Enable "modify" in write tool (#1044)

6 files changed, 626 insertions, 287 deletions

diff --git a/packages/core/src/core/coreToolScheduler.ts b/packages/core/src/core/coreToolScheduler.ts
index d17e1511..14a39792 100644
--- a/packages/core/src/core/coreToolScheduler.ts
+++ b/packages/core/src/core/coreToolScheduler.ts
@@ -13,14 +13,17 @@ import {
   ToolResult,
   ToolRegistry,
   ApprovalMode,
-  EditTool,
-  EditToolParams,
   EditorType,
   Config,
   logToolCall,
 } from '../index.js';
 import { Part, PartListUnion } from '@google/genai';
 import { getResponseTextFromParts } from '../utils/generateContentResponseUtilities.js';
+import {
+  isModifiableTool,
+  ModifyContext,
+  modifyWithEditor,
+} from '../tools/modifiable-tool.js';
 
 export type ValidatingToolCall = {
   status: 'validating';
@@ -525,8 +528,8 @@ export class CoreToolScheduler {
       );
     } else if (outcome === ToolConfirmationOutcome.ModifyWithEditor) {
       const waitingToolCall = toolCall as WaitingToolCall;
-      if (waitingToolCall?.confirmationDetails?.type === 'edit') {
-        const editTool = waitingToolCall.tool as EditTool;
+      if (isModifiableTool(waitingToolCall.tool)) {
+        const modifyContext = waitingToolCall.tool.getModifyContext(signal);
         const editorType = this.getPreferredEditor();
         if (!editorType) {
           return;
@@ -535,22 +538,22 @@ export class CoreToolScheduler {
         this.setStatusInternal(callId, 'awaiting_approval', {
           ...waitingToolCall.confirmationDetails,
           isModifying: true,
-        });
+        } as ToolCallConfirmationDetails);
 
-        const modifyResults = await editTool.onModify(
-          waitingToolCall.request.args as unknown as EditToolParams,
-          signal,
+        const { updatedParams, updatedDiff } = await modifyWithEditor<
+          typeof waitingToolCall.request.args
+        >(
+          waitingToolCall.request.args,
+          modifyContext as ModifyContext<typeof waitingToolCall.request.args>,
           editorType,
+          signal,
         );
-
-        if (modifyResults) {
-          this.setArgsInternal(callId, modifyResults.updatedParams);
-          this.setStatusInternal(callId, 'awaiting_approval', {
-            ...waitingToolCall.confirmationDetails,
-            fileDiff: modifyResults.updatedDiff,
-            isModifying: false,
-          });
-        }
+        this.setArgsInternal(callId, updatedParams);
+        this.setStatusInternal(callId, 'awaiting_approval', {
+          ...waitingToolCall.confirmationDetails,
+          fileDiff: updatedDiff,
+          isModifying: false,
+        } as ToolCallConfirmationDetails);
       }
     } else {
       this.setStatusInternal(callId, 'scheduled');
diff --git a/packages/core/src/tools/edit.test.ts b/packages/core/src/tools/edit.test.ts
index dd3b481d..7dff5bf9 100644
--- a/packages/core/src/tools/edit.test.ts
+++ b/packages/core/src/tools/edit.test.ts
@@ -605,141 +605,4 @@ describe('EditTool', () => {
       );
     });
   });
-
-  describe('onModify', () => {
-    const testFile = 'some_file.txt';
-    let filePath: string;
-    const diffDir = path.join(os.tmpdir(), 'gemini-cli-edit-tool-diffs');
-
-    beforeEach(() => {
-      filePath = path.join(rootDir, testFile);
-      mockOpenDiff.mockClear();
-    });
-
-    afterEach(() => {
-      fs.rmSync(diffDir, { recursive: true, force: true });
-    });
-
-    it('should create temporary files, call openDiff, and return updated params with diff', async () => {
-      const originalContent = 'original content';
-      const params: EditToolParams = {
-        file_path: filePath,
-        old_string: originalContent,
-        new_string: 'modified content',
-      };
-
-      fs.writeFileSync(filePath, originalContent, 'utf8');
-
-      const result = await tool.onModify(
-        params,
-        new AbortController().signal,
-        'vscode',
-      );
-
-      expect(mockOpenDiff).toHaveBeenCalledTimes(1);
-      const [oldPath, newPath] = mockOpenDiff.mock.calls[0];
-      expect(oldPath).toMatch(
-        /gemini-cli-edit-tool-diffs[/\\]gemini-cli-edit-some_file\.txt-old-\d+/,
-      );
-      expect(newPath).toMatch(
-        /gemini-cli-edit-tool-diffs[/\\]gemini-cli-edit-some_file\.txt-new-\d+/,
-      );
-
-      expect(result).toBeDefined();
-      expect(result!.updatedParams).toEqual({
-        file_path: filePath,
-        old_string: originalContent,
-        new_string: 'modified content',
