fix(core): prevent omission placeholder deletions in replace/write_file (#19870)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/core/src/tools/definitions/__snapshots__/coreToolsModelSnapshots.test.ts.snap

@@ -547,7 +547,7 @@ A good instruction should concisely answer:
         "type": "string",
       },
       "new_string": {
-        "description": "The exact literal text to replace \`old_string\` with, preferably unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic.",
+        "description": "The exact literal text to replace \`old_string\` with, preferably unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide exact literal code.",
         "type": "string",
       },
       "old_string": {
@@ -665,7 +665,7 @@ exports[`coreTools snapshots for specific models > Model: gemini-2.5-pro > snaps
   "parametersJsonSchema": {
     "properties": {
       "content": {
-        "description": "The content to write to the file.",
+        "description": "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content.",
         "type": "string",
       },
       "file_path": {
@@ -1312,7 +1312,7 @@ The user has the ability to modify the \`new_string\` content. If modified, this
         "type": "string",
       },
       "new_string": {
-        "description": "The exact literal text to replace \`old_string\` with, unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic.",
+        "description": "The exact literal text to replace \`old_string\` with, unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide exact literal code.",
         "type": "string",
       },
       "old_string": {
@@ -1429,7 +1429,7 @@ The user has the ability to modify \`content\`. If modified, this will be stated
   "parametersJsonSchema": {
     "properties": {
       "content": {
-        "description": "The content to write to the file.",
+        "description": "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content.",
         "type": "string",
       },
       "file_path": {

packages/core/src/tools/definitions/model-family-sets/default-legacy.ts

@@ -71,7 +71,8 @@ export const DEFAULT_LEGACY_SET: CoreToolSet = {
           type: 'string',
         },
         content: {
-          description: 'The content to write to the file.',
+          description:
+            "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content.",
           type: 'string',
         },
       },
@@ -332,7 +333,7 @@ A good instruction should concisely answer:
         },
         new_string: {
           description:
-            'The exact literal text to replace `old_string` with, preferably unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic.',
+            "The exact literal text to replace `old_string` with, preferably unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide exact literal code.",
           type: 'string',
         },
         expected_replacements: {

packages/core/src/tools/definitions/model-family-sets/gemini-3.ts

@@ -73,7 +73,8 @@ The user has the ability to modify \`content\`. If modified, this will be stated
           type: 'string',
         },
         content: {
-          description: 'The content to write to the file.',
+          description:
+            "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content.",
           type: 'string',
         },
       },
@@ -310,7 +311,7 @@ The user has the ability to modify the \`new_string\` content. If modified, this
         },
         new_string: {
           description:
-            'The exact literal text to replace `old_string` with, unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic.',
+            "The exact literal text to replace `old_string` with, unescaped. Provide the EXACT text. Ensure the resulting code is correct and idiomatic. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide exact literal code.",
           type: 'string',
         },
         expected_replacements: {

packages/core/src/tools/edit.test.ts

@@ -607,6 +607,53 @@ function doIt() {
       };
       expect(tool.validateToolParams(params)).toMatch(/Path not in workspace/);
     });
+
+    it('should reject omission placeholder in new_string when old_string does not contain that placeholder', () => {
+      const params: EditToolParams = {
+        file_path: path.join(rootDir, 'test.txt'),
+        instruction: 'An instruction',
+        old_string: 'old content',
+        new_string: '(rest of methods ...)',
+      };
+      expect(tool.validateToolParams(params)).toBe(
+        "`new_string` contains an omission placeholder (for example 'rest of methods ...'). Provide exact literal replacement text.",
+      );
+    });
+
+    it('should reject new_string when it contains an additional placeholder not present in old_string', () => {
+      const params: EditToolParams = {
+        file_path: path.join(rootDir, 'test.txt'),
+        instruction: 'An instruction',
+        old_string: '(rest of methods ...)',
+        new_string: `(rest of methods ...)
+(unchanged code ...)`,
+      };
+      expect(tool.validateToolParams(params)).toBe(
+        "`new_string` contains an omission placeholder (for example 'rest of methods ...'). Provide exact literal replacement text.",
+      );
+    });
+
+    it('should allow omission placeholders when all are already present in old_string', () => {
+      const params: EditToolParams = {
+        file_path: path.join(rootDir, 'test.txt'),
+        instruction: 'An instruction',
+        old_string: `(rest of methods ...)
+(unchanged code ...)`,
+        new_string: `(unchanged code ...)
+(rest of methods ...)`,
+      };
+      expect(tool.validateToolParams(params)).toBeNull();
+    });
+
+    it('should allow normal code that contains placeholder text in a string literal', () => {
+      const params: EditToolParams = {
+        file_path: path.join(rootDir, 'test.ts'),
+        instruction: 'Update string literal',
+        old_string: 'const msg = "old";',
+        new_string: 'const msg = "(rest of methods ...)";',
+      };
+      expect(tool.validateToolParams(params)).toBeNull();
+    });
   });
 
