fix(core): mitigate data corruption during write_file on massive text blocks

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

@@ -112,7 +112,7 @@ export const DEFAULT_LEGACY_SET: CoreToolSet = {
     name: WRITE_FILE_TOOL_NAME,
     description: `Writes content to a specified file in the local filesystem.
 
-      The user has the ability to modify \`content\`. If modified, this will be stated in the response.`,
+      The user has the ability to modify \`content\`. If modified, this will be stated in the response. WARNING: Do NOT use this tool if the file contains massive literal text sequences (like a 6000+ character string, large arrays, or inline base64 images), as LLMs are prone to corrupting or truncating such sequences during a full file rewrite. Use the '${EDIT_TOOL_NAME}' tool instead.`,
     parametersJsonSchema: {
       type: 'object',
       properties: {
@@ -122,7 +122,7 @@ export const DEFAULT_LEGACY_SET: CoreToolSet = {
         },
         [WRITE_FILE_PARAM_CONTENT]: {
           description:
-            "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content.",
+            "The content to write to the file. Do not use omission placeholders like '(rest of methods ...)', '...', or 'unchanged code'; provide complete literal content. WARNING: Do not use this tool to rewrite files containing massive literal text blocks (e.g., inline base64 images or >6000 character strings) because you may corrupt them. Use the replace tool instead.",
           type: 'string',
         },
       },

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

@@ -119,7 +119,7 @@ export const GEMINI_3_SET: CoreToolSet = {
 
   write_file: {
     name: WRITE_FILE_TOOL_NAME,
-    description: `Writes the complete content to a file, automatically creating missing parent directories. Overwrites existing files. The user has the ability to modify 'content' before it is saved. Best for new or small files; use '${EDIT_TOOL_NAME}' for targeted edits to large files to minimize token usage and simplify reviews.`,
+    description: `Writes the complete content to a file, automatically creating missing parent directories. Overwrites existing files. The user has the ability to modify 'content' before it is saved. Best for new or small files; use '${EDIT_TOOL_NAME}' for targeted edits to large files to minimize token usage and simplify reviews. WARNING: Do NOT use this tool if the file contains massive literal text sequences (like a 6000+ character string, large arrays, or inline base64 images), as LLMs are prone to corrupting or truncating such sequences during a full file rewrite. Use the '${EDIT_TOOL_NAME}' tool instead.`,
     parametersJsonSchema: {
       type: 'object',
       properties: {
@@ -129,7 +129,7 @@ export const GEMINI_3_SET: CoreToolSet = {
         },
         [WRITE_FILE_PARAM_CONTENT]: {
           description:
-            "The complete content to write. Provide the full file; do not use placeholders like '// ... rest of code'.",
+            "The complete content to write. Provide the full file; do not use placeholders like '// ... rest of code'. WARNING: Do not use this tool to rewrite files containing massive literal text blocks (e.g., inline base64 images or >6000 character strings) because you may corrupt them. Use the replace tool instead.",
           type: 'string',
         },
       },

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

@@ -9,6 +9,8 @@ import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
 
 describe('detectOmissionPlaceholders', () => {
   it('detects standalone placeholder lines', () => {
+    expect(detectOmissionPlaceholders('...')).toEqual([' ...']);
+    expect(detectOmissionPlaceholders('// ...')).toEqual([' ...']);
     expect(detectOmissionPlaceholders('(rest of methods ...)')).toEqual([
       'rest of methods ...',
     ]);

packages/core/src/tools/omissionPlaceholderDetector.ts

@@ -5,6 +5,7 @@
  */
 
 const OMITTED_PREFIXES = new Set([
+  '',
   'rest of',
   'rest of method',
   'rest of methods',