diff --git a/README.md b/README.md
index 4d288f9..7515b0b 100644
--- a/README.md
+++ b/README.md
@@ -253,15 +253,20 @@ await crypto.decryptMessage(...)
 import { downloadUnsealed } from "@yeez-tech/meta-encryptor";
 
 await downloadUnsealed({
-  url: "https://example.com/encrypted file",
-  privateKeyHex: "YOUR_PRIVATE_KEY_HEX",
+  url: "https://example.com/encrypted-file",
+  privateKey: "YOUR_PRIVATE_KEY_HEX",
   filename: "decrypted-file.txt",
-  progressHandler: (totalItems, processedItems, readBytes, writeBytes) => {
-    console.log(`Progress: ${processedItems}/${totalItems}`);
+  onProgress: (total, processed, readBytes, writeBytes) => {
+    console.log(`Progress: ${processed}/${total}`);
   },
+  // 可选：覆盖明文大小上限（字节）；默认桌面 1 GiB、移动端 200 MiB
+  // desktopLimit: 2 * 1024 * 1024 * 1024,
+  // mobileLimit: 500 * 1024 * 1024,
 });
 ```
 
+更完整的参数说明见 [`src/browser/README.md`](./src/browser/README.md)。
+
 ##### sealedFileVersion
 
 返回封装文件的版本号。
diff --git a/jest.config.browser.mjs b/jest.config.browser.mjs
index 8a05d22..87e41c8 100644
--- a/jest.config.browser.mjs
+++ b/jest.config.browser.mjs
@@ -7,6 +7,7 @@ export default {
     "<rootDir>/test/downloadFunctions.spec.mjs",
     "<rootDir>/test/downloadCallbacks.spec.mjs",
     "<rootDir>/test/downloadUnsealedCallbacks.spec.mjs",
+    "<rootDir>/test/downloadUnsealedSizeLimit.spec.mjs",
     "<rootDir>/test/progress.spec.mjs",
   ],
   
diff --git a/package.json b/package.json
index af974fe..c04e4d9 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@yeez-tech/meta-encryptor",
-    "version": "5.0.7",
+    "version": "5.1.0",
     "description": "Data Seal/Unseal for Fidelius",
     "main": "./build/commonjs/index.node.cjs",
     "module": "./build/es/index.browser.js",
diff --git a/src/browser/README.md b/src/browser/README.md
index f30d979..08b98ac 100644
--- a/src/browser/README.md
+++ b/src/browser/README.md
@@ -50,6 +50,10 @@ npm install @yeez-tech/meta-encryptor
 | `onDownloadReady` | function | ❌ | 下载就绪回调：HTTP 流首个数据块进入管道时触发（HEAD+tail 完成后），可用于关闭准备蒙层 |
 | `onSuccess`  | function | ❌   | 成功回调 `(data: { filename }) => void`                      |
 | `onError`    | function | ❌   | 错误回调 `(error: Error) => void`                            |
+| `desktopLimit` | number | ❌ | 桌面端明文大小上限（字节），默认 `1 GiB`（`1024 * 1024 * 1024`） |
+| `mobileLimit`  | number | ❌ | 移动端明文大小上限（字节），默认 `200 MiB`（`200 * 1024 * 1024`） |
+
+超过对应上限时抛出 `MetaEncryptorError`（`code: ERR_FILE_TOO_LARGE`）。不传或非法值时回退到平台默认值。
 
 #### 返回值
 
@@ -88,6 +92,19 @@ try {
 
 ## 使用示例
 
+### 自定义大小上限
+
+```javascript
+await downloadUnsealed({
+  url,
+  privateKey,
+  filename,
+  // 可选：覆盖默认上限（字节）
+  desktopLimit: 2 * 1024 * 1024 * 1024, // 桌面 2 GiB
+  mobileLimit: 500 * 1024 * 1024,       // 移动端 500 MiB
+});
+```
+
 ### 示例 1：基本使用
 
