diff --git a/apps/api/src/opendirector_api/localization.py b/apps/api/src/opendirector_api/localization.py
index 0586213f..0b1965bd 100644
--- a/apps/api/src/opendirector_api/localization.py
+++ b/apps/api/src/opendirector_api/localization.py
@@ -63,6 +63,7 @@
     "capability_unsupported": "This capability is not supported yet.",
     "operation_rejected": "The operation was rejected. Reload the item and try again.",
     "storage_busy": "Project storage is busy. Try again shortly.",
+    "project_open_elsewhere": "This project is open in another window. Close it there and try again.",
     "local_only": "Only this computer can access the project service.",
     "cross_origin": "File access from another site is not allowed.",
     "invalid_local_request": "This is not a valid local app request.",
diff --git a/packages/core/src/opendirector/infrastructure/local_projects.py b/packages/core/src/opendirector/infrastructure/local_projects.py
index bac7d0ca..2e515e9d 100644
--- a/packages/core/src/opendirector/infrastructure/local_projects.py
+++ b/packages/core/src/opendirector/infrastructure/local_projects.py
@@ -123,7 +123,7 @@ def _open(self, db, project: dict, session: str):
         identifier(session)
         project_id = project["id"]
         if db.execute("SELECT 1 FROM sessions WHERE project_id=? AND id<>? AND expires>?", (project_id, session, time.time())).fetchone():
-            raise ProjectError("该项目正在其他窗口中打开，请先关闭后再试。", 409, code="storage_busy")
+            raise ProjectError("该项目正在其他窗口中打开，请先关闭后再试。", 409, code="project_open_elsewhere")
         lease = db.execute("SELECT path FROM sessions WHERE id=? AND project_id=? AND expires>?", (session, project_id, time.time())).fetchone()
         if lease and lease["path"] != project["path"]:
             raise ProjectError("项目目录已在应用外改名或移动，请复制未保存内容并刷新页面。", 409)
diff --git a/scripts/run-browser-suite.mjs b/scripts/run-browser-suite.mjs
index 70e56877..b64bb957 100644
--- a/scripts/run-browser-suite.mjs
+++ b/scripts/run-browser-suite.mjs
@@ -33,6 +33,7 @@ const workflows = [
     name: "project list overflow",
     script: "tests/browser/project_list_overflow.mjs",
   },
+  { name: "session lease", script: "tests/browser/session_lease.mjs" },
   { name: "mp4 export", script: "tests/browser/mp4_export.mjs" },
   { name: "editor save", script: "tests/browser/editor_save.mjs" },
   { name: "canvas crop", script: "tests/browser/canvas_crop.mjs" },
diff --git a/src/backend/api.ts b/src/backend/api.ts
index d83cf6f8..4d63e7dd 100644
--- a/src/backend/api.ts
+++ b/src/backend/api.ts
@@ -5,6 +5,75 @@ import { tNow } from "../i18n/messages.ts";
 
 export const sessionId = crypto.randomUUID();
 
+const SESSION_KEY = "opendirector.session.v1";
+const lockName = (id: string) => `opendirector.session.${id}`;
+
+interface SessionLocks {
+  request(name: string, callback: () => Promise<void>): Promise<void>;
+  query(): Promise<{ held?: { name?: string }[] }>;
+}
+
+/** Release this tab's previous page lease after a reload or crash.
+ *
+ * Each page load uses a new session. When the previous page could not send its
+ * release (crash, killed process, lost keepalive), its lease would otherwise
+ * block the reloaded tab as "another window" until it expires. The previous
+ * session is released only when no live page still holds its Web Lock, so a
+ * duplicated tab, which copies sessionStorage, never releases a live window.
+ * Without Web Locks nothing is released and the lease simply expires. */
+export async function releaseStaleSession(
+  previous: string | null,
+  current: string,
+  locks: SessionLocks | undefined,
+  release: (id: string) => Promise<unknown>,
+  pause = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms)),
+) {
+  if (!locks) return false;
+  // Hold this page's lock for its lifetime; the browser drops it on unload.
+  void locks.request(lockName(current), () => new Promise<void>(() => {})).catch(() => {});
+  if (!previous || previous === current) return false;
+  for (let attempt = 0; attempt < 8; attempt++) {
+    // A reloading page may still be tearing down; look again briefly.
