diff --git a/packages/runtime-host/package.json b/packages/runtime-host/package.json
index 16fb19dff98..747a0778135 100644
--- a/packages/runtime-host/package.json
+++ b/packages/runtime-host/package.json
@@ -26,6 +26,7 @@
     "clean": "node ../../scripts/clean-paths.mjs dist tsconfig.tsbuildinfo",
     "build": "tsc -p tsconfig.json",
     "benchmark:session-copy": "node scripts/session-copy-cost-benchmark.mjs",
+    "benchmark:hydration-e2e": "node scripts/hydration-e2e-benchmark.mjs",
     "typecheck": "tsc -p tsconfig.json --noEmit",
     "test:dist": "node --test \"dist/**/*.test.js\""
   },
diff --git a/packages/runtime-host/scripts/hydration-e2e-benchmark.mjs b/packages/runtime-host/scripts/hydration-e2e-benchmark.mjs
new file mode 100644
index 00000000000..adc73217233
--- /dev/null
+++ b/packages/runtime-host/scripts/hydration-e2e-benchmark.mjs
@@ -0,0 +1,301 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+// End-to-end hydration measurement related to the #4677 budget item:
+// "long Session @30ms RTT: open to visible tail p95 <= 800ms".
+//
+// IMPORTANT - proxy metric, not the visible point: the #4677 800ms budget
+// applies to the Desktop visible tail (renderer paint). This script stops
+// its `hostDecodeTail` timer immediately after
+// `ClientSessionSubscription.decodeTranscriptPage`; no
+// `DesktopTranscriptReplica` installation, Electron IPC, renderer update
+// or paint has occurred. The `wire` wrapper also invokes Host handlers
+// directly rather than exercising protocol serialization/transport.
+// Treat `hostDecodeTail` and `fullHydration` as Host-client decode
+// proxies with no budget verdict here; measuring the actual visible
+// point needs a Desktop/Electron harness extension (open acceptance
+// work).
+//
+// Faithful path: a real execution composition (FakeBackend), turns seeded
+// through `turn.start`, then per round the client RPCs
+// `subscription.open` (tail bootstrap) and `session.transcript.page`
+// with REAL injected round-trip latency: every handler call is one wire
+// round trip modeled as a 15ms request flight + measured server CPU +
+// 15ms response flight. The tail is decoded with the production
+// ClientSessionSubscription decode path.
+//
+// Usage (from packages/runtime-host, after a workspace build):
+//   node scripts/hydration-e2e-benchmark.mjs
+//   TURNS=1000 ROUNDS=10 RTT_MS=30 node scripts/hydration-e2e-benchmark.mjs
+
+import { performance } from 'node:perf_hooks';
+import { mkdtemp, rm } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { randomUUID } from 'node:crypto';
+import { FakeBackend } from '@maka/runtime/test-only/fake-backend';
+import { SessionManager } from '@maka/runtime/session-manager';
+import { resolveStorageRoot, tryAcquireInteractiveRootOwner } from '@maka/storage/root-authority';
+import { ClientSessionSubscription } from '../dist/client/session-subscription.js';
+import { createExecutionRuntimeHostComposition } from '../dist/server/execution-composition.js';
+
+const RTT_MS = Number(process.env.RTT_MS ?? 30);
+const ROUNDS = Number(process.env.ROUNDS ?? 10);
+const TURNS = Number(process.env.TURNS ?? 1000);
+const TURN_TEXT_BYTES = Number(process.env.TURN_TEXT_BYTES ?? 40);
+// The #4677 800ms budget belongs to the Desktop visible tail (renderer
+// paint), which this script does not measure; no PASS/FAIL verdict is
+// emitted against it here (see header note).
+
+const HOST_EPOCH = 'hydration-benchmark';
+const CONNECTION_ID = 'hydration-benchmark-client';
+const operationContext = {
+  hostEpoch: HOST_EPOCH,
+  connectionId: CONNECTION_ID,
+  principal: 'local_os_user',
+  principalKind: 'local_owner',
+  acquireResidency: () => ({ release() {} }),
+};
+const flightMs = RTT_MS / 2;
+const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+// Every wire RPC is one round trip: request flight, server work, response flight.
+const wire = async (serverWork) => {
+  await sleep(flightMs);
+  const result = await serverWork();
+  await sleep(flightMs);
+  return result;
+};
+
+const root = await mkdtemp(join(tmpdir(), 'maka-hydration-benchmark-'));
+const capability = await resolveStorageRoot({ path: root, kind: 'interactive' });
+const owner = await tryAcquireInteractiveRootOwner(capability);
+if (!owner) throw new Error(`Could not acquire hydration storage root: ${root}`);
+
+// Capture the composition's SessionManager (same trick as startup-profile.mjs).
