diff --git a/cocoscreator_projects/YouleNexus/assets/framework/platform/runtime-session.ts b/cocoscreator_projects/YouleNexus/assets/framework/platform/runtime-session.ts
index a943911..b6f6468 100644
--- a/cocoscreator_projects/YouleNexus/assets/framework/platform/runtime-session.ts
+++ b/cocoscreator_projects/YouleNexus/assets/framework/platform/runtime-session.ts
@@ -1,7 +1,7 @@
 import type { GameHost } from '../sdk/contracts/index.ts';
 import {
   GameSessionHost,
-  GameSessionOpenFault,
+  isGameSessionOpenFault,
 } from '../sdk/runtime/game-session-host.ts';
 import {
   parseLoginResponse,
@@ -170,7 +170,7 @@ export class RuntimeSession {
       });
       if (reconnect.present) this.options.gameSession.restore(reconnect.value);
     } catch (error) {
-      const errors = error instanceof GameSessionOpenFault ? [...error.errors] : [error];
+      const errors = isGameSessionOpenFault(error) ? [...error.errors] : [error];
       this.throwFault(operation, [...errors, ...this.releaseGame()]);
     }
   }
diff --git a/cocoscreator_projects/YouleNexus/assets/framework/sdk/runtime/game-session-host.ts b/cocoscreator_projects/YouleNexus/assets/framework/sdk/runtime/game-session-host.ts
index ded9da3..e1bbe3e 100644
--- a/cocoscreator_projects/YouleNexus/assets/framework/sdk/runtime/game-session-host.ts
+++ b/cocoscreator_projects/YouleNexus/assets/framework/sdk/runtime/game-session-host.ts
@@ -7,6 +7,9 @@ import type {
 } from '../contracts/index.ts';
 
 const RESERVED_ROUTES = new Set(['platform', 'agent', 'room']);
+const GAME_SESSION_OPEN_FAULT_BRAND = Symbol.for(
+  'youle.framework.sdk.GameSessionOpenFault',
+);
 
 export type GameSessionState =
   | 'idle'
@@ -22,10 +25,33 @@ export class GameSessionOpenFault extends Error {
   constructor(primary: unknown, cleanup: unknown) {
     super('Game session open failed and module cleanup also failed');
     this.name = 'GameSessionOpenFault';
+    Object.defineProperty(this, GAME_SESSION_OPEN_FAULT_BRAND, { value: true });
     this.errors = Object.freeze([primary, cleanup]);
   }
 }
 
