From 804712322579c9745254f4688940b0f32c8360e4 Mon Sep 17 00:00:00 2001
From: Coco Sheng <cocosheng@google.com>
Date: Wed, 20 May 2026 13:36:48 -0400
Subject: [PATCH] fix(core): mitigate data corruption during write_file on
 massive text blocks

---
 .../src/tools/definitions/model-family-sets/default-legacy.ts | 4 ++--
 .../core/src/tools/definitions/model-family-sets/gemini-3.ts  | 4 ++--
 packages/core/src/tools/omissionPlaceholderDetector.test.ts   | 2 ++
 packages/core/src/tools/omissionPlaceholderDetector.ts        | 1 +
 4 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/packages/core/src/tools/definitions/model-family-sets/default-legacy.ts b/packages/core/src/tools/definitions/model-family-sets/default-legacy.ts
index 3dfe8dd40e..07bfc1189e 100644
--- a/packages/core/src/tools/definitions/model-family-sets/default-legacy.ts
+++ b/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',
         },
       },
diff --git a/packages/core/src/tools/definitions/model-family-sets/gemini-3.ts b/packages/core/src/tools/definitions/model-family-sets/gemini-3.ts
index 57a897f9ee..b4a9268667 100644
--- a/packages/core/src/tools/definitions/model-family-sets/gemini-3.ts
+++ b/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',
         },
       },
diff --git a/packages/core/src/tools/omissionPlaceholderDetector.test.ts b/packages/core/src/tools/omissionPlaceholderDetector.test.ts
index 4e574d5e22..edee2d1a07 100644
--- a/packages/core/src/tools/omissionPlaceholderDetector.test.ts
+++ b/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 ...',
     ]);
diff --git a/packages/core/src/tools/omissionPlaceholderDetector.ts b/packages/core/src/tools/omissionPlaceholderDetector.ts
index 7057a7f09d..be9a5b582b 100644
--- a/packages/core/src/tools/omissionPlaceholderDetector.ts
+++ b/packages/core/src/tools/omissionPlaceholderDetector.ts
@@ -5,6 +5,7 @@
  */
 
 const OMITTED_PREFIXES = new Set([
+  '',
   'rest of',
   'rest of method',
   'rest of methods',