   describe('execute', () => {

packages/core/src/tools/edit.ts

@@ -54,6 +54,7 @@ import { debugLogger } from '../utils/debugLogger.js';
 import levenshtein from 'fast-levenshtein';
 import { EDIT_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
+import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
 
 const ENABLE_FUZZY_MATCH_RECOVERY = true;
 const FUZZY_MATCH_THRESHOLD = 0.1; // Allow up to 10% weighted difference
@@ -973,6 +974,19 @@ export class EditTool
     }
     params.file_path = filePath;
 
+    const newPlaceholders = detectOmissionPlaceholders(params.new_string);
+    if (newPlaceholders.length > 0) {
+      const oldPlaceholders = new Set(
+        detectOmissionPlaceholders(params.old_string),
+      );
+
+      for (const placeholder of newPlaceholders) {
+        if (!oldPlaceholders.has(placeholder)) {
+          return "`new_string` contains an omission placeholder (for example 'rest of methods ...'). Provide exact literal replacement text.";
+        }
+      }
+    }
+
     return this.config.validatePathAccess(params.file_path);
   }
 

packages/core/src/tools/omissionPlaceholderDetector.test.ts

@@ -0,0 +1,63 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect, it } from 'vitest';
+import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
+
+describe('detectOmissionPlaceholders', () => {
+  it('detects standalone placeholder lines', () => {
+    expect(detectOmissionPlaceholders('(rest of methods ...)')).toEqual([
+      'rest of methods ...',
+    ]);
+    expect(detectOmissionPlaceholders('(rest of code ...)')).toEqual([
+      'rest of code ...',
+    ]);
+    expect(detectOmissionPlaceholders('(unchanged code ...)')).toEqual([
+      'unchanged code ...',
+    ]);
+    expect(detectOmissionPlaceholders('// rest of methods ...')).toEqual([
+      'rest of methods ...',
+    ]);
+  });
+
+  it('detects case-insensitive placeholders', () => {
+    expect(detectOmissionPlaceholders('(Rest Of Methods ...)')).toEqual([
+      'rest of methods ...',
+    ]);
+  });
+
+  it('detects multiple placeholder lines in one input', () => {
+    const text = `class Example {
+  run() {}
+  (rest of methods ...)
+  (unchanged code ...)
+}`;
+    expect(detectOmissionPlaceholders(text)).toEqual([
+      'rest of methods ...',
+      'unchanged code ...',
+    ]);
+  });
+
+  it('does not detect placeholders embedded in normal code', () => {
+    expect(
+      detectOmissionPlaceholders(
+        'const note = "(rest of methods ...)";\nconsole.log(note);',
+      ),
+    ).toEqual([]);
+  });
+
+  it('does not detect omission phrase when inline in a comment', () => {
+    expect(
+      detectOmissionPlaceholders('return value; // rest of methods ...'),
+    ).toEqual([]);
+  });
+
+  it('does not detect unrelated ellipsis text', () => {
+    expect(detectOmissionPlaceholders('const message = "loading...";')).toEqual(
+      [],
+    );
+  });
+});