+    if (attempt) await pause(250);
+    const state = await locks.query().catch(() => null);
+    if (!state) return false;
+    if (!state.held?.some((lock) => lock.name === lockName(previous))) {
+      await release(previous).catch(() => {});
+      return true;
+    }
+  }
+  return false;
+}
+
+function tabStorage(): Storage | null {
+  try {
+    return globalThis.sessionStorage ?? null;
+  } catch {
+    return null;
+  }
+}
+
+const previousSession = tabStorage()?.getItem(SESSION_KEY) ?? null;
+/** This page replaced an earlier page of the same tab (reload or reconnect). */
+export const reloadedTab = previousSession !== null;
+try {
+  tabStorage()?.setItem(SESSION_KEY, sessionId);
+} catch {
+  // Private modes may refuse storage; the lease then just expires.
+}
+
+/** Requests wait until a stale lease from this tab's previous page is gone. */
+const sessionReady = releaseStaleSession(
+  previousSession,
+  sessionId,
+  (globalThis.navigator as { locks?: SessionLocks } | undefined)?.locks,
+  (id) =>
+    fetch(`/api/creation/sessions/${id}`, {
+      method: "DELETE",
+      headers: { "X-OpenDirector-Request": "1" },
+    }),
+).catch(() => false);
+
 export class ApiError extends Error {
   status: number | null;
   code: string;
@@ -41,6 +110,7 @@ export async function request<T>(
   path: string,
   options: RequestInit = {},
 ): Promise<T> {
+  await sessionReady;
   let response: Response;
   try {
     response = await fetch(`/api${path}`, {
diff --git a/src/backend/creationClient.ts b/src/backend/creationClient.ts
index 6bfa53af..232bd37e 100644
--- a/src/backend/creationClient.ts
+++ b/src/backend/creationClient.ts
@@ -16,6 +16,7 @@ import {
   ApiError,
   headers,
   json,
+  reloadedTab,
   request,
   responseError,
   storageRequest,
@@ -160,13 +161,16 @@ export class CreationClient {
         this.install(canvas ? canvas.snapshot : (opened as ProjectSnapshot));
         break;
       } catch (error) {
-        // The old page's keepalive release can arrive just after a reload's
-        // first open. Genuine concurrent windows retain their lease protection.
+        // A storage lock timeout is retried. After a reload, the old page's
+        // keepalive release can also arrive just after this first open, so a
+        // reloaded tab briefly waits for its own lease too. A fresh window
+        // refused by another live window (`project_open_elsewhere`) fails at once.
         if (
           attempt >= 7 ||
           !(error instanceof ApiError) ||
           error.status !== 409 ||
-          error.code !== "storage_busy"
+          (error.code !== "storage_busy" &&
+            !(reloadedTab && error.code === "project_open_elsewhere"))
         )
           throw error;
         await new Promise((resolve) =>
diff --git a/tests/backendWorkspace.test.ts b/tests/backendWorkspace.test.ts
index 819dfc5c..b9b729b5 100644
--- a/tests/backendWorkspace.test.ts
+++ b/tests/backendWorkspace.test.ts
@@ -660,7 +660,7 @@ test("a long read that lost its lease fails instead of adopting it", async (t) =
       (options.method ?? "GET") === "PUT"
     )
       return Response.json(
-        { detail: "工程正在其他窗口中打开", code: "storage_busy" },
+        { detail: "工程正在其他窗口中打开", code: "project_open_elsewhere" },
         { status: 409 },
       );
     return plain(url, options);
diff --git a/tests/browser/manifest.json b/tests/browser/manifest.json
index a6e8c1ab..343f7569 100644
--- a/tests/browser/manifest.json
+++ b/tests/browser/manifest.json
@@ -340,6 +340,13 @@
         "production"
       ]
     },
+    {
+      "file": "session_lease.mjs",
+      "lanes": [
+        "gate"
+      ],
+      "note": "Reload after a lost lease release reopens at once; a second window is refused with project_open_elsewhere (#277)."