-      });
-      expect(result!.updatedDiff).toEqual(`Index: some_file.txt
-===================================================================
---- some_file.txt\tCurrent
-+++ some_file.txt\tProposed
-@@ -1,1 +1,1 @@
--original content
-\\ No newline at end of file
-+modified content
-\\ No newline at end of file
-`);
-
-      // Verify temp files are cleaned up
-      expect(fs.existsSync(oldPath)).toBe(false);
-      expect(fs.existsSync(newPath)).toBe(false);
-    });
-
-    it('should handle non-existent files and return updated params', async () => {
-      const params: EditToolParams = {
-        file_path: filePath,
-        old_string: '',
-        new_string: 'new file content',
-      };
-
-      const result = await tool.onModify(
-        params,
-        new AbortController().signal,
-        'vscode',
-      );
-
-      expect(mockOpenDiff).toHaveBeenCalledTimes(1);
-
-      const [oldPath, newPath] = mockOpenDiff.mock.calls[0];
-
-      expect(result).toBeDefined();
-      expect(result!.updatedParams).toEqual({
-        file_path: filePath,
-        old_string: '',
-        new_string: 'new file content',
-      });
-      expect(result!.updatedDiff).toContain('new file content');
-
-      // Verify temp files are cleaned up
-      expect(fs.existsSync(oldPath)).toBe(false);
-      expect(fs.existsSync(newPath)).toBe(false);
-    });
-
-    it('should clean up previous temp files before creating new ones', async () => {
-      const params: EditToolParams = {
-        file_path: filePath,
-        old_string: 'old',
-        new_string: 'new',
-      };
-
-      fs.writeFileSync(filePath, 'some old content', 'utf8');
-
-      // Call onModify first time
-      const result1 = await tool.onModify(
-        params,
-        new AbortController().signal,
-        'vscode',
-      );
-      const firstCall = mockOpenDiff.mock.calls[0];
-      const firstOldPath = firstCall[0];
-      const firstNewPath = firstCall[1];
-
-      expect(result1).toBeDefined();
-      expect(fs.existsSync(firstOldPath)).toBe(false);
-      expect(fs.existsSync(firstNewPath)).toBe(false);
-
-      // Ensure different timestamps so that the file names are different for testing.
-      await new Promise((resolve) => setTimeout(resolve, 2));
-
-      const result2 = await tool.onModify(
-        params,
-        new AbortController().signal,
-        'vscode',
-      );
-      const secondCall = mockOpenDiff.mock.calls[1];
-      const secondOldPath = secondCall[0];
-      const secondNewPath = secondCall[1];
-
-      // Call onModify second time
-      expect(result2).toBeDefined();
-      expect(fs.existsSync(secondOldPath)).toBe(false);
-      expect(fs.existsSync(secondNewPath)).toBe(false);
-
-      // Verify different file names were used
-      expect(firstOldPath).not.toBe(secondOldPath);
-      expect(firstNewPath).not.toBe(secondNewPath);
-    });
-  });
 });
diff --git a/packages/core/src/tools/edit.ts b/packages/core/src/tools/edit.ts
index 39352121..3317d460 100644
--- a/packages/core/src/tools/edit.ts
+++ b/packages/core/src/tools/edit.ts
@@ -6,7 +6,6 @@
 
 import * as fs from 'fs';
 import * as path from 'path';
-import * as os from 'os';
 import * as Diff from 'diff';
 import {
   BaseTool,
@@ -23,9 +22,8 @@ import { GeminiClient } from '../core/client.js';
 import { Config, ApprovalMode } from '../config/config.js';
 import { ensureCorrectEdit } from '../utils/editCorrector.js';
 import { DEFAULT_DIFF_OPTIONS } from './diffOptions.js';
-import { openDiff } from '../utils/editor.js';
 import { ReadFileTool } from './read-file.js';
-import { EditorType } from '../utils/editor.js';
+import { ModifiableTool, ModifyContext } from './modifiable-tool.js';
 
 /**
  * Parameters for the Edit tool
@@ -64,13 +62,14 @@ interface CalculatedEdit {
 /**
  * Implementation of the Edit tool logic
  */
-export class EditTool extends BaseTool<EditToolParams, ToolResult> {
+export class EditTool
+  extends BaseTool<EditToolParams, ToolResult>
+  implements ModifiableTool<EditToolParams>
+{
   static readonly Name = 'replace';
   private readonly config: Config;
   private readonly rootDirectory: string;
   private readonly client: GeminiClient;
-  private tempOldDiffPath?: string;
-  private tempNewDiffPath?: string;
 
   /**
    * Creates a new instance of the EditLogic
@@ -433,132 +432,6 @@ Expectation for required parameters:
   }
 
   /**
-   * Creates temp files for the current and proposed file contents and opens a diff tool.
