fix(core): preserve escape sequences in string literals for modern models (#28299)

Co-authored-by: David Pierce <davidapierce@google.com>

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

@@ -85,7 +85,7 @@ const mockConfigInternal = {
   getIdeMode: vi.fn(() => false),
   getWorkspaceContext: () => new WorkspaceContext(rootDir, [plansDir]),
   getApiKey: () => 'test-key',
-  getModel: () => 'test-model',
+  getModel: () => 'gemini-1.5-flash',
   getSandbox: () => false,
   getDebugMode: () => false,
   getQuestion: () => undefined,
@@ -107,7 +107,7 @@ const mockConfigInternal = {
   isInteractive: () => false,
   getDisableLLMCorrection: vi.fn(() => true),
   isPlanMode: vi.fn(() => false),
-  getActiveModel: () => 'test-model',
+  getActiveModel: () => 'gemini-1.5-flash',
   storage: {
     getProjectTempDir: vi.fn().mockReturnValue('/tmp/project'),
   },

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

@@ -55,7 +55,12 @@ import { WRITE_FILE_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
 import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
 import { resolveAndValidatePlanPath } from '../utils/planUtils.js';
-import { isGemini3Model } from '../config/models.js';
+import {
+  isGemini3Model,
+  isGemini2Model,
+  isCustomModel,
+  resolveModel,
+} from '../config/models.js';
 import { discoverJitContext, appendJitContext } from './jit-context.js';
 
 /**
@@ -193,7 +198,13 @@ export async function getCorrectedFileContent(
     }
   }
 
-  const aggressiveUnescape = !isGemini3Model(config.getActiveModel());
+  const activeModel = config.getActiveModel();
+  const resolvedModel = resolveModel(activeModel, false, false, true, config);
+
+  const aggressiveUnescape =
+    !isGemini3Model(resolvedModel, config) &&
+    !isGemini2Model(resolvedModel) &&
+    !isCustomModel(resolvedModel, config);
 
   correctedContent = await ensureCorrectFileContent(
     proposedContent,

packages/core/src/utils/editCorrector.test.ts

@@ -245,5 +245,20 @@ describe('editCorrector', () => {
       expect(result).toBe(content);
       expect(mockGenerateJson).not.toHaveBeenCalled();
     });
+
+    it('should preserve \\n inside string literals even when aggressiveUnescape is false (b-496211054)', async () => {
+      const content =
+        'fmt.Printf("OpenFile with FailIfExists failed: %v\\n", err)';
+
+      const result = await ensureCorrectFileContent(
+        content,
+        mockBaseLlmClientInstance,
+        abortSignal,
+        true, // disableLLMCorrection
+        false, // aggressiveUnescape (now false for Gemini 2.5/3.x/Custom)
+      );
+
+      expect(result).toBe(content);
+    });
   });
 });