 ```javascript
diff --git a/src/browser/downloadUnsealed.js b/src/browser/downloadUnsealed.js
index 9352a89..b395c16 100644
--- a/src/browser/downloadUnsealed.js
+++ b/src/browser/downloadUnsealed.js
@@ -88,6 +88,8 @@ async function inspectSealed(url, log) {
  * @param {Function} [options.onDownloadReady] - HTTP 流首个数据块进入管道时触发（可关蒙层）
  * @param {Function} [options.onSuccess]
  * @param {Function} [options.onError]
+ * @param {number} [options.desktopLimit] - desktop plaintext size limit in bytes (default 1 GiB)
+ * @param {number} [options.mobileLimit] - mobile plaintext size limit in bytes (default 200 MiB)
  * @returns {Promise<void>}
  */
 export async function downloadUnsealed({
@@ -98,7 +100,11 @@ export async function downloadUnsealed({
   onProgress,
   onDownloadReady,
   onSuccess,
-  onError
+  onError,
+  /** Override desktop plaintext size limit (bytes). Default 1 GiB. */
+  desktopLimit,
+  /** Override mobile plaintext size limit (bytes). Default 200 MiB. */
+  mobileLimit,
 }) {
   const log = makeLogger(onLog);
   const key = privateKey.trim()
@@ -116,7 +122,10 @@ export async function downloadUnsealed({
     log(`Plaintext size=${meta.plaintextSize} bytes, sealed=${meta.sealedContentSize} bytes, totalSize=${meta.totalSize}`);
 
     const mobile = isMobile()
-    const limit = mobile ? MOBILE_LIMIT : DESKTOP_LIMIT
+    const configured = mobile ? mobileLimit : desktopLimit
+    const platformDefault = mobile ? MOBILE_LIMIT : DESKTOP_LIMIT
+    const limit =
+      typeof configured === 'number' && configured > 0 ? configured : platformDefault
     const mode = mobile ? 'mobile' : 'desktop'
     const ua = typeof navigator !== 'undefined' ? navigator.userAgent : 'n/a';
     log(`Detected ${mode} (ua=${ua}), limit ${(limit / 1024 / 1024).toFixed(0)} MB`);
diff --git a/src/browser/stream_download.js b/src/browser/stream_download.js
index 94eb67c..a5d7949 100644
--- a/src/browser/stream_download.js
+++ b/src/browser/stream_download.js
@@ -3,6 +3,8 @@ import { HttpSealedFileStream } from './HttpSealedFileStream.js'
 import { MetaEncryptorError } from '../common/errors.js';
 import { createProgressTransformer, createDownloadReadyTransformer } from '../common/progress.js';
 
+const STREAMSAVER_LOAD_TIMEOUT_MS = 8000;
+
 async function ensureStreamSaver(log) {
   if (typeof window === 'undefined') return null;
   if (window.streamSaver?.createWriteStream) return window.streamSaver;
@@ -12,8 +14,9 @@ async function ensureStreamSaver(log) {
     await new Promise((resolve, reject) => {
       const s = document.createElement('script');
       s.src = 'https://cdn.jsdelivr.net/npm/streamsaver@2.0.3/StreamSaver.min.js';
-      s.onload = resolve;
-      s.onerror = reject;
+      const timer = setTimeout(() => reject(new Error('StreamSaver load timeout')), STREAMSAVER_LOAD_TIMEOUT_MS);
+      s.onload = () => { clearTimeout(timer); resolve(); };
+      s.onerror = (err) => { clearTimeout(timer); reject(err); };
       document.head.appendChild(s);
     });
     if (window.streamSaver?.createWriteStream) {
diff --git a/src/common/locale.js b/src/common/locale.js
index 0e6f960..5a1772b 100644
--- a/src/common/locale.js
+++ b/src/common/locale.js
@@ -1,5 +1,5 @@
-import en from '../locales/en.json';
-import zhCN from '../locales/zh-CN.json';
+import en from '../locales/en.json' with { type: 'json' };
+import zhCN from '../locales/zh-CN.json' with { type: 'json' };
 