-   * When the diff tool is closed, the tool will check if the file has been modified and provide the updated params.
-   * @returns Updated params and diff if the file has been modified, undefined otherwise.
-   */
-  async onModify(
-    params: EditToolParams,
-    _abortSignal: AbortSignal,
-    editorType: EditorType,
-  ): Promise<
-    { updatedParams: EditToolParams; updatedDiff: string } | undefined
-  > {
-    const { oldPath, newPath } = this.createTempFiles(params);
-    this.tempOldDiffPath = oldPath;
-    this.tempNewDiffPath = newPath;
-
-    await openDiff(this.tempOldDiffPath, this.tempNewDiffPath, editorType);
-    return await this.getUpdatedParamsIfModified(params, _abortSignal);
-  }
-
-  private async getUpdatedParamsIfModified(
-    params: EditToolParams,
-    _abortSignal: AbortSignal,
-  ): Promise<
-    { updatedParams: EditToolParams; updatedDiff: string } | undefined
-  > {
-    if (!this.tempOldDiffPath || !this.tempNewDiffPath) return undefined;
-    let oldContent = '';
-    let newContent = '';
-    try {
-      oldContent = fs.readFileSync(this.tempOldDiffPath, 'utf8');
-    } catch (err) {
-      if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
-      oldContent = '';
-    }
-    try {
-      newContent = fs.readFileSync(this.tempNewDiffPath, 'utf8');
-    } catch (err) {
-      if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
-      newContent = '';
-    }
-
-    // Combine the edits into a single edit
-    const updatedParams: EditToolParams = {
-      ...params,
-      old_string: oldContent,
-      new_string: newContent,
-    };
-
-    const updatedDiff = Diff.createPatch(
-      path.basename(params.file_path),
-      oldContent,
-      newContent,
-      'Current',
-      'Proposed',
-      DEFAULT_DIFF_OPTIONS,
-    );
-
-    this.deleteTempFiles();
-    return { updatedParams, updatedDiff };
-  }
-
-  private createTempFiles(params: EditToolParams): Record<string, string> {
-    this.deleteTempFiles();
-
-    const tempDir = os.tmpdir();
-    const diffDir = path.join(tempDir, 'gemini-cli-edit-tool-diffs');
-
-    if (!fs.existsSync(diffDir)) {
-      fs.mkdirSync(diffDir, { recursive: true });
-    }
-
-    const fileName = path.basename(params.file_path);
-    const timestamp = Date.now();
-    const tempOldPath = path.join(
-      diffDir,
-      `gemini-cli-edit-${fileName}-old-${timestamp}`,
-    );
-    const tempNewPath = path.join(
-      diffDir,
-      `gemini-cli-edit-${fileName}-new-${timestamp}`,
-    );
-
-    let currentContent = '';
-    try {
-      currentContent = fs.readFileSync(params.file_path, 'utf8');
-    } catch (err) {
-      if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
-      currentContent = '';
-    }
-
-    let proposedContent = currentContent;
-    proposedContent = this._applyReplacement(
-      proposedContent,
-      params.old_string,
-      params.new_string,
-      params.old_string === '' && currentContent === '',
-    );
-
-    fs.writeFileSync(tempOldPath, currentContent, 'utf8');
-    fs.writeFileSync(tempNewPath, proposedContent, 'utf8');
-    return {
-      oldPath: tempOldPath,
-      newPath: tempNewPath,
-    };
-  }
-
-  private deleteTempFiles(): void {
-    try {
-      if (this.tempOldDiffPath) {
-        fs.unlinkSync(this.tempOldDiffPath);
-        this.tempOldDiffPath = undefined;
-      }
-    } catch {
-      console.error(`Error deleting temp diff file: `, this.tempOldDiffPath);
-    }
-    try {
-      if (this.tempNewDiffPath) {
-        fs.unlinkSync(this.tempNewDiffPath);
-        this.tempNewDiffPath = undefined;
-      }
-    } catch {
-      console.error(`Error deleting temp diff file: `, this.tempNewDiffPath);
-    }
-  }
-
-  /**
    * Creates parent directories if they don't exist
    */
   private ensureParentDirectoriesExist(filePath: string): void {
