diff --git a/src/filesystem/__tests__/lib.test.ts b/src/filesystem/__tests__/lib.test.ts
index e0ae61224f..6f7fde6307 100644
--- a/src/filesystem/__tests__/lib.test.ts
+++ b/src/filesystem/__tests__/lib.test.ts
@@ -303,10 +303,43 @@ describe('Lib Functions', () => {
     describe('writeFileContent', () => {
       it('writes file content', async () => {
         mockFs.writeFile.mockResolvedValueOnce(undefined);
-        
+        mockFs.stat.mockResolvedValueOnce({ size: Buffer.byteLength('new content', 'utf-8') } as any);
+
         await writeFileContent('/test/file.txt', 'new content');
-        
+
         expect(mockFs.writeFile).toHaveBeenCalledWith('/test/file.txt', 'new content', { encoding: "utf-8", flag: 'wx' });
+        expect(mockFs.stat).toHaveBeenCalledWith('/test/file.txt');
+      });
+
+      // Regression for #4138: write_file silently returns success on Windows
+      // even though no bytes reach disk. The post-write stat verification turns
+      // that silent failure into an explicit error.
+      it('throws when post-write stat finds the file missing (silent write failure)', async () => {
+        mockFs.writeFile.mockResolvedValueOnce(undefined);
+        const enoent = Object.assign(new Error('ENOENT: no such file or directory'), { code: 'ENOENT' });
+        mockFs.stat.mockRejectedValueOnce(enoent);
+
+        await expect(writeFileContent('/test/file.txt', 'payload')).rejects.toThrow(
+          /reported success but the file is missing on disk/
+        );
+      });
+
+      it('throws when post-write stat reports a size mismatch', async () => {
+        mockFs.writeFile.mockResolvedValueOnce(undefined);
+        // Content is 7 bytes ('payload'); pretend disk shows 0 bytes.
+        mockFs.stat.mockResolvedValueOnce({ size: 0 } as any);
+
+        await expect(writeFileContent('/test/file.txt', 'payload')).rejects.toThrow(
+          /on-disk size is 0 bytes; expected 7 bytes/
+        );
+      });
+
+      it('verifies size correctly for multibyte UTF-8 content', async () => {
+        const content = 'héllo 🌍'; // 12 UTF-8 bytes
+        mockFs.writeFile.mockResolvedValueOnce(undefined);
+        mockFs.stat.mockResolvedValueOnce({ size: Buffer.byteLength(content, 'utf-8') } as any);
+
+        await expect(writeFileContent('/test/file.txt', content)).resolves.toBeUndefined();
       });
     });
 
diff --git a/src/filesystem/lib.ts b/src/filesystem/lib.ts
index ce4af9f38a..c2978f7065 100644
--- a/src/filesystem/lib.ts
+++ b/src/filesystem/lib.ts
@@ -182,6 +182,28 @@ export async function writeFileContent(filePath: string, content: string): Promi
       throw error;
     }
   }
+
+  // Post-write verification: stat the target and confirm size matches the
+  // UTF-8 byte length of the written content. Catches silent write failures
+  // where writeFile resolves successfully but the bytes never reach disk
+  // (observed on Windows in #4138; the underlying cause varies by host but
+  // the failure mode — silent success with no file on disk — is the same).
+  const expectedSize = Buffer.byteLength(content, 'utf-8');
+  let stats: Awaited<ReturnType<typeof fs.stat>>;
+  try {
+    stats = await fs.stat(filePath);
+  } catch (statError) {
+    throw new Error(
+      `Write to ${filePath} reported success but the file is missing on disk. ` +
+      `This indicates a silent write failure in the host filesystem layer.`
+    );
+  }
+  if (stats.size !== expectedSize) {
+    throw new Error(
+      `Write to ${filePath} reported success but on-disk size is ${stats.size} bytes; ` +
+      `expected ${expectedSize} bytes.`
+    );
+  }
 }