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

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

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);
   }