a token.
+      let reportA = await request
+        .post('/host-shell')
+        .send({ data: { attributes: { hash: 'aaa111' } } });
+      assert.strictEqual(reportA.status, 204, 'host-shell report accepted');
+
+      // Now heartbeats echo it.
+      let second = await heartbeat();
+      assert.strictEqual(
+        second.headers[headerKey],
+        'aaa111',
+        'heartbeat echoes the reported host-shell token',
+      );
+
+      // A changed token is echoed; a repeat of the same token is a no-op.
+      await request
+        .post('/host-shell')
+        .send({ data: { attributes: { hash: 'bbb222' } } });
+      let third = await heartbeat();
+      assert.strictEqual(
+        third.headers[headerKey],
+        'bbb222',
+        'heartbeat echoes the updated host-shell token',
+      );
+
+      // A missing hash is rejected.
+      let bad = await request
+        .post('/host-shell')
+        .send({ data: { attributes: {} } });
+      assert.strictEqual(bad.status, 400, 'host-shell report requires a hash');
+    });
+
     test('health includes active servers with affinities and last used times', async function (assert) {
       process.env.PRERENDER_MULTIPLEX = '2';
       let { app } = buildPrerenderManagerApp();