fix: Address review comments for BOM encoding support

- Edit tool now respects defaultFileEncoding for new files
- Edit tool preserves BOM character for existing files without re-adding
- AcpFileSystemService detects BOM through ACP client with fallback
- Use line: null, limit: 1 for efficient BOM detection
- Add unit tests for AcpFileSystemService.detectFileBOM
- Add unit tests for EditTool BOM handling

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/acp-integration/service/filesystem.test.ts

@@ -17,6 +17,93 @@ const createFallback = (): FileSystemService => ({
 });
 
 describe('AcpFileSystemService', () => {
+  describe('detectFileBOM', () => {
+    it('detects BOM through ACP client when content starts with U+FEFF', async () => {
+      const client = {
+        readTextFile: vi
+          .fn()
+          .mockResolvedValue({ content: '\ufeff// BOM file' }),
+      } as unknown as import('../acp.js').Client;
+
+      const svc = new AcpFileSystemService(
+        client,
+        'session-1',
+        { readTextFile: true, writeTextFile: true },
+        createFallback(),
+      );
+
+      const result = await svc.detectFileBOM('/test/file.txt');
+      expect(result).toBe(true);
+      expect(client.readTextFile).toHaveBeenCalledWith({
+        path: '/test/file.txt',
+        sessionId: 'session-1',
+        line: null,
+        limit: 1,
+      });
+    });
+
+    it('detects no BOM through ACP client when content does not start with U+FEFF', async () => {
+      const client = {
+        readTextFile: vi.fn().mockResolvedValue({ content: '// No BOM file' }),
+      } as unknown as import('../acp.js').Client;
+
+      const svc = new AcpFileSystemService(
+        client,
+        'session-2',
+        { readTextFile: true, writeTextFile: true },
+        createFallback(),
+      );
+
+      const result = await svc.detectFileBOM('/test/file.txt');
+      expect(result).toBe(false);
+    });
+
+    it('falls back to local filesystem when ACP client fails', async () => {
+      const client = {
+        readTextFile: vi.fn().mockRejectedValue(new Error('Network error')),
+      } as unknown as import('../acp.js').Client;
+
+      const fallback = createFallback();
+      (fallback.detectFileBOM as ReturnType<typeof vi.fn>).mockResolvedValue(
+        true,
+      );
+
+      const svc = new AcpFileSystemService(
+        client,
+        'session-3',
+        { readTextFile: true, writeTextFile: true },
+        fallback,
+      );
+
+      const result = await svc.detectFileBOM('/test/file.txt');
+      expect(result).toBe(true);
+      expect(fallback.detectFileBOM).toHaveBeenCalledWith('/test/file.txt');
+    });
+
+    it('falls back to local filesystem when readTextFile capability is disabled', async () => {
+      const client = {
+        readTextFile: vi.fn(),
+      } as unknown as import('../acp.js').Client;
+
+      const fallback = createFallback();
+      (fallback.detectFileBOM as ReturnType<typeof vi.fn>).mockResolvedValue(
+        false,
+      );
+
+      const svc = new AcpFileSystemService(
+        client,
+        'session-4',
+        { readTextFile: false, writeTextFile: true },
+        fallback,
+      );
+
+      const result = await svc.detectFileBOM('/test/file.txt');
+      expect(result).toBe(false);
+      expect(fallback.detectFileBOM).toHaveBeenCalledWith('/test/file.txt');
+      expect(client.readTextFile).not.toHaveBeenCalled();
+    });
+  });
+
   describe('readTextFile ENOENT handling', () => {
     it('converts RESOURCE_NOT_FOUND error to ENOENT', async () => {
       const resourceNotFoundError = {

packages/cli/src/acp-integration/service/filesystem.ts

@@ -74,7 +74,22 @@ export class AcpFileSystemService implements FileSystemService {
   }
 
   async detectFileBOM(filePath: string): Promise<boolean> {
-    // Always use fallback for BOM detection
+    // Try to detect BOM through ACP client first by reading first line
+    if (this.capabilities.readTextFile) {
+      try {
+        const response = await this.client.readTextFile({
+          path: filePath,
+          sessionId: this.sessionId,
+          line: null,
+          limit: 1,
+        });
+        // Check if content starts with BOM character (U+FEFF)
+        return response.content.charCodeAt(0) === 0xfeff;
+      } catch {
+        // Fall through to fallback if ACP read fails
+      }
+    }
+    // Fall back to local filesystem detection
     return this.fallback.detectFileBOM(filePath);
   }