+export interface GameSessionOpenFaultShape {
+  readonly errors: readonly [unknown, unknown];
+}
+
+/** Recognizes open faults across realms and duplicate module instances. */
+export function isGameSessionOpenFault(value: unknown): value is GameSessionOpenFaultShape {
+  if (value === null || typeof value !== 'object') return false;
+
+  try {
+    const brand = ownDataDescriptor(value, GAME_SESSION_OPEN_FAULT_BRAND);
+    const errors = ownDataDescriptor(value, 'errors');
+    if (brand?.value !== true || !errors || !Array.isArray(errors.value)) return false;
+
+    const errorList = errors.value;
+    return ownDataDescriptor(errorList, 'length')?.value === 2
+      && ownDataDescriptor(errorList, '0') !== undefined
+      && ownDataDescriptor(errorList, '1') !== undefined;
+  } catch {
+    return false;
+  }
+}
+
 /** Validates the compile-time game identity before it reaches the runtime. */
 export function assertGameEntry(entry: GameEntry): void {
   assertNonEmptyText(entry.key, 'key');
@@ -195,3 +221,10 @@ function disposeModuleIfPresent(module: unknown): void {
   const dispose = (module as Record<string, unknown>).dispose;
   if (typeof dispose === 'function') dispose.call(module);
 }
+
+function ownDataDescriptor(target: object, key: PropertyKey): PropertyDescriptor | undefined {
+  const descriptor = Object.getOwnPropertyDescriptor(target, key);
+  return descriptor && Object.prototype.hasOwnProperty.call(descriptor, 'value')
+    ? descriptor
+    : undefined;
+}
diff --git a/cocoscreator_projects/framework-tests/platform/runtime-session.test.ts b/cocoscreator_projects/framework-tests/platform/runtime-session.test.ts
index b209c5d..2fce325 100644
--- a/cocoscreator_projects/framework-tests/platform/runtime-session.test.ts
+++ b/cocoscreator_projects/framework-tests/platform/runtime-session.test.ts
@@ -1,6 +1,7 @@
 import { readFileSync } from 'node:fs';
 import assert from 'node:assert/strict';
 import { test } from 'node:test';
+import { runInNewContext } from 'node:vm';
 import type {
   GameEntry,
   GameHost,
@@ -8,7 +9,10 @@ import type {
   PlatformGameSnapshot,
   PlatformToGameEvent,
 } from '../../YouleNexus/assets/framework/sdk/contracts/index.ts';
-import { GameSessionHost } from '../../YouleNexus/assets/framework/sdk/runtime/game-session-host.ts';
+import {
+  GameSessionHost,
+  GameSessionOpenFault,
+} from '../../YouleNexus/assets/framework/sdk/runtime/game-session-host.ts';
 import { RuntimeSession } from '../../YouleNexus/assets/framework/platform/runtime-session.ts';
 import { PlatformStore } from '../../YouleNexus/assets/framework/platform/stores/platform-store.ts';
 import { selectGameSnapshot } from '../../YouleNexus/assets/framework/platform/stores/selectors.ts';
@@ -383,3 +387,84 @@ test('RuntimeSession flattens attach cleanup failures before appending lease inv
     [setup.errors.attach, setup.errors.dispose, setup.errors.invalidate],
   );
 });
+
+test('RuntimeSession flattens a foreign-realm branded open fault before lease invalidation', () => {
+  const setup = makeLeaseSession();
+  const primary = new Error('foreign primary');
+  const cleanup = new Error('foreign cleanup');
+  const foreignFault = runInNewContext(`(() => {
+    const fault = new Error('foreign open fault');
+    Object.defineProperties(fault, {
+      [Symbol.for('youle.framework.sdk.GameSessionOpenFault')]: { value: true },
+      errors: { value: [primary, cleanup] },
+    });
+    return fault;
+  })()`, { primary, cleanup });
+  assert.equal(foreignFault instanceof GameSessionOpenFault, false);
+  setup.setFailures('invalidate');
+  setup.gameSession.open = () => { throw foreignFault; };
+
+  const error = caughtError(() => setup.session.handleLogin(fixture('player-login-room.json')));
+
+  assert.equal(setup.session.state, 'faulted');
+  assert.equal(setup.session.fault, error);
+  assert.equal(setup.gameSession.state, 'idle');
+  assert.equal(setup.leases[0]?.invalidations, 1);
+  assert.deepEqual(
+    (error as Error & { errors?: readonly unknown[] }).errors,
+    [primary, cleanup, setup.errors.invalidate],
+  );
+});
+
+test('RuntimeSession keeps ordinary aggregates and unbranded or accessor spoofs as single errors', () => {
+  const primary = new Error('user primary');
+  const cleanup = new Error('user cleanup');
+  const ordinaryAggregate = runInNewContext(
+    'new AggregateError([primary, cleanup], "user aggregate")',
+    { primary, cleanup },
+  );
+  const unbranded = { errors: [primary, cleanup] };
+  let accessorReads = 0;
+  const accessorSpoof = new Error('accessor spoof');
+  Object.defineProperties(accessorSpoof, {
+    [Symbol.for('youle.framework.sdk.GameSessionOpenFault')]: { value: true },
+    errors: {
+      get() {
+        accessorReads += 1;
+        throw new Error('errors getter must not run');
+      },
+    },
+  });
+  const brandAccessorSpoof = new Error('brand accessor spoof');
+  Object.defineProperties(brandAccessorSpoof, {
+    [Symbol.for('youle.framework.sdk.GameSessionOpenFault')]: {
+      get() {
+        accessorReads += 1;
+        throw new Error('brand getter must not run');
+      },
+    },
+    errors: { value: [primary, cleanup] },
+  });
+
+  for (const candidate of [
+    ordinaryAggregate,
+    unbranded,
+    accessorSpoof,
+    brandAccessorSpoof,
+  ]) {
+    const setup = makeLeaseSession();
+    setup.setFailures('invalidate');
+    setup.gameSession.open = () => { throw candidate; };
+
+    const error = caughtError(() => setup.session.handleLogin(fixture('player-login-room.json')));
+    const errors = (error as Error & { errors?: readonly unknown[] }).errors;
+
+    assert.equal(errors?.length, 2);
+    assert.equal(errors?.[0], candidate);
+    assert.equal(errors?.[1], setup.errors.invalidate);
+    assert.equal(setup.session.state, 'faulted');
+    assert.equal(setup.gameSession.state, 'idle');
+    assert.equal(setup.leases[0]?.invalidations, 1);
+  }
+  assert.equal(accessorReads, 0);
+});
diff --git a/cocoscreator_projects/framework-tests/sdk/game-session-host.test.ts b/cocoscreator_projects/framework-tests/sdk/game-session-host.test.ts
index 51afc26..4a7e821 100644
--- a/cocoscreator_projects/framework-tests/sdk/game-session-host.test.ts
+++ b/cocoscreator_projects/framework-tests/sdk/game-session-host.test.ts
@@ -1,5 +1,6 @@
 import assert from 'node:assert/strict';
 import { test } from 'node:test';
+import { runInNewContext } from 'node:vm';
 import type {
   GameEntry,
   GameHost,
@@ -11,7 +12,11 @@ import type {
 import {
   assertGameEntry,
   GameSessionHost,
+  GameSessionOpenFault,
 } from '../../YouleNexus/assets/framework/sdk/runtime/game-session-host.ts';
+import * as gameSessionRuntime from '../../YouleNexus/assets/framework/sdk/runtime/game-session-host.ts';
+
+const OPEN_FAULT_BRAND_KEY = 'youle.framework.sdk.GameSessionOpenFault';
 