+let manager;
+const originalRecover = SessionManager.prototype.recoverInterruptedSessionsStrict;
+SessionManager.prototype.recoverInterruptedSessionsStrict = async function (stores) {
+  manager = this;
+  return originalRecover.call(this, stores);
+};
+const composition = await createExecutionRuntimeHostComposition(
+  {
+    owner,
+    hostEpoch: HOST_EPOCH,
+    acquireResidency: () => ({ release() {} }),
+    retainUntilProcessExit: () => undefined,
+    requestDrain: () => undefined,
+  },
+  {},
+  { primaryBackendFactory: (context) => new FakeBackend(context) },
+);
+await composition.recover();
+SessionManager.prototype.recoverInterruptedSessionsStrict = originalRecover;
+if (!manager) throw new Error('Composition did not construct a SessionManager');
+// Attach the benchmark's client connection to continuity, like a real
+// transport accept does (frames are dropped; this harness reads via RPCs).
+const connection = composition.continuity.attachConnection(CONNECTION_ID, {
+  send: async () => {},
+});
+await configureDefaultModel(composition.handlers);
+const call = async (operation, input) => {
+  const outcome = await composition.handlers[operation](input, operationContext);
+  if (!outcome.ok) throw new Error(`${operation} failed: ${JSON.stringify(outcome.error)}`);
+  return outcome.result;
+};
+
+try {
+  const created = await call('session.create', {
+    sessionId: randomUUID(),
+    workspace: { kind: 'host_path', path: root },
+    name: `hydration-${TURNS}turns`,
+    modelTarget: { kind: 'default' },
+  });
+  const sessionId = created.id;
+  const seedStart = performance.now();
+  for (let turn = 0; turn < TURNS; turn += 1) {
+    const turnId = `turn-${turn}`;
+    const started = await call('turn.start', {
+      sessionId,
+      turnId,
+      content: { text: `hydration turn ${turn} ${'x'.repeat(TURN_TEXT_BYTES)}` },
+    });
+    await waitForCompletedTurn(sessionId, turnId);
+    if (turn % 50 === 0 || turn === TURNS - 1) {
+      process.stdout.write(
+        `seeded ${turn + 1}/${TURNS} turns (${((performance.now() - seedStart) / 1000).toFixed(0)}s)\n`,
+      );
+    }
+  }
+  const seedSeconds = (performance.now() - seedStart) / 1000;
+
+  const samples = [];
+  for (let round = 0; round < ROUNDS; round += 1) {
+    samples.push(await openAndMeasure(sessionId));
+  }
+  report({ TURNS, TURN_TEXT_BYTES, RTT_MS, seedSeconds, samples });
+} finally {
+  await composition.close?.();
+  await rm(root, { recursive: true, force: true }).catch(() => {});
+}
+
+// One round = one client opening the session over an RTT_MS wire.
+
+async function waitForCompletedTurn(sessionId, turnId) {
+  const deadline = performance.now() + 30_000;
+  while (performance.now() < deadline) {
+    const messages = await manager.getMessages(sessionId);
+    const state = messages.findLast(
+      (message) => message.type === 'turn_state' && message.turnId === turnId,
+    );
+    if (state?.status === 'completed') return;
+    if (state?.status === 'failed' || state?.status === 'aborted') {
+      throw new Error(`Hydration turn ${turnId} ended as ${state.status}`);
+    }
+    await new Promise((resolve) => setTimeout(resolve, 5));
+  }
+  throw new Error(`Timed out waiting for hydration turn ${turnId}`);
+}
+
+async function openAndMeasure(sessionId) {
+  const openStart = performance.now();
+  const open = await wire(() =>
+    call('subscription.open', {
+      sessionId,
+      transcript: { kind: 'tail', maxBytes: 16 * 1024 },
+    }),
+  );
+  if (!open.transcript) throw new Error('subscription.open returned no transcript bootstrap');
+  const bootstrap = open.transcript;
+  let wireBytes = bootstrap.durable.rawBytes;
+  let pageRequests = 0;
+
+  const subscription = new ClientSessionSubscription(
+    open,
+    async () => call('subscription.close', { subscriptionId: open.subscriptionId }),
+    async (request) => {
+      pageRequests += 1;
+      const page = await wire(() =>
+        call('session.transcript.page', { subscriptionId: open.subscriptionId, ...request }),
+      );
+      wireBytes += page.rawBytes;
+      return page;
+    },
+  );
+
+  // Host-client decode proxy for the visible tail: production decode
+  // path over the bootstrap page; stops here, before any replica
+  // installation / IPC / renderer work (see header note).