+    },
     {
       "file": "project_title_enter.mjs",
       "previewSafe": true,
diff --git a/tests/browser/session_lease.mjs b/tests/browser/session_lease.mjs
new file mode 100644
index 00000000..f1777268
--- /dev/null
+++ b/tests/browser/session_lease.mjs
@@ -0,0 +1,77 @@
+/** A tab reload after a lost lease release reopens its own project at once,
+ * while a second live window is still refused with an explicit message. */
+import assert from "node:assert/strict";
+import { launchBrowser } from "./engine.mjs";
+
+const base = process.env.CREATION_WEB_URL ?? "http://127.0.0.1:5179";
+const api = process.env.CREATION_API_URL ?? "http://127.0.0.1:8019";
+const headers = { "X-OpenDirector-Request": "1", "Content-Type": "application/json" };
+async function call(path, body) {
+  const response = await fetch(api + path, { headers, ...(body ? { method: "POST", body: JSON.stringify(body) } : {}) });
+  assert.ok(response.ok, await response.clone().text());
+  return response.json();
+}
+assert.equal((await call("/api/creation-test-health")).isolated, true);
+const saved = { "zh-CN": "已保存到本机", "en-US": "Saved on this device" };
+
+const browser = await launchBrowser();
+try {
+  for (const locale of ["en-US", "zh-CN"]) {
+    const id = crypto.randomUUID();
+    await call(`/api/creation/projects/${id}/commands`, { projectId: id, commandId: crypto.randomUUID(),
+      type: "project.create", payload: { name: `Session lease ${locale} ${id.slice(0, 8)}` } });
+    const url = `${base}/ai-tool/ai-canvas/${id}`;
+    const context = await browser.newContext({ locale, viewport: { width: 1280, height: 800 } });
+    await context.addInitScript((value) => {
+      localStorage.setItem("opendirector.modelSetup.v1", "dismissed");
+      localStorage.setItem("opendirector.locale.v1", value);
+    }, locale);
+    const page = await context.newPage();
+    page.setDefaultTimeout(20000);
+    const openStatuses = [];
+    page.on("console", (m) => { if (process.env.DEBUG && m.type() === "error") console.log("console:", m.text()); });
+    page.on("response", (response) => {
+      if (process.env.DEBUG && response.url().includes("/api/creation/") && !response.url().includes("/assets/")) console.log(response.request().method(), response.status(), response.url().slice(-60));
+      if (response.url().includes("/open-canvas")) openStatuses.push(response.status());
+    });
+    await page.goto(url);
+    await page.getByText(saved[locale], { exact: true }).waitFor({ state: "attached" });
+
+    // A lost pagehide release (crash, killed process): the old page's DELETE
+    // never leaves it. Only this page's fetch is replaced; the reload is fresh.
+    const stale = await page.evaluate(() => {
+      const original = window.fetch;
+      window.fetch = (input, init) =>
+        String(input).includes("/creation/sessions/") && init?.method === "DELETE"
+          ? Promise.reject(new TypeError("dropped"))
+          : original(input, init);
+      return sessionStorage.getItem("opendirector.session.v1");
+    });
+    assert.ok(stale, "the tab records its session");
+    if (process.env.DEBUG) console.log("--- reload");
+    const started = Date.now();
+    await page.reload();
+    // The old page's lease was still live, so the reload itself released it.
+    assert.notEqual(await page.evaluate(() => sessionStorage.getItem("opendirector.session.v1")), stale);
+    await page.getByText(saved[locale], { exact: true }).waitFor({ state: "attached" });
+    const elapsed = Date.now() - started;
+    assert.ok(elapsed < 15000, `reload reopened after ${elapsed} ms`);
+    assert.deepEqual(openStatuses.filter((status) => status === 409), [], `${locale}: reload must not be refused`);
+
+    // A genuinely separate live window still cannot open the same project.