@@ -567,4 +440,39 @@ Expectation for required parameters:
       fs.mkdirSync(dirName, { recursive: true });
     }
   }
+
+  getModifyContext(_: AbortSignal): ModifyContext<EditToolParams> {
+    return {
+      getFilePath: (params: EditToolParams) => params.file_path,
+      getCurrentContent: async (params: EditToolParams): Promise<string> => {
+        try {
+          return fs.readFileSync(params.file_path, 'utf8');
+        } catch (err) {
+          if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
+          return '';
+        }
+      },
+      getProposedContent: async (params: EditToolParams): Promise<string> => {
+        try {
+          const currentContent = fs.readFileSync(params.file_path, 'utf8');
+          return this._applyReplacement(
+            currentContent,
+            params.old_string,
+            params.new_string,
+            params.old_string === '' && currentContent === '',
+          );
+        } catch (err) {
+          if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
+          return '';
+        }
+      },
+      createUpdatedParams: (
+        modifiedProposedContent: string,
+        originalParams: EditToolParams,
+      ): EditToolParams => ({
+        ...originalParams,
+        new_string: modifiedProposedContent,
+      }),
+    };
+  }
 }
diff --git a/packages/core/src/tools/modifiable-tool.test.ts b/packages/core/src/tools/modifiable-tool.test.ts
new file mode 100644
index 00000000..56c27fe0
--- /dev/null
+++ b/packages/core/src/tools/modifiable-tool.test.ts
@@ -0,0 +1,367 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  vi,
+  describe,
+  it,
+  expect,
+  beforeEach,
+  afterEach,
+  type Mock,
+} from 'vitest';
+import {
+  modifyWithEditor,
+  ModifyContext,
+  ModifiableTool,
+  isModifiableTool,
+} from './modifiable-tool.js';
+import { EditorType } from '../utils/editor.js';
+import fs from 'fs';
+import os from 'os';
+import * as path from 'path';
+
+// Mock dependencies
+const mockOpenDiff = vi.hoisted(() => vi.fn());
+const mockCreatePatch = vi.hoisted(() => vi.fn());
+
+vi.mock('../utils/editor.js', () => ({
+  openDiff: mockOpenDiff,
+}));
+
+vi.mock('diff', () => ({
+  createPatch: mockCreatePatch,
+}));
+
+vi.mock('fs');
+vi.mock('os');
+
+interface TestParams {
+  filePath: string;
+  someOtherParam: string;
+  modifiedContent?: string;
+}
+
+describe('modifyWithEditor', () => {
+  let tempDir: string;
+  let mockModifyContext: ModifyContext<TestParams>;
+  let mockParams: TestParams;
+  let currentContent: string;
+  let proposedContent: string;
+  let modifiedContent: string;
+  let abortSignal: AbortSignal;
+
+  beforeEach(() => {
+    vi.resetAllMocks();
+
+    tempDir = '/tmp/test-dir';
+    abortSignal = new AbortController().signal;
+
+    currentContent = 'original content\nline 2\nline 3';
+    proposedContent = 'modified content\nline 2\nline 3';
+    modifiedContent = 'user modified content\nline 2\nline 3\nnew line';
+    mockParams = {
+      filePath: path.join(tempDir, 'test.txt'),
+      someOtherParam: 'value',
+    };
+
+    mockModifyContext = {
+      getFilePath: vi.fn().mockReturnValue(mockParams.filePath),
+      getCurrentContent: vi.fn().mockResolvedValue(currentContent),
+      getProposedContent: vi.fn().mockResolvedValue(proposedContent),
+      createUpdatedParams: vi
+        .fn()
+        .mockImplementation((modifiedContent, originalParams) => ({
+          ...originalParams,
+          modifiedContent,
+        })),
+    };
+
+    (os.tmpdir as Mock).mockReturnValue(tempDir);
+
+    (fs.existsSync as Mock).mockReturnValue(true);
+    (fs.mkdirSync as Mock).mockImplementation(() => undefined);
+    (fs.writeFileSync as Mock).mockImplementation(() => {});
+    (fs.unlinkSync as Mock).mockImplementation(() => {});
+
+    (fs.readFileSync as Mock).mockImplementation((filePath: string) => {
+      if (filePath.includes('-new-')) {