 const _bundled = { en, 'zh-CN': zhCN };
 let _messages = null;
diff --git a/test/downloadFunctions.spec.mjs b/test/downloadFunctions.spec.mjs
index b935d21..dc797e6 100644
--- a/test/downloadFunctions.spec.mjs
+++ b/test/downloadFunctions.spec.mjs
@@ -62,7 +62,7 @@ describe('blobDownloadAndDecrypt', () => {
       );
 
       expect(result).toEqual({ ok: true });
-      expect(logs.some(m => m.includes('下载完成'))).toBe(true);
+      expect(logs.some(m => m.includes('Download complete'))).toBe(true);
       expect(blobContent).not.toBeNull();
     } finally {
       URL.createObjectURL = origCOU;
@@ -80,7 +80,7 @@ describe('blobDownloadAndDecrypt', () => {
     await expect(
       blobDownloadAndDecrypt('http://mock/file', 'a'.repeat(64), 'out.bin', { log: (m) => logs.push(m) })
     ).rejects.toThrow('fetch fail');
-    expect(logs.some(m => m.includes('失败'))).toBe(true);
+    expect(logs.some(m => /failed/i.test(m))).toBe(true);
   });
 });
 
@@ -98,16 +98,30 @@ describe('streamDownloadAndDecrypt', () => {
     );
 
     expect(result).toEqual({ ok: true });
-    expect(logs.some(m => m.includes('下载完成'))).toBe(true);
+    expect(logs.some(m => m.includes('Download complete'))).toBe(true);
     expect(chunks.length).toBe(1);
     expect(new TextDecoder().decode(chunks[0])).toBe('hello from mock');
   });
 