+    const other = await context.newPage();
+    await other.addInitScript(() => sessionStorage.clear());
+    const refused = other.waitForResponse((response) => response.url().includes("/open-canvas"));
+    await other.goto(url);
+    const response = await refused;
+    assert.equal(response.status(), 409);
+    const body = await response.json();
+    assert.equal(body.code, "project_open_elsewhere");
+    if (locale === "en-US") assert.match(body.detail, /another window/);
+    else assert.match(body.detail, /其他窗口/);
+    console.log(`PASS ${locale}: reload reopened in ${elapsed} ms; second window refused: ${body.detail}`);
+    await context.close();
+  }
+} finally {
+  await browser.close();
+}
diff --git a/tests/sessionLease.test.ts b/tests/sessionLease.test.ts
new file mode 100644
index 00000000..581b842f
--- /dev/null
+++ b/tests/sessionLease.test.ts
@@ -0,0 +1,67 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { releaseStaleSession } from "../src/backend/api.ts";
+
+function locks(held: string[]) {
+  const requested: string[] = [];
+  return {
+    requested,
+    request: async (name: string) => {
+      requested.push(name);
+    },
+    query: async () => ({ held: held.map((name) => ({ name })) }),
+  };
+}
+const noPause = async () => {};
+
+test("a reloaded tab releases its previous page's unreleased lease", async () => {
+  const released: string[] = [];
+  const browser = locks([]);
+  assert.equal(
+    await releaseStaleSession("old", "new", browser, async (id) => released.push(id), noPause),
+    true,
+  );
+  assert.deepEqual(released, ["old"]);
+  assert.deepEqual(browser.requested, ["opendirector.session.new"]);
+});
+
+test("a duplicated tab never releases the lease of a live window", async () => {
+  const released: string[] = [];
+  const browser = locks(["opendirector.session.old"]);
+  assert.equal(
+    await releaseStaleSession("old", "new", browser, async (id) => released.push(id), noPause),
+    false,
+  );
+  assert.deepEqual(released, []);
+});
+
+test("a first load or a browser without Web Locks releases nothing", async () => {
+  const released: string[] = [];
+  const release = async (id: string) => released.push(id);
+  assert.equal(await releaseStaleSession(null, "new", locks([]), release, noPause), false);
+  assert.equal(await releaseStaleSession("old", "new", undefined, release, noPause), false);
+  assert.deepEqual(released, []);
+});
+
+test("a failed release does not block the page", async () => {
+  assert.equal(
+    await releaseStaleSession("old", "new", locks([]), async () => {
+      throw new Error("offline");
+    }, noPause),
+    true,
+  );
+});
+
+test("a previous page that is still unloading is released once its lock is gone", async () => {
+  const released: string[] = [];
+  let checks = 0;
+  const browser = {
+    request: async () => {},
+    query: async () => ({ held: ++checks < 3 ? [{ name: "opendirector.session.old" }] : [] }),
+  };
+  assert.equal(
+    await releaseStaleSession("old", "new", browser, async (id) => released.push(id), noPause),
+    true,
+  );
+  assert.deepEqual(released, ["old"]);
+});
diff --git a/tests/test_creation_storage.py b/tests/test_creation_storage.py
index 7dab2b13..f54fccae 100644
--- a/tests/test_creation_storage.py
+++ b/tests/test_creation_storage.py
@@ -193,8 +193,10 @@ def test_leases_expiry_external_moves_and_closed_rename_guards(projects):
     created = create(projects)
     session, other = str(uuid4()), str(uuid4())
     projects.open(created.projectId, session)
-    with pytest.raises(ProjectError, match='其他窗口'):
+    with pytest.raises(ProjectError, match='其他窗口') as busy:
         projects.open(created.projectId, other)
+    # Another window is not a lock timeout: it must not share the retryable code.
+    assert busy.value.code == 'project_open_elsewhere'
     with pytest.raises(ProjectError, match='关闭'):
         projects.execute(rename(created))
     (projects.root / 'Example').rename(projects.root / 'Moved outside app')
diff --git a/tests/test_localization.py b/tests/test_localization.py
index 121f8c53..deafa035 100644
--- a/tests/test_localization.py
+++ b/tests/test_localization.py
@@ -94,3 +94,10 @@ def test_streaming_and_background_jobs_use_the_same_language_rule() -> None:
         )
         == "The item changed or is busy. Reload it and try again."
     )
+
+
+def test_another_window_is_not_reported_as_busy_storage() -> None:
+    chinese = "该项目正在其他窗口中打开，请先关闭后再试。"
+    assert localized_error(request("zh-CN"), chinese=chinese, status=409, code="project_open_elsewhere") == chinese
+    english = localized_error(request("en-US"), chinese=chinese, status=409, code="project_open_elsewhere")
+    assert "another window" in english and "busy" not in english