diff --git a/apps/web-wasm/src/App.vue b/apps/web-wasm/src/App.vue
index d586840c..86f31109 100644
--- a/apps/web-wasm/src/App.vue
+++ b/apps/web-wasm/src/App.vue
@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, onMounted, ref } from 'vue'
+import { computed, onBeforeUnmount, onMounted, ref } from 'vue'
 import { useColorMode } from '@vueuse/core'
 import { storeToRefs } from 'pinia'
 import { useRoute } from 'vue-router'
@@ -13,6 +13,13 @@ import { reportError } from '@/services/log-report'
 import { useSessionStore } from '@/stores/session'
 import { useSettingsStore } from '@/stores/settings'
 import { PLATFORM_CAPABILITIES } from '@/utils/platform-capabilities'
+import {
+  WebWasmWorkspaceBusyError,
+  acquireWebWasmWorkspaceLease,
+  handleWebWasmWorkspacePageHide,
+  type WebWasmLockManager,
+  type WebWasmWorkspaceLease,
+} from './workspace-lock'
 
 const { t } = useI18n()
 const route = useRoute()
@@ -35,6 +42,7 @@ const toaster = {
 }
 
 let initInProgress = false
+let workspaceLease: WebWasmWorkspaceLease | undefined
 
 async function initializeApp() {
   if (initInProgress || isInitialized.value) return
@@ -42,6 +50,7 @@ async function initializeApp() {
   initError.value = null
 
   try {
+    workspaceLease ??= await acquireWebWasmWorkspaceLease(navigator.locks as WebWasmLockManager | undefined)
     await initializeAppRuntime({
       capabilities: PLATFORM_CAPABILITIES,
       initializeServices: () => initServices(),
@@ -52,14 +61,35 @@ async function initializeApp() {
     })
   } catch (error) {
     console.error('Web WASM application initialization failed', error)
-    initError.value = error instanceof Error ? error.message : String(error)
+    initError.value =
+      error instanceof WebWasmWorkspaceBusyError
+        ? t('common.webWasmWorkspaceBusy')
+        : error instanceof Error
+          ? error.message
+          : String(error)
     reportError(initError.value, error instanceof Error ? error.stack : undefined)
   } finally {
     initInProgress = false
   }
 }
 
-onMounted(initializeApp)
+function releaseWorkspaceLease() {
+  workspaceLease?.release()
+  workspaceLease = undefined
+}
+
+function handlePageHide(event: PageTransitionEvent) {
+  workspaceLease = handleWebWasmWorkspacePageHide(event, workspaceLease)
+}
+
+onMounted(() => {
+  window.addEventListener('pagehide', handlePageHide)
+  void initializeApp()
+})
+onBeforeUnmount(() => {
+  window.removeEventListener('pagehide', handlePageHide)
+  releaseWorkspaceLease()
+})
 </script>
 
