feat: preserve EOL in files (#16087)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Adib234 <30782825+Adib234@users.noreply.github.com>
Co-authored-by: Jack Wotherspoon <jackwoth@google.com>

packages/core/src/tools/write-file.ts

@@ -7,6 +7,7 @@
 import fs from 'node:fs';
 import fsPromises from 'node:fs/promises';
 import path from 'node:path';
+import os from 'node:os';
 import * as Diff from 'diff';
 import { WRITE_FILE_TOOL_NAME } from './tool-names.js';
 import type { Config } from '../config/config.js';
@@ -33,6 +34,7 @@ import {
   ensureCorrectEdit,
   ensureCorrectFileContent,
 } from '../utils/editCorrector.js';
+import { detectLineEnding } from '../utils/textUtils.js';
 import { DEFAULT_DIFF_OPTIONS, getDiffStat } from './diffOptions.js';
 import type {
   ModifiableDeclarativeTool,
@@ -301,9 +303,19 @@ class WriteFileToolInvocation extends BaseToolInvocation<
         await fsPromises.mkdir(dirName, { recursive: true });
       }
 
+      let finalContent = fileContent;
+      const useCRLF =
+        !isNewFile && originalContent
+          ? detectLineEnding(originalContent) === '\r\n'
+          : os.EOL === '\r\n';
+
+      if (useCRLF) {
+        finalContent = finalContent.replace(/\r?\n/g, '\r\n');
+      }
+
       await this.config
         .getFileSystemService()
-        .writeTextFile(this.resolvedPath, fileContent);
+        .writeTextFile(this.resolvedPath, finalContent);
 
       // Generate diff for display result
       const fileName = path.basename(this.resolvedPath);