+        return modifiedContent;
+      }
+      return currentContent;
+    });
+
+    mockCreatePatch.mockReturnValue('mock diff content');
+    mockOpenDiff.mockResolvedValue(undefined);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  describe('successful modification', () => {
+    it('should successfully modify content with VSCode editor', async () => {
+      const result = await modifyWithEditor(
+        mockParams,
+        mockModifyContext,
+        'vscode' as EditorType,
+        abortSignal,
+      );
+
+      expect(mockModifyContext.getCurrentContent).toHaveBeenCalledWith(
+        mockParams,
+      );
+      expect(mockModifyContext.getProposedContent).toHaveBeenCalledWith(
+        mockParams,
+      );
+      expect(mockModifyContext.getFilePath).toHaveBeenCalledWith(mockParams);
+
+      expect(fs.writeFileSync).toHaveBeenCalledTimes(2);
+      expect(fs.writeFileSync).toHaveBeenNthCalledWith(
+        1,
+        expect.stringContaining(
+          path.join(tempDir, 'gemini-cli-tool-modify-diffs'),
+        ),
+        currentContent,
+        'utf8',
+      );
+      expect(fs.writeFileSync).toHaveBeenNthCalledWith(
+        2,
+        expect.stringContaining(
+          path.join(tempDir, 'gemini-cli-tool-modify-diffs'),
+        ),
+        proposedContent,
+        'utf8',
+      );
+
+      expect(mockOpenDiff).toHaveBeenCalledWith(
+        expect.stringContaining('-old-'),
+        expect.stringContaining('-new-'),
+        'vscode',
+      );
+
+      expect(fs.readFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('-old-'),
+        'utf8',
+      );
+      expect(fs.readFileSync).toHaveBeenCalledWith(
+        expect.stringContaining('-new-'),
+        'utf8',
+      );
+
+      expect(mockModifyContext.createUpdatedParams).toHaveBeenCalledWith(
+        modifiedContent,
+        mockParams,
+      );
+
+      expect(mockCreatePatch).toHaveBeenCalledWith(
+        path.basename(mockParams.filePath),
+        currentContent,
+        modifiedContent,
+        'Current',
+        'Proposed',
+        expect.objectContaining({
+          context: 3,
+          ignoreWhitespace: true,
+        }),
+      );
+
+      expect(fs.unlinkSync).toHaveBeenCalledTimes(2);
+      expect(fs.unlinkSync).toHaveBeenNthCalledWith(
+        1,
+        expect.stringContaining('-old-'),
+      );
+      expect(fs.unlinkSync).toHaveBeenNthCalledWith(
+        2,
+        expect.stringContaining('-new-'),
+      );
+
+      expect(result).toEqual({
+        updatedParams: {
+          ...mockParams,
+          modifiedContent,
+        },
+        updatedDiff: 'mock diff content',
+      });
+    });
+
+    it('should create temp directory if it does not exist', async () => {
+      (fs.existsSync as Mock).mockReturnValue(false);
+
+      await modifyWithEditor(
+        mockParams,
+        mockModifyContext,
+        'vscode' as EditorType,
+        abortSignal,
+      );
+
+      expect(fs.mkdirSync).toHaveBeenCalledWith(
+        path.join(tempDir, 'gemini-cli-tool-modify-diffs'),
+        { recursive: true },
+      );
+    });
+
+    it('should not create temp directory if it already exists', async () => {
+      (fs.existsSync as Mock).mockReturnValue(true);
+
+      await modifyWithEditor(
+        mockParams,
+        mockModifyContext,
+        'vscode' as EditorType,
+        abortSignal,
+      );
+
+      expect(fs.mkdirSync).not.toHaveBeenCalled();
+    });
+  });
+
+  it('should handle missing old temp file gracefully', async () => {
+    (fs.readFileSync as Mock).mockImplementation((filePath: string) => {
+      if (filePath.includes('-old-')) {
+        const error = new Error('ENOENT: no such file or directory');
+        (error as NodeJS.ErrnoException).code = 'ENOENT';
+        throw error;
+      }
+      return modifiedContent;
+    });
+