 <template>
diff --git a/apps/web-wasm/src/workspace-lock.test.ts b/apps/web-wasm/src/workspace-lock.test.ts
new file mode 100644
index 00000000..5a20263b
--- /dev/null
+++ b/apps/web-wasm/src/workspace-lock.test.ts
@@ -0,0 +1,71 @@
+import assert from 'node:assert/strict'
+import { describe, it } from 'node:test'
+import {
+  WebWasmWorkspaceBusyError,
+  acquireWebWasmWorkspaceLease,
+  handleWebWasmWorkspacePageHide,
+  type WebWasmLockManager,
+  type WebWasmWorkspaceLease,
+} from './workspace-lock'
+
+describe('acquireWebWasmWorkspaceLease', () => {
+  it('rejects a second Web WASM page while the workspace lock is occupied', async () => {
+    const lockManager: WebWasmLockManager = {
+      async request(_name, options, callback) {
+        assert.deepEqual(options, { mode: 'exclusive', ifAvailable: true })
+        await callback(null)
+      },
+    }
+
+    await assert.rejects(acquireWebWasmWorkspaceLease(lockManager), WebWasmWorkspaceBusyError)
+  })
+
+  it('holds the workspace lock until the lease is released', async () => {
+    let requestCompleted = false
+    const lockManager: WebWasmLockManager = {
+      async request(_name, _options, callback) {
+        await callback({})
+        requestCompleted = true
+      },
+    }
+
+    const lease = await acquireWebWasmWorkspaceLease(lockManager)
+    assert.equal(requestCompleted, false)
+
+    lease.release()
+    await new Promise((resolve) => setTimeout(resolve, 0))
+    assert.equal(requestCompleted, true)
+
+    lease.release()
+  })
+})
+
+describe('handleWebWasmWorkspacePageHide', () => {
+  it('keeps the workspace lease while the page enters the back-forward cache', () => {
+    let releaseCount = 0
+    const lease: WebWasmWorkspaceLease = {
+      release() {
+        releaseCount++
+      },
+    }
+
+    const retainedLease = handleWebWasmWorkspacePageHide({ persisted: true }, lease)
+
+    assert.equal(retainedLease, lease)
+    assert.equal(releaseCount, 0)
+  })
+
+  it('releases and clears the workspace lease when the page is discarded', () => {
+    let releaseCount = 0
+    const lease: WebWasmWorkspaceLease = {
+      release() {
+        releaseCount++
+      },
+    }
+
+    const retainedLease = handleWebWasmWorkspacePageHide({ persisted: false }, lease)
+
+    assert.equal(retainedLease, undefined)
+    assert.equal(releaseCount, 1)
+  })
+})
diff --git a/apps/web-wasm/src/workspace-lock.ts b/apps/web-wasm/src/workspace-lock.ts
new file mode 100644
index 00000000..003511cb
--- /dev/null
+++ b/apps/web-wasm/src/workspace-lock.ts
@@ -0,0 +1,75 @@
+import { handleWebWasmPageHide } from './runtime-lifecycle'
+
+export const WEB_WASM_WORKSPACE_LOCK = 'chatlab-web-wasm-opfs-workspace'
+
+export interface WebWasmLockManager {
+  request(
+    name: string,
+    options: { mode: 'exclusive'; ifAvailable: true },
+    callback: (lock: object | null) => Promise<void>
+  ): Promise<void>
+}
+
+export interface WebWasmWorkspaceLease {
+  release(): void
+}
+
+export class WebWasmWorkspaceBusyError extends Error {
+  readonly code = 'OPFS_WORKSPACE_BUSY'
+
+  constructor() {
+    super('The Web WASM workspace is already open in another tab')
+    this.name = 'WebWasmWorkspaceBusyError'
+  }
+}
+
+export function handleWebWasmWorkspacePageHide(
+  event: Pick<PageTransitionEvent, 'persisted'>,
+  lease: WebWasmWorkspaceLease | undefined
+): WebWasmWorkspaceLease | undefined {
+  let retainedLease = lease
+  handleWebWasmPageHide(event, () => {
+    lease?.release()
+    retainedLease = undefined
+  })
+  return retainedLease
+}
+
+export async function acquireWebWasmWorkspaceLease(
+  lockManager: WebWasmLockManager | undefined
+): Promise<WebWasmWorkspaceLease> {
+  if (!lockManager) return { release: () => undefined }
+
+  let releaseLock: () => void = () => undefined
+  const lockReleased = new Promise<void>((resolve) => {
+    releaseLock = resolve
+  })
+  let released = false
+  const lease: WebWasmWorkspaceLease = {
+    release() {
+      if (released) return
+      released = true
+      releaseLock()
+    },
+  }
+
+  let resolveAcquired!: (lease: WebWasmWorkspaceLease) => void
+  let rejectAcquired!: (error: unknown) => void
+  const acquired = new Promise<WebWasmWorkspaceLease>((resolve, reject) => {
+    resolveAcquired = resolve
+    rejectAcquired = reject
+  })
+
+  void lockManager
+    .request(WEB_WASM_WORKSPACE_LOCK, { mode: 'exclusive', ifAvailable: true }, async (lock) => {
+      if (!lock) {
+        rejectAcquired(new WebWasmWorkspaceBusyError())
+        return
+      }
+      resolveAcquired(lease)
+      await lockReleased
+    })
+    .catch(rejectAcquired)
+
+  return acquired
+}
diff --git a/packages/web-runtime/src/sqlite/opfs.test.ts b/packages/web-runtime/src/sqlite/opfs.test.ts
new file mode 100644
index 00000000..c831e9c6
--- /dev/null
+++ b/packages/web-runtime/src/sqlite/opfs.test.ts
@@ -0,0 +1,26 @@
+import assert from 'node:assert/strict'
+import { describe, it } from 'node:test'
+import { WebRuntimeError } from '../runtime-error'
+import { normalizeOpfsPoolInitializationError } from './opfs'
+
+describe('normalizeOpfsPoolInitializationError', () => {
+  it('maps an occupied OPFS access handle to a stable runtime error', () => {
+    const cause = new DOMException(
+      'Access Handles cannot be created if there is another open Access Handle',
+      'NoModificationAllowedError'
+    )
+
+    const error = normalizeOpfsPoolInitializationError(cause)
+
+    assert.ok(error instanceof WebRuntimeError)
+    assert.equal(error.code, 'OPFS_WORKSPACE_BUSY')
+    assert.match(error.message, /another Web WASM tab/i)
+    assert.equal(error.cause, cause)
+  })
+
+  it('preserves unrelated initialization errors', () => {
+    const cause = new Error('unexpected failure')
+
+    assert.equal(normalizeOpfsPoolInitializationError(cause), cause)
+  })
+})
diff --git a/packages/web-runtime/src/sqlite/opfs.ts b/packages/web-runtime/src/sqlite/opfs.ts
index f5e32ff5..7556d801 100644
--- a/packages/web-runtime/src/sqlite/opfs.ts
+++ b/packages/web-runtime/src/sqlite/opfs.ts
@@ -36,8 +36,24 @@ export async function initializeOpfsSqlite(
     initialCapacity: 8,
     forceReinitIfPreviouslyFailed: true,
   }
-  const pool = await sqlite3.installOpfsSAHPoolVfs(poolOptions)
+  let pool: SAHPoolUtil
+  try {
+    pool = await sqlite3.installOpfsSAHPoolVfs(poolOptions)
+  } catch (error) {
+    throw normalizeOpfsPoolInitializationError(error)
+  }
   onStage?.('opfs-pool-ready')
 
