From ef59c6675ebec030ca40be41f2760ce1abdff66a Mon Sep 17 00:00:00 2001
From: Zio Wang <zio.wang-m5@osl.com>
Date: Tue, 11 Aug 2026 17:34:54 +0800
Subject: [PATCH] fix(modernjs-v3): compute Content-Length in bytes for served
 bundles

---
 .../modernjs-v3-content-length-bytes.md       |  5 +++
 .../src/server/staticMiddleware.spec.ts       | 43 +++++++++++++++++++
 .../src/server/staticMiddleware.ts            |  5 ++-
 3 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/modernjs-v3-content-length-bytes.md

diff --git a/.changeset/modernjs-v3-content-length-bytes.md b/.changeset/modernjs-v3-content-length-bytes.md
new file mode 100644
index 00000000000..c49459c809b
--- /dev/null
+++ b/.changeset/modernjs-v3-content-length-bytes.md
@@ -0,0 +1,5 @@
+---
+'@module-federation/modern-js-v3': patch
+---
+
+Fix truncated remote chunks served from `/bundles` by computing `Content-Length` in bytes instead of characters. Chunks containing non-ASCII characters were cut short, causing `SyntaxError` when an SSR consumer evaluated them.
diff --git a/packages/modernjs-v3/src/server/staticMiddleware.spec.ts b/packages/modernjs-v3/src/server/staticMiddleware.spec.ts
index d37cd778b61..ea576ec9911 100644
--- a/packages/modernjs-v3/src/server/staticMiddleware.spec.ts
+++ b/packages/modernjs-v3/src/server/staticMiddleware.spec.ts
@@ -193,6 +193,49 @@ describe('staticMiddleware', () => {
       expect(result).toBe('empty-response');
       expect(nextSpy).not.toHaveBeenCalled();
     });
+
+    it('should set Content-Length in bytes for non-ASCII content', async () => {
+      // Multi-byte characters make byte length differ from string length.
+      // Using the string length here truncates the response body.
+      const mockFileContent = 'console.log("中文注释");';
+      const mockFileResult = {
+        content: mockFileContent,
+        lastModified: Date.now(),
+      };
+
+      mockContext.req.path = '/bundles/non-ascii.js';
+      (access as any).mockResolvedValue(undefined);
+      (fileCache.getFile as any).mockResolvedValue(mockFileResult);
+
+      await middleware(mockContext, nextSpy);
+
+      expect(Buffer.byteLength(mockFileContent)).toBeGreaterThan(
+        mockFileContent.length,
+      );
+      expect(mockContext.header).toHaveBeenCalledWith(
+        'Content-Length',
+        String(Buffer.byteLength(mockFileContent)),
+      );
+    });
+
+    it('should set Content-Length in bytes for ASCII content', async () => {
+      const mockFileContent = 'console.log("ascii only");';
+      const mockFileResult = {
+        content: mockFileContent,
+        lastModified: Date.now(),
+      };
+
+      mockContext.req.path = '/bundles/ascii.js';
+      (access as any).mockResolvedValue(undefined);
+      (fileCache.getFile as any).mockResolvedValue(mockFileResult);
+
+      await middleware(mockContext, nextSpy);
+
+      expect(mockContext.header).toHaveBeenCalledWith(
+        'Content-Length',
+        String(mockFileContent.length),
+      );
+    });
   });
 
   describe('asset prefix handling', () => {
diff --git a/packages/modernjs-v3/src/server/staticMiddleware.ts b/packages/modernjs-v3/src/server/staticMiddleware.ts
index fd0b909fc54..37cd175802b 100644
--- a/packages/modernjs-v3/src/server/staticMiddleware.ts
+++ b/packages/modernjs-v3/src/server/staticMiddleware.ts
@@ -63,7 +63,10 @@ const createStaticMiddleware = (options: {
     }
 
     c.header('Content-Type', 'application/javascript');
-    c.header('Content-Length', String(fileResult.content.length));
+    // The file is read as a UTF-8 string, so `content.length` is the number of
+    // characters. Content-Length must be the number of bytes, otherwise any
+    // chunk containing non-ASCII characters is truncated by the difference.
+    c.header('Content-Length', String(Buffer.byteLength(fileResult.content)));
     return c.body(fileResult.content, 200);
   };
 };