+    const result = await modifyWithEditor(
+      mockParams,
+      mockModifyContext,
+      'vscode' as EditorType,
+      abortSignal,
+    );
+
+    expect(mockCreatePatch).toHaveBeenCalledWith(
+      path.basename(mockParams.filePath),
+      '',
+      modifiedContent,
+      'Current',
+      'Proposed',
+      expect.objectContaining({
+        context: 3,
+        ignoreWhitespace: true,
+      }),
+    );
+
+    expect(result.updatedParams).toBeDefined();
+    expect(result.updatedDiff).toBe('mock diff content');
+  });
+
+  it('should handle missing new temp file gracefully', async () => {
+    (fs.readFileSync as Mock).mockImplementation((filePath: string) => {
+      if (filePath.includes('-new-')) {
+        const error = new Error('ENOENT: no such file or directory');
+        (error as NodeJS.ErrnoException).code = 'ENOENT';
+        throw error;
+      }
+      return currentContent;
+    });
+
+    const result = await modifyWithEditor(
+      mockParams,
+      mockModifyContext,
+      'vscode' as EditorType,
+      abortSignal,
+    );
+
+    expect(mockCreatePatch).toHaveBeenCalledWith(
+      path.basename(mockParams.filePath),
+      currentContent,
+      '',
+      'Current',
+      'Proposed',
+      expect.objectContaining({
+        context: 3,
+        ignoreWhitespace: true,
+      }),
+    );
+
+    expect(result.updatedParams).toBeDefined();
+    expect(result.updatedDiff).toBe('mock diff content');
+  });
+
+  it('should clean up temp files even if editor fails', async () => {
+    const editorError = new Error('Editor failed to open');
+    mockOpenDiff.mockRejectedValue(editorError);
+
+    await expect(
+      modifyWithEditor(
+        mockParams,
+        mockModifyContext,
+        'vscode' as EditorType,
+        abortSignal,
+      ),
+    ).rejects.toThrow('Editor failed to open');
+
+    expect(fs.unlinkSync).toHaveBeenCalledTimes(2);
+  });
+
+  it('should handle temp file cleanup errors gracefully', async () => {
+    const consoleErrorSpy = vi
+      .spyOn(console, 'error')
+      .mockImplementation(() => {});
+    (fs.unlinkSync as Mock).mockImplementation((_filePath: string) => {
+      throw new Error('Failed to delete file');
+    });
+
+    await modifyWithEditor(
+      mockParams,
+      mockModifyContext,
+      'vscode' as EditorType,
+      abortSignal,
+    );
+
+    expect(consoleErrorSpy).toHaveBeenCalledTimes(2);
+    expect(consoleErrorSpy).toHaveBeenCalledWith(
+      expect.stringContaining('Error deleting temp diff file:'),
+    );
+
+    consoleErrorSpy.mockRestore();
+  });
+
+  it('should create temp files with correct naming', async () => {
+    const testFilePath = path.join(tempDir, 'subfolder', 'test-file.txt');
+    mockModifyContext.getFilePath = vi.fn().mockReturnValue(testFilePath);
+
+    await modifyWithEditor(
+      mockParams,
+      mockModifyContext,
+      'vscode' as EditorType,
+      abortSignal,
+    );
+
+    const writeFileCalls = (fs.writeFileSync as Mock).mock.calls;
+    expect(writeFileCalls).toHaveLength(2);
+
+    const oldFilePath = writeFileCalls[0][0];
+    const newFilePath = writeFileCalls[1][0];
+
+    expect(oldFilePath).toMatch(/gemini-cli-modify-test-file\.txt-old-\d+$/);
+    expect(newFilePath).toMatch(/gemini-cli-modify-test-file\.txt-new-\d+$/);
+    expect(oldFilePath).toContain(`${tempDir}/gemini-cli-tool-modify-diffs/`);
+    expect(newFilePath).toContain(`${tempDir}/gemini-cli-tool-modify-diffs/`);
+  });
+});
+
+describe('isModifiableTool', () => {
+  it('should return true for objects with getModifyContext method', () => {
+    const mockTool = {
+      name: 'test-tool',
+      getModifyContext: vi.fn(),
+    } as unknown as ModifiableTool<TestParams>;
+
+    expect(isModifiableTool(mockTool)).toBe(true);
+  });
+
+  it('should return false for objects without getModifyContext method', () => {
+    const mockTool = {
