feat: Add flow to allow modifying edits during edit tool call (#808)

packages/cli/src/config/config.ts

@@ -172,6 +172,7 @@ export async function loadCliConfig(
     fileFilteringRespectGitIgnore: settings.fileFiltering?.respectGitIgnore,
     fileFilteringAllowBuildArtifacts:
       settings.fileFiltering?.allowBuildArtifacts,
+    enableModifyWithExternalEditors: settings.enableModifyWithExternalEditors,
   });
 }
 

packages/cli/src/config/settings.ts

@@ -37,6 +37,7 @@ export interface Settings {
   contextFileName?: string;
   accessibility?: AccessibilitySettings;
   telemetry?: boolean;
+  enableModifyWithExternalEditors?: boolean;
 
   // Git-aware file filtering settings
   fileFiltering?: {

packages/cli/src/ui/App.tsx

@@ -355,6 +355,7 @@ export const App = ({ config, settings, startupWarnings = [] }: AppProps) => {
                 key={h.id}
                 item={h}
                 isPending={false}
+                config={config}
               />
             )),
           ]}
@@ -370,6 +371,7 @@ export const App = ({ config, settings, startupWarnings = [] }: AppProps) => {
               // HistoryItemDisplay. Refactor later. Use a fake id for now.
               item={{ ...item, id: 0 }}
               isPending={true}
+              config={config}
             />
           ))}
         </Box>

packages/cli/src/ui/components/HistoryItemDisplay.tsx

@@ -15,17 +15,20 @@ import { ToolGroupMessage } from './messages/ToolGroupMessage.js';
 import { GeminiMessageContent } from './messages/GeminiMessageContent.js';
 import { Box } from 'ink';
 import { AboutBox } from './AboutBox.js';
+import { Config } from '@gemini-cli/core';
 
 interface HistoryItemDisplayProps {
   item: HistoryItem;
   availableTerminalHeight: number;
   isPending: boolean;
+  config?: Config;
 }
 
 export const HistoryItemDisplay: React.FC<HistoryItemDisplayProps> = ({
   item,
   availableTerminalHeight,
   isPending,
+  config,
 }) => (
   <Box flexDirection="column" key={item.id}>
     {/* Render standard message types */}
@@ -60,6 +63,7 @@ export const HistoryItemDisplay: React.FC<HistoryItemDisplayProps> = ({
         toolCalls={item.tools}
         groupId={item.id}
         availableTerminalHeight={availableTerminalHeight}
+        config={config}
       />
     )}
   </Box>

packages/cli/src/ui/components/messages/ToolConfirmationMessage.tsx

@@ -13,6 +13,8 @@ import {
   ToolConfirmationOutcome,
   ToolExecuteConfirmationDetails,
   ToolMcpConfirmationDetails,
+  checkHasEditor,
+  Config,
 } from '@gemini-cli/core';
 import {
   RadioButtonSelect,
@@ -21,11 +23,12 @@ import {
 
 export interface ToolConfirmationMessageProps {
   confirmationDetails: ToolCallConfirmationDetails;
+  config?: Config;
 }
 
 export const ToolConfirmationMessage: React.FC<
   ToolConfirmationMessageProps
-> = ({ confirmationDetails }) => {
+> = ({ confirmationDetails, config }) => {
   const { onConfirm } = confirmationDetails;
 
   useInput((_, key) => {
@@ -44,6 +47,24 @@ export const ToolConfirmationMessage: React.FC<
   >();
 
   if (confirmationDetails.type === 'edit') {
+    if (confirmationDetails.isModifying) {
+      return (
+        <Box
+          minWidth="90%"
+          borderStyle="round"
+          borderColor={Colors.Gray}
+          justifyContent="space-around"
+          padding={1}
+          overflow="hidden"
+        >
+          <Text>Modify in progress: </Text>
+          <Text color={Colors.AccentGreen}>
+            Save and close external editor to continue
+          </Text>
+        </Box>
+      );
+    }
+
     // Body content is now the DiffRenderer, passing filename to it
     // The bordered box is removed from here and handled within DiffRenderer
     bodyContent = (
@@ -63,8 +84,28 @@ export const ToolConfirmationMessage: React.FC<
         label: 'Yes, allow always',
         value: ToolConfirmationOutcome.ProceedAlways,
       },
-      { label: 'No (esc)', value: ToolConfirmationOutcome.Cancel },
     );
+
+    // Conditionally add editor options if editors are installed
+    const notUsingSandbox = !process.env.SANDBOX;
+    const externalEditorsEnabled =
+      config?.getEnableModifyWithExternalEditors() ?? false;
+
+    if (checkHasEditor('vscode') && notUsingSandbox && externalEditorsEnabled) {
+      options.push({
+        label: 'Modify with VS Code',
+        value: ToolConfirmationOutcome.ModifyVSCode,
+      });
+    }
+
+    if (checkHasEditor('vim') && externalEditorsEnabled) {
+      options.push({
+        label: 'Modify with vim',
+        value: ToolConfirmationOutcome.ModifyVim,
+      });
+    }
+
+    options.push({ label: 'No (esc)', value: ToolConfirmationOutcome.Cancel });
   } else if (confirmationDetails.type === 'exec') {
     const executionProps =
       confirmationDetails as ToolExecuteConfirmationDetails;