 function makeHost(): GameHost {
   const snapshot: PlatformGameSnapshot = {
@@ -221,6 +226,61 @@ test('GameSessionHost preserves attach then dispose failures by identity and occ
   assert.equal(sessions.state, 'idle');
 });
 
+test('open fault guard recognizes the global brand across realms without reading accessors', () => {
+  type OpenFaultShape = { readonly errors: readonly unknown[] };
+  type OpenFaultGuard = (value: unknown) => value is OpenFaultShape;
+  const exported = (gameSessionRuntime as unknown as Record<string, unknown>)
+    .isGameSessionOpenFault;
+  assert.equal(typeof exported, 'function');
+  const isGameSessionOpenFault = exported as OpenFaultGuard;
+  const primary = new Error('primary');
+  const cleanup = new Error('cleanup');
+  const sameRealm = new GameSessionOpenFault(primary, cleanup);
+  const foreignRealm = runInNewContext(`(() => {
+    const fault = new Error('foreign open fault');
+    Object.defineProperties(fault, {
+      [Symbol.for(${JSON.stringify(OPEN_FAULT_BRAND_KEY)})]: { value: true },
+      errors: { value: [primary, cleanup] },
+    });
+    return fault;
+  })()`, { primary, cleanup });
+  const ordinaryAggregate = runInNewContext(
+    'new AggregateError([primary, cleanup], "user aggregate")',
+    { primary, cleanup },
+  );
+  const unbranded = { errors: [primary, cleanup] };
+  let accessorReads = 0;
+  const accessorSpoof = new Error('accessor spoof');
+  Object.defineProperties(accessorSpoof, {
+    [Symbol.for(OPEN_FAULT_BRAND_KEY)]: { value: true },
+    errors: {
+      get() {
+        accessorReads += 1;
+        throw new Error('errors getter must not run');
+      },
+    },
+  });
+  const brandAccessorSpoof = new Error('brand accessor spoof');
+  Object.defineProperties(brandAccessorSpoof, {
+    [Symbol.for(OPEN_FAULT_BRAND_KEY)]: {
+      get() {
+        accessorReads += 1;
+        throw new Error('brand getter must not run');
+      },
+    },
+    errors: { value: [primary, cleanup] },
+  });
+
+  assert.equal(foreignRealm instanceof GameSessionOpenFault, false);
+  assert.equal(isGameSessionOpenFault(sameRealm), true);
+  assert.equal(isGameSessionOpenFault(foreignRealm), true);
+  assert.equal(isGameSessionOpenFault(ordinaryAggregate), false);
+  assert.equal(isGameSessionOpenFault(unbranded), false);
+  assert.equal(isGameSessionOpenFault(accessorSpoof), false);
+  assert.equal(isGameSessionOpenFault(brandAccessorSpoof), false);
+  assert.equal(accessorReads, 0);
+});
+
 test('GameSessionHost disposes a module exactly once and becomes terminal after dispose', () => {
   const log: string[] = [];
   const sessions = new GameSessionHost(makeEntry(() => makeModule(log)));