+      name: 'test-tool',
+    } as unknown as ModifiableTool<TestParams>;
+
+    expect(isModifiableTool(mockTool)).toBe(false);
+  });
+});
diff --git a/packages/core/src/tools/modifiable-tool.ts b/packages/core/src/tools/modifiable-tool.ts
new file mode 100644
index 00000000..96fe176c
--- /dev/null
+++ b/packages/core/src/tools/modifiable-tool.ts
@@ -0,0 +1,163 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { EditorType } from '../utils/editor.js';
+import os from 'os';
+import path from 'path';
+import fs from 'fs';
+import * as Diff from 'diff';
+import { openDiff } from '../utils/editor.js';
+import { DEFAULT_DIFF_OPTIONS } from './diffOptions.js';
+import { isNodeError } from '../utils/errors.js';
+import { Tool } from './tools.js';
+
+/**
+ * A tool that supports a modify operation.
+ */
+export interface ModifiableTool<ToolParams> extends Tool<ToolParams> {
+  getModifyContext(abortSignal: AbortSignal): ModifyContext<ToolParams>;
+}
+
+export interface ModifyContext<ToolParams> {
+  getFilePath: (params: ToolParams) => string;
+
+  getCurrentContent: (params: ToolParams) => Promise<string>;
+
+  getProposedContent: (params: ToolParams) => Promise<string>;
+
+  createUpdatedParams: (
+    modifiedProposedContent: string,
+    originalParams: ToolParams,
+  ) => ToolParams;
+}
+
+export interface ModifyResult<ToolParams> {
+  updatedParams: ToolParams;
+  updatedDiff: string;
+}
+
+export function isModifiableTool<TParams>(
+  tool: Tool<TParams>,
+): tool is ModifiableTool<TParams> {
+  return 'getModifyContext' in tool;
+}
+
+function createTempFilesForModify(
+  currentContent: string,
+  proposedContent: string,
+  file_path: string,
+): { oldPath: string; newPath: string } {
+  const tempDir = os.tmpdir();
+  const diffDir = path.join(tempDir, 'gemini-cli-tool-modify-diffs');
+
+  if (!fs.existsSync(diffDir)) {
+    fs.mkdirSync(diffDir, { recursive: true });
+  }
+
+  const fileName = path.basename(file_path);
+  const timestamp = Date.now();
+  const tempOldPath = path.join(
+    diffDir,
+    `gemini-cli-modify-${fileName}-old-${timestamp}`,
+  );
+  const tempNewPath = path.join(
+    diffDir,
+    `gemini-cli-modify-${fileName}-new-${timestamp}`,
+  );
+
+  fs.writeFileSync(tempOldPath, currentContent, 'utf8');
+  fs.writeFileSync(tempNewPath, proposedContent, 'utf8');
+
+  return { oldPath: tempOldPath, newPath: tempNewPath };
+}
+
+function getUpdatedParams<ToolParams>(
+  tmpOldPath: string,
+  tempNewPath: string,
+  originalParams: ToolParams,
+  modifyContext: ModifyContext<ToolParams>,
+): { updatedParams: ToolParams; updatedDiff: string } {
+  let oldContent = '';
+  let newContent = '';
+
+  try {
+    oldContent = fs.readFileSync(tmpOldPath, 'utf8');
+  } catch (err) {
+    if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
+    oldContent = '';
+  }
+
+  try {
+    newContent = fs.readFileSync(tempNewPath, 'utf8');
+  } catch (err) {
+    if (!isNodeError(err) || err.code !== 'ENOENT') throw err;
+    newContent = '';
+  }
+
+  const updatedParams = modifyContext.createUpdatedParams(
+    newContent,
+    originalParams,
+  );
+  const updatedDiff = Diff.createPatch(
+    path.basename(modifyContext.getFilePath(originalParams)),
+    oldContent,
+    newContent,
+    'Current',
+    'Proposed',
+    DEFAULT_DIFF_OPTIONS,
+  );
+
+  return { updatedParams, updatedDiff };
+}
+
+function deleteTempFiles(oldPath: string, newPath: string): void {
+  try {
+    fs.unlinkSync(oldPath);
+  } catch {
+    console.error(`Error deleting temp diff file: ${oldPath}`);
+  }
+
+  try {
+    fs.unlinkSync(newPath);
+  } catch {
+    console.error(`Error deleting temp diff file: ${newPath}`);
+  }
+}
+
+/**
+ * Triggers an external editor for the user to modify the proposed content,
+ * and returns the updated tool parameters and the diff after the user has modified the proposed content.