packages/cli/src/ui/components/messages/ToolGroupMessage.tsx

@@ -10,17 +10,20 @@ import { IndividualToolCallDisplay, ToolCallStatus } from '../../types.js';
 import { ToolMessage } from './ToolMessage.js';
 import { ToolConfirmationMessage } from './ToolConfirmationMessage.js';
 import { Colors } from '../../colors.js';
+import { Config } from '@gemini-cli/core';
 
 interface ToolGroupMessageProps {
   groupId: number;
   toolCalls: IndividualToolCallDisplay[];
   availableTerminalHeight: number;
+  config?: Config;
 }
 
 // Main component renders the border and maps the tools using ToolMessage
 export const ToolGroupMessage: React.FC<ToolGroupMessageProps> = ({
   toolCalls,
   availableTerminalHeight,
+  config,
 }) => {
   const hasPending = !toolCalls.every(
     (t) => t.status === ToolCallStatus.Success,
@@ -80,6 +83,7 @@ export const ToolGroupMessage: React.FC<ToolGroupMessageProps> = ({
               tool.confirmationDetails && (
                 <ToolConfirmationMessage
                   confirmationDetails={tool.confirmationDetails}
+                  config={config}
                 />
               )}
           </Box>

packages/cli/src/ui/components/shared/text-buffer.ts

@@ -1440,7 +1440,7 @@ export interface TextBuffer {
     key: Record<string, boolean>,
   ) => boolean;
   /**
-   * Opens the current buffer contents in the user’s preferred terminal text
+   * Opens the current buffer contents in the user's preferred terminal text
    * editor ($VISUAL or $EDITOR, falling back to "vi").  The method blocks
    * until the editor exits, then reloads the file and replaces the in‑memory
    * buffer with whatever the user saved.
@@ -1451,7 +1451,7 @@ export interface TextBuffer {
    *
    * Note: We purposefully rely on the *synchronous* spawn API so that the
    * calling process genuinely waits for the editor to close before
-   * continuing.  This mirrors Git’s behaviour and simplifies downstream
+   * continuing.  This mirrors Git's behaviour and simplifies downstream
    * control‑flow (callers can simply `await` the Promise).
    */
   openInExternalEditor: (opts?: { editor?: string }) => Promise<void>;

packages/core/src/config/config.ts

@@ -78,6 +78,7 @@ export interface ConfigParameters {
   telemetryLogUserPromptsEnabled?: boolean;
   fileFilteringRespectGitIgnore?: boolean;
   fileFilteringAllowBuildArtifacts?: boolean;
+  enableModifyWithExternalEditors?: boolean;
 }
 
 export class Config {
@@ -106,6 +107,7 @@ export class Config {
   private readonly geminiIgnorePatterns: string[] = [];
   private readonly fileFilteringRespectGitIgnore: boolean;
   private readonly fileFilteringAllowBuildArtifacts: boolean;
+  private readonly enableModifyWithExternalEditors: boolean;
   private fileDiscoveryService: FileDiscoveryService | null = null;
 
   constructor(params: ConfigParameters) {
@@ -135,6 +137,8 @@ export class Config {
       params.fileFilteringRespectGitIgnore ?? true;
     this.fileFilteringAllowBuildArtifacts =
       params.fileFilteringAllowBuildArtifacts ?? false;
+    this.enableModifyWithExternalEditors =
+      params.enableModifyWithExternalEditors ?? false;
 
     if (params.contextFileName) {
       setGeminiMdFilename(params.contextFileName);
@@ -266,6 +270,10 @@ export class Config {
     return this.fileFilteringAllowBuildArtifacts;
   }
 
