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

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 +607,64 @@ 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 the same 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 allow omission placeholder when the same placeholder already exists 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 ...)',
+      };
+      expect(tool.validateToolParams(params)).toBeNull();
+    });
+
+    it('should allow normal code that contains ellipsis 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();
+    });
+
+    it('should reject multiline omission placeholder edits that hide unchanged methods (regression #19858)', () => {
+      const params: EditToolParams = {
+        file_path: path.join(rootDir, 'service.ts'),
+        instruction: 'Rename one method and keep rest unchanged',
+        old_string: `class Service {
+  run() {
+    return "run";
+  }
+
+  helper() {
+    return "helper";
+  }
+}`,
+        new_string: `class Service {
+  execute() {
+    return "run";
+  }
+
+  (rest of methods ...)
+}`,
+      };
+      expect(tool.validateToolParams(params)).toBe(
+        "`new_string` contains an omission placeholder (for example 'rest of methods ...'). Provide exact literal replacement text.",
+      );
+    });
   });
 
   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 { detectOmissionPlaceholder } from './omissionPlaceholderDetector.js';
 
 const ENABLE_FUZZY_MATCH_RECOVERY = true;
 const FUZZY_MATCH_THRESHOLD = 0.1; // Allow up to 10% weighted difference
@@ -973,6 +974,24 @@ export class EditTool
     }
     params.file_path = filePath;
 
+    const newPlaceholderMatch = detectOmissionPlaceholder(params.new_string);
+    if (newPlaceholderMatch.found) {
+      const oldPlaceholderMatch = detectOmissionPlaceholder(params.old_string);
+      const normalizedNewPlaceholder = newPlaceholderMatch.match
+        ?.trim()
+        .toLowerCase();
+      const normalizedOldPlaceholder = oldPlaceholderMatch.match
+        ?.trim()
+        .toLowerCase();
+
+      if (
+        !oldPlaceholderMatch.found ||
+        normalizedOldPlaceholder !== normalizedNewPlaceholder
+      ) {
+        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 +1,74 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect, it } from 'vitest';
+import { detectOmissionPlaceholder } from './omissionPlaceholderDetector.js';
+
+describe('detectOmissionPlaceholder', () => {
+  it('detects standalone placeholder lines', () => {
+    expect(detectOmissionPlaceholder('(rest of methods ...)')).toEqual({
+      found: true,
+      match: '(rest of methods ...)',
+    });
+    expect(detectOmissionPlaceholder('(rest of code ...)')).toEqual({
+      found: true,
+      match: '(rest of code ...)',
+    });
+    expect(detectOmissionPlaceholder('(unchanged code ...)')).toEqual({
+      found: true,
+      match: '(unchanged code ...)',
+    });
+    expect(detectOmissionPlaceholder('// rest of methods ...')).toEqual({
+      found: true,
+      match: '// rest of methods ...',
+    });
+  });
+
+  it('detects case-insensitive placeholders', () => {
+    expect(detectOmissionPlaceholder('(Rest Of Methods ...)')).toEqual({
+      found: true,
+      match: '(Rest Of Methods ...)',
+    });
+  });
+
+  it('detects placeholder lines inside larger multiline text', () => {
+    const text = `class Example {\n  methodA() {}\n  (rest of methods ...)\n}`;
+    expect(detectOmissionPlaceholder(text)).toEqual({
+      found: true,
+      match: '(rest of methods ...)',
+    });
+  });
+
+  it('does not detect placeholders embedded in normal code', () => {
+    expect(
+      detectOmissionPlaceholder(
+        'const note = "(rest of methods ...)";\nconsole.log(note);',
+      ),
+    ).toEqual({
+      found: false,
+    });
+  });
+
+  it('does not detect text without ellipsis marker', () => {
+    expect(detectOmissionPlaceholder('(rest of methods)')).toEqual({
+      found: false,
+    });
+  });
+
+  it('does not detect unrelated ellipsis text', () => {
+    expect(detectOmissionPlaceholder('const message = "loading...";')).toEqual({
+      found: false,
+    });
+  });
+
+  it('does not detect omission phrase when it is inline code/comment context', () => {
+    expect(
+      detectOmissionPlaceholder('return value; // rest of methods ...'),
+    ).toEqual({
+      found: false,
+    });
+  });
+});

packages/core/src/tools/omissionPlaceholderDetector.ts

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export interface OmissionPlaceholderDetectionResult {
+  found: boolean;
+  match?: string;
+}
+
+const STANDALONE_OMISSION_LINE_PATTERN =
+  /^(?:\/\/\s*)?\(?\s*(?:rest\s+of(?:\s+(?:methods?|code))?|unchanged\s+(?:code|methods?))\s*\.{3,}\s*\)?$/i;
+
+/**
+ * Detects shorthand omission placeholders such as:
+ * - (rest of methods ...)
+ * - (rest of code ...)
+ * - (unchanged code ...)
+ * - // rest of methods ...
+ */
+export function detectOmissionPlaceholder(
+  text: string,
+): OmissionPlaceholderDetectionResult {
+  const lines = text.split(/\r?\n/);
+
+  for (const rawLine of lines) {
+    const line = rawLine.trim();
+    if (!line) {
+      continue;
+    }
+
+    if (!line.includes('...')) {
+      continue;
+    }
+
+    if (
+      !/rest\s+of/i.test(line) &&
+      !/unchanged\s+(?:code|methods?)/i.test(line)
+    ) {
+      continue;
+    }
+
+    if (STANDALONE_OMISSION_LINE_PATTERN.test(line)) {
+      return { found: true, match: line };
+    }
+  }
+
+  return { found: false };
+}

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 allow content with ellipsis 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);
+    });
+
+    it('should reject omission placeholder within multiline file content (regression #19858)', () => {
+      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.",
+      );
+    });
   });
 
   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 { detectOmissionPlaceholder } from './omissionPlaceholderDetector.js';
 
 /**
  * Parameters for the WriteFile tool
@@ -486,6 +487,11 @@ export class WriteFileTool
       }`;
     }
 
+    const omissionPlaceholderMatch = detectOmissionPlaceholder(params.content);
+    if (omissionPlaceholderMatch.found) {
+      return "`content` contains an omission placeholder (for example 'rest of methods ...'). Provide complete file content.";
+    }
+
     return null;
   }