   return { sqlite3, pool }
 }
+
+export function normalizeOpfsPoolInitializationError(error: unknown): unknown {
+  if (error instanceof Error && error.name === 'NoModificationAllowedError') {
+    return new WebRuntimeError(
+      'OPFS_WORKSPACE_BUSY',
+      'The Web WASM workspace is already open in another Web WASM tab. Close the other tab and retry.',
+      { cause: error }
+    )
+  }
+  return error
+}
diff --git a/src/i18n/locales/en-US/common.json b/src/i18n/locales/en-US/common.json
index 01ae39b5..a9228a21 100644
--- a/src/i18n/locales/en-US/common.json
+++ b/src/i18n/locales/en-US/common.json
@@ -11,6 +11,7 @@
   "done": "Done",
   "retry": "Retry",
   "initFailed": "App initialization failed",
+  "webWasmWorkspaceBusy": "Web WASM is already open in another tab. Close the other ChatLab Web WASM tab and retry.",
   "loading": "Loading...",
   "initializing": "Initializing...",
   "noResults": "No results found",
diff --git a/src/i18n/locales/ja-JP/common.json b/src/i18n/locales/ja-JP/common.json
index 77545a7f..09c4f877 100644
--- a/src/i18n/locales/ja-JP/common.json
+++ b/src/i18n/locales/ja-JP/common.json
@@ -11,6 +11,7 @@
   "done": "完了",
   "retry": "リトライ",
   "initFailed": "アプリの初期化に失敗しました",
+  "webWasmWorkspaceBusy": "Web WASM は別のタブで開かれています。もう一方の ChatLab Web WASM タブを閉じてから再試行してください。",
   "loading": "読み込み中...",
   "initializing": "初期化中...",
   "noResults": "結果が見つかりません",
diff --git a/src/i18n/locales/zh-CN/common.json b/src/i18n/locales/zh-CN/common.json
index 773e242d..6e2ccc51 100644
--- a/src/i18n/locales/zh-CN/common.json
+++ b/src/i18n/locales/zh-CN/common.json
@@ -11,6 +11,7 @@
   "done": "完成",
   "retry": "重试",
   "initFailed": "应用初始化失败",
+  "webWasmWorkspaceBusy": "Web WASM 已在另一个标签页中打开。请关闭另一个 ChatLab Web WASM 标签页后重试。",
   "loading": "加载中...",
   "initializing": "初始化中...",
   "noResults": "无匹配结果",
diff --git a/src/i18n/locales/zh-TW/common.json b/src/i18n/locales/zh-TW/common.json
index e10bd914..e6395b54 100644
--- a/src/i18n/locales/zh-TW/common.json
+++ b/src/i18n/locales/zh-TW/common.json
@@ -11,6 +11,7 @@
   "done": "完成",
   "retry": "重試",
   "initFailed": "應用程式初始化失敗",
+  "webWasmWorkspaceBusy": "Web WASM 已在另一個分頁中開啟。請關閉另一個 ChatLab Web WASM 分頁後重試。",
   "loading": "載入中...",
   "initializing": "初始化中...",
   "noResults": "無匹配結果",