packages/core/src/tools/omissionPlaceholderDetector.ts

@@ -0,0 +1,106 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+const OMITTED_PREFIXES = new Set([
+  'rest of',
+  'rest of method',
+  'rest of methods',
+  'rest of code',
+  'unchanged code',
+  'unchanged method',
+  'unchanged methods',
+]);
+
+function isAllDots(str: string): boolean {
+  if (str.length === 0) {
+    return false;
+  }
+  for (let i = 0; i < str.length; i++) {
+    if (str[i] !== '.') {
+      return false;
+    }
+  }
+  return true;
+}
+
+function normalizeWhitespace(input: string): string {
+  const segments: string[] = [];
+  let current = '';
+
+  for (const char of input) {
+    if (char === ' ' || char === '\t' || char === '\n' || char === '\r') {
+      if (current.length > 0) {
+        segments.push(current);
+        current = '';
+      }
+      continue;
+    }
+    current += char;
+  }
+
+  if (current.length > 0) {
+    segments.push(current);
+  }
+
+  return segments.join(' ');
+}
+
+function normalizePlaceholder(line: string): string | null {
+  let text = line.trim();
+  if (!text) {
+    return null;
+  }
+
+  if (text.startsWith('//')) {
+    text = text.slice(2).trim();
+  }
+
+  if (text.startsWith('(') && text.endsWith(')')) {
+    text = text.slice(1, -1).trim();
+  }
+
+  const ellipsisStart = text.indexOf('...');
+  if (ellipsisStart < 0) {
+    return null;
+  }
+
+  const prefixRaw = text.slice(0, ellipsisStart).trim().toLowerCase();
+  const suffixRaw = text.slice(ellipsisStart + 3).trim();
+  const prefix = normalizeWhitespace(prefixRaw);
+
+  if (!OMITTED_PREFIXES.has(prefix)) {
+    return null;
+  }
+
+  if (suffixRaw.length > 0 && !isAllDots(suffixRaw)) {
+    return null;
+  }
+
+  return `${prefix} ...`;
+}
+
+/**
+ * Detects shorthand omission placeholders such as:
+ * - (rest of methods ...)
+ * - (rest of code ...)
+ * - (unchanged code ...)
+ * - // rest of methods ...
+ *
+ * Returns all placeholders found as normalized tokens.
+ */
+export function detectOmissionPlaceholders(text: string): string[] {
+  const lines = text.replaceAll('\r\n', '\n').split('\n');
+  const matches: string[] = [];
+
+  for (const rawLine of lines) {
+    const normalized = normalizePlaceholder(rawLine);
+    if (normalized) {
+      matches.push(normalized);
+    }
+  }
+
+  return matches;
+}

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

@@ -310,6 +310,42 @@ describe('WriteFileTool', () => {
       };
       expect(() => tool.build(params)).toThrow(`Missing or empty "file_path"`);
     });
+
+    it('should throw an error if content includes an omission placeholder', () => {
+      const params = {
+        file_path: path.join(rootDir, 'placeholder.txt'),
+        content: '(rest of methods ...)',
+      };
+      expect(() => tool.build(params)).toThrow(
+        "`content` contains an omission placeholder (for example 'rest of methods ...'). Provide complete file content.",
+      );
+    });
+
+    it('should throw an error when multiline content includes omission placeholders', () => {
+      const params = {
+        file_path: path.join(rootDir, 'service.ts'),
+        content: `class Service {
+  execute() {
+    return "run";
+  }
+
+  // rest of methods ...
+}`,
+      };
+      expect(() => tool.build(params)).toThrow(
+        "`content` contains an omission placeholder (for example 'rest of methods ...'). Provide complete file content.",
+      );
+    });
+
+    it('should allow content with placeholder text in a normal string literal', () => {
+      const params = {
+        file_path: path.join(rootDir, 'valid-content.ts'),
+        content: 'const note = "(rest of methods ...)";',
+      };
+      const invocation = tool.build(params);
+      expect(invocation).toBeDefined();
+      expect(invocation.params).toEqual(params);
+    });
   });
 
   describe('getCorrectedFileContent', () => {

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

@@ -47,6 +47,7 @@ import type { MessageBus } from '../confirmation-bus/message-bus.js';
 import { debugLogger } from '../utils/debugLogger.js';
 import { WRITE_FILE_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
+import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
 
 /**
  * Parameters for the WriteFile tool
@@ -486,6 +487,11 @@ export class WriteFileTool
       }`;
     }
 
+    const omissionPlaceholders = detectOmissionPlaceholders(params.content);
+    if (omissionPlaceholders.length > 0) {
+      return "`content` contains an omission placeholder (for example 'rest of methods ...'). Provide complete file content.";
+    }
+
     return null;
   }