+ */
+export async function modifyWithEditor<ToolParams>(
+  originalParams: ToolParams,
+  modifyContext: ModifyContext<ToolParams>,
+  editorType: EditorType,
+  _abortSignal: AbortSignal,
+): Promise<ModifyResult<ToolParams>> {
+  const currentContent = await modifyContext.getCurrentContent(originalParams);
+  const proposedContent =
+    await modifyContext.getProposedContent(originalParams);
+
+  const { oldPath, newPath } = createTempFilesForModify(
+    currentContent,
+    proposedContent,
+    modifyContext.getFilePath(originalParams),
+  );
+
+  try {
+    await openDiff(oldPath, newPath, editorType);
+    const result = getUpdatedParams(
+      oldPath,
+      newPath,
+      originalParams,
+      modifyContext,
+    );
+
+    return result;
+  } finally {
+    deleteTempFiles(oldPath, newPath);
+  }
+}
diff --git a/packages/core/src/tools/write-file.ts b/packages/core/src/tools/write-file.ts
index dc634cc8..b9e07034 100644
--- a/packages/core/src/tools/write-file.ts
+++ b/packages/core/src/tools/write-file.ts
@@ -25,6 +25,7 @@ import {
 } from '../utils/editCorrector.js';
 import { GeminiClient } from '../core/client.js';
 import { DEFAULT_DIFF_OPTIONS } from './diffOptions.js';
+import { ModifiableTool, ModifyContext } from './modifiable-tool.js';
 
 /**
  * Parameters for the WriteFile tool
@@ -51,7 +52,10 @@ interface GetCorrectedFileContentResult {
 /**
  * Implementation of the WriteFile tool logic
  */
-export class WriteFileTool extends BaseTool<WriteFileToolParams, ToolResult> {
+export class WriteFileTool
+  extends BaseTool<WriteFileToolParams, ToolResult>
+  implements ModifiableTool<WriteFileToolParams>
+{
   static readonly Name: string = 'write_file';
   private readonly client: GeminiClient;
 
@@ -336,4 +340,35 @@ export class WriteFileTool extends BaseTool<WriteFileToolParams, ToolResult> {
     }
     return { originalContent, correctedContent, fileExists };
   }
+
+  getModifyContext(
+    abortSignal: AbortSignal,
+  ): ModifyContext<WriteFileToolParams> {
+    return {
+      getFilePath: (params: WriteFileToolParams) => params.file_path,
+      getCurrentContent: async (params: WriteFileToolParams) => {
+        const correctedContentResult = await this._getCorrectedFileContent(
+          params.file_path,
+          params.content,
+          abortSignal,
+        );
+        return correctedContentResult.originalContent;
+      },
+      getProposedContent: async (params: WriteFileToolParams) => {
+        const correctedContentResult = await this._getCorrectedFileContent(
+          params.file_path,
+          params.content,
+          abortSignal,
+        );
+        return correctedContentResult.correctedContent;
+      },
+      createUpdatedParams: (
+        modifiedProposedContent: string,
+        originalParams: WriteFileToolParams,
+      ) => ({
+        ...originalParams,
+        content: modifiedProposedContent,
+      }),
+    };
+  }
 }