   test('throws when no writable and no StreamSaver', async () => {
+    // jsdom does not load external <script> tags; force onerror so we do not hang.
+    const appendChild = document.head.appendChild.bind(document.head);
+    document.head.appendChild = (node) => {
+      if (node?.tagName === 'SCRIPT') {
+        queueMicrotask(() => node.onerror?.(new Event('error')));
+        return node;
+      }
+      return appendChild(node);
+    };
+
     const logs = [];
-    await expect(
-      streamDownloadAndDecrypt('http://mock/file', 'a'.repeat(64), 'out.bin', { log: (m) => logs.push(m) })
-    ).rejects.toThrow('StreamSaver');
-    expect(logs.some(m => m.includes('失败'))).toBe(true);
+    try {
+      await expect(
+        streamDownloadAndDecrypt('http://mock/file', 'a'.repeat(64), 'out.bin', { log: (m) => logs.push(m) })
+      ).rejects.toMatchObject({ code: 'ERR_NO_STREAM_WRITABLE' });
+      expect(logs.some(m => /failed/i.test(m))).toBe(true);
+    } finally {
+      document.head.appendChild = appendChild;
+    }
   });
 });
diff --git a/test/downloadUnsealedSizeLimit.spec.mjs b/test/downloadUnsealedSizeLimit.spec.mjs
new file mode 100644
index 0000000..afe4986
--- /dev/null
+++ b/test/downloadUnsealedSizeLimit.spec.mjs
@@ -0,0 +1,112 @@
+/**
+ * downloadUnsealed size-limit overrides.
+ *
+ *   node --experimental-vm-modules node_modules/.bin/jest --config jest.config.browser.mjs test/downloadUnsealedSizeLimit.spec.mjs
+ */
+import { jest } from '@jest/globals';
+import { webcrypto as nodeWebcrypto } from 'crypto';
+import { HeaderSize } from '../src/common/limits.js';
+
+globalThis.crypto = nodeWebcrypto;
+
+jest.unstable_mockModule('../src/browser/stream_download.js', () => ({
+  streamDownloadAndDecrypt: jest.fn(async () => {}),
+}));
+jest.unstable_mockModule('../src/browser/blob_download.js', () => ({
+  blobDownloadAndDecrypt: jest.fn(async () => {}),
+}));
+
+const { downloadUnsealed } = await import('../src/browser/downloadUnsealed.js');
+const { MetaEncryptorError } = await import('../src/common/errors.js');
+const { streamDownloadAndDecrypt } = await import('../src/browser/stream_download.js');
+
+function makeInspectDiskBuf(contentBytes = 200) {
+  const header = Buffer.alloc(HeaderSize);
+  Buffer.from('1fe2ef7f3ed18847', 'hex').copy(header, 0);
+  header.writeUInt32LE(2, 8);
+  header.writeUInt32LE(0, 16);
+  header.writeUInt32LE(1, 24);
+  return Buffer.concat([Buffer.alloc(contentBytes, 0xab), header]);
+}
+
+function createMockFetch(diskBuf) {
+  const totalSize = diskBuf.length;
+  return jest.fn(async (url, init = {}) => {
+    if (init.method === 'HEAD') {
+      return {
+        ok: true,
+        status: 200,
+        url,
+        headers: {
+          get: (k) => {
+            const key = String(k).toLowerCase();
+            if (key === 'content-length') return String(totalSize);
+            if (key === 'accept-ranges') return 'bytes';
+            return null;
+          },
+        },
+      };
+    }
+    const range = init.headers?.Range || init.headers?.range || '';
+    const m = /bytes=(\d+)-(\d+)/.exec(range);
+    const start = m ? Number(m[1]) : 0;
+    const end = m ? Number(m[2]) : totalSize - 1;
+    const slice = diskBuf.subarray(start, end + 1);
+    return {
+      ok: true,
+      status: 206,
+      url,
+      headers: {
+        get: (k) => {
+          const key = String(k).toLowerCase();
+          if (key === 'content-length') return String(slice.length);
+          if (key === 'content-range') return `bytes ${start}-${end}/${totalSize}`;
+          return null;
+        },
+      },
+      arrayBuffer: async () => slice.buffer.slice(slice.byteOffset, slice.byteOffset + slice.byteLength),
+    };
+  });
+}
+
+describe('downloadUnsealed size limit config', () => {
+  const key = 'a'.repeat(64);
+
+  beforeEach(() => {
+    streamDownloadAndDecrypt.mockClear();
+    Object.defineProperty(globalThis, 'navigator', {
+      value: { userAgent: 'Mozilla/5.0 (Macintosh)' },
+      configurable: true,
+    });
+  });
+
+  test('rejects when plaintext exceeds default desktop 1GiB', async () => {
+    // plaintextSize = sealedContentSize - itemNumber*100
+    // sealedContentSize = totalSize - HeaderSize - blockNumber*BlockInfoSize
+    // Make a large content so plaintextSize > 1GiB is hard in unit test;
+    // instead spy by using a tiny custom desktopLimit.
+    const diskBuf = makeInspectDiskBuf(500);
+    globalThis.fetch = createMockFetch(diskBuf);
+    await expect(
+      downloadUnsealed({
+        url: 'https://example.com/file',
+        privateKey: key,
+        filename: 'out.bin',
+        desktopLimit: 1, // 1 byte — force reject
+      })
+    ).rejects.toMatchObject({ code: 'ERR_FILE_TOO_LARGE' });
+    expect(streamDownloadAndDecrypt).not.toHaveBeenCalled();
+  });
+
+  test('allows oversized (vs default) when desktopLimit raised', async () => {
+    const diskBuf = makeInspectDiskBuf(500);
+    globalThis.fetch = createMockFetch(diskBuf);
+    await downloadUnsealed({
+      url: 'https://example.com/file',
+      privateKey: key,
+      filename: 'out.bin',
+      desktopLimit: 10 * 1024 * 1024 * 1024,
+    });
+    expect(streamDownloadAndDecrypt).toHaveBeenCalled();
+  });
+});