+  const tail = await subscription.decodeTranscriptPage(bootstrap.durable, (value) => value);
+  const hostDecodeTailMs = performance.now() - openStart;
+
+  // Full hydration: page in the entire transcript through cursors.
+  const materialized = await subscription.loadTranscript((value) => value);
+  const fullHydrationMs = performance.now() - openStart;
+  await subscription.close();
+
+  return {
+    hostDecodeTailMs,
+    fullHydrationMs,
+    pageRequests,
+    tailMessages: tail.messages.length,
+    materializedMessages: materialized.length,
+    tailKiB: bootstrap.durable.rawBytes / 1024,
+    wireMiB: wireBytes / (1024 * 1024),
+  };
+}
+
+function report({ TURNS, TURN_TEXT_BYTES, RTT_MS, seedSeconds, samples }) {
+  const pick = (key) => samples.map((sample) => sample[key]);
+  const mean = (values) => values.reduce((sum, value) => sum + value, 0) / values.length;
+  const hostDecodeTail = stats(pick('hostDecodeTailMs'));
+  const fullHydration = stats(pick('fullHydrationMs'));
+  console.log(
+    `\nfixture: ${TURNS} turns (~${TURN_TEXT_BYTES}B user text/turn), ` +
+      `seed ${seedSeconds.toFixed(0)}s, ${ROUNDS} rounds @ ${RTT_MS}ms RTT`,
+  );
+  // Both rows are Host-client decode proxies (see header note); the
+  // #4677 800ms visible-tail budget is not judged here.
+  console.table([
+    {
+      metric: 'hostDecodeTail (Host-client decode proxy)',
+      medianMs: hostDecodeTail.medianMs.toFixed(1),
+      p95Ms: hostDecodeTail.p95Ms.toFixed(1),
+      maxMs: hostDecodeTail.maxMs.toFixed(1),
+    },
+    {
+      metric: 'fullHydration (Host-client decode proxy)',
+      medianMs: fullHydration.medianMs.toFixed(1),
+      p95Ms: fullHydration.p95Ms.toFixed(1),
+      maxMs: fullHydration.maxMs.toFixed(1),
+    },
+  ]);
+  console.log(
+    `tail: ${mean(pick('tailKiB')).toFixed(2)} KiB / ${Math.round(mean(pick('tailMessages')))} msgs, ` +
+      `pages(older): median ${[...pick('pageRequests')].sort((a, b) => a - b)[Math.floor(ROUNDS / 2)]}, ` +
+      `total wire: ${mean(pick('wireMiB')).toFixed(2)} MiB, ` +
+      `materialized: ${samples[0].materializedMessages} msgs`,
+  );
+}
+
+function stats(values) {
+  const sorted = [...values].sort((a, b) => a - b);
+  const p95Index = Math.min(sorted.length - 1, Math.ceil(sorted.length * 0.95) - 1);
+  return {
+    medianMs: sorted[Math.floor(sorted.length / 2)],
+    p95Ms: sorted[p95Index],
+    maxMs: sorted[sorted.length - 1],
+    minMs: sorted[0],
+  };
+}
+
+async function configureDefaultModel(handlers) {
+  const callModel = async (operation, input) => {
+    const outcome = await handlers[operation](input, operationContext);
+    if (!outcome.ok) throw new Error(`${operation} failed: ${JSON.stringify(outcome.error)}`);
+    return outcome.result;
+  };
+  const catalog = await callModel('connection.catalog.query', { kind: 'start' });
+  const created = await callModel('connection.catalog.create', {
+    expectedCatalogRevision: catalog.revision,
+    connection: {
+      slug: 'hydration-benchmark',
+      name: 'Hydration benchmark',
+      providerType: 'custom',
+      defaultApiProtocol: 'openai-chat',
+      baseUrl: 'https://hydration-benchmark.invalid/v1',
+      enabled: true,
+      enabledModelIds: ['hydration-test-model'],
+    },
+  });
+  if (created.kind !== 'committed') throw new Error('Benchmark connection must commit');
+  await callModel('credential.vault.set', {
+    locator: {
+      scope: 'connection',
+      connectionId: created.connection.connectionId,
+      kind: 'api_key',
+    },
+    expected: null,
+    secret: 'hydration-benchmark-key',
+  });
+  await callModel('connection.catalog.set-default-target', {
+    expectedCatalogRevision: created.catalogRevision,
+    target: { connectionId: created.connection.connectionId, modelId: 'hydration-test-model' },
+  });
+}