+  getEnableModifyWithExternalEditors(): boolean {
+    return this.enableModifyWithExternalEditors;
+  }
+
   async getFileService(): Promise<FileDiscoveryService> {
     if (!this.fileDiscoveryService) {
       this.fileDiscoveryService = new FileDiscoveryService(this.targetDir);

packages/core/src/core/coreToolScheduler.ts

@@ -13,6 +13,8 @@ import {
   ToolResult,
   ToolRegistry,
   ApprovalMode,
+  EditTool,
+  EditToolParams,
 } from '../index.js';
 import { Part, PartListUnion } from '@google/genai';
 import { getResponseTextFromParts } from '../utils/generateContentResponseUtilities.js';
@@ -358,6 +360,16 @@ export class CoreToolScheduler {
     this.checkAndNotifyCompletion();
   }
 
+  private setArgsInternal(targetCallId: string, args: unknown): void {
+    this.toolCalls = this.toolCalls.map((call) => {
+      if (call.request.callId !== targetCallId) return call;
+      return {
+        ...call,
+        request: { ...call.request, args: args as Record<string, unknown> },
+      };
+    });
+  }
+
   private isRunning(): boolean {
     return this.toolCalls.some(
       (call) =>
@@ -471,6 +483,33 @@ export class CoreToolScheduler {
         'cancelled',
         'User did not allow tool call',
       );
+    } else if (
+      outcome === ToolConfirmationOutcome.ModifyVSCode ||
+      outcome === ToolConfirmationOutcome.ModifyVim
+    ) {
+      const waitingToolCall = toolCall as WaitingToolCall;
+      if (waitingToolCall?.confirmationDetails?.type === 'edit') {
+        const editTool = waitingToolCall.tool as EditTool;
+        this.setStatusInternal(callId, 'awaiting_approval', {
+          ...waitingToolCall.confirmationDetails,
+          isModifying: true,
+        });
+
+        const modifyResults = await editTool.onModify(
+          waitingToolCall.request.args as unknown as EditToolParams,
+          this.abortController.signal,
+          outcome,
+        );
+
+        if (modifyResults) {
+          this.setArgsInternal(callId, modifyResults.updatedParams);
+          this.setStatusInternal(callId, 'awaiting_approval', {
+            ...waitingToolCall.confirmationDetails,
+            fileDiff: modifyResults.updatedDiff,
+            isModifying: false,
+          });
+        }
+      }
     } else {
       this.setStatusInternal(callId, 'scheduled');
     }

packages/core/src/index.ts

@@ -25,6 +25,7 @@ export * from './utils/errors.js';
 export * from './utils/getFolderStructure.js';
 export * from './utils/memoryDiscovery.js';
 export * from './utils/gitIgnoreParser.js';
+export * from './utils/editor.js';
 
 // Export services
 export * from './services/fileDiscoveryService.js';

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

@@ -8,6 +8,7 @@
 
 const mockEnsureCorrectEdit = vi.hoisted(() => vi.fn());
 const mockGenerateJson = vi.hoisted(() => vi.fn());
+const mockOpenDiff = vi.hoisted(() => vi.fn());
 
 vi.mock('../utils/editCorrector.js', () => ({
   ensureCorrectEdit: mockEnsureCorrectEdit,
@@ -19,6 +20,10 @@ vi.mock('../core/client.js', () => ({
   })),
 }));
 
+vi.mock('../utils/editor.js', () => ({
+  openDiff: mockOpenDiff,
+}));
+
 import { describe, it, expect, beforeEach, afterEach, vi, Mock } from 'vitest';
 import { EditTool, EditToolParams } from './edit.js';
 import { FileDiff } from './tools.js';
@@ -27,6 +32,7 @@ import fs from 'fs';
 import os from 'os';
 import { ApprovalMode, Config } from '../config/config.js';
 import { Content, Part, SchemaUnion } from '@google/genai';
+import { ToolConfirmationOutcome } from './tools.js';
 
 describe('EditTool', () => {
   let tool: EditTool;
@@ -715,4 +721,140 @@ function makeRequest() {
       );
     });
   });
+
+  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,
+        edits: [
+          { old_string: originalContent, new_string: 'modified content' },
+        ],
+      };
+
+      fs.writeFileSync(filePath, originalContent, 'utf8');
+
+      const result = await tool.onModify(
+        params,
+        new AbortController().signal,
+        ToolConfirmationOutcome.ModifyVSCode,
+      );
+
+      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,
+        edits: [
+          { 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,
+        edits: [{ old_string: '', new_string: 'new file content' }],
+      };
+
+      const result = await tool.onModify(
+        params,
+        new AbortController().signal,
+        ToolConfirmationOutcome.ModifyVSCode,
+      );
+
+      expect(mockOpenDiff).toHaveBeenCalledTimes(1);
+
+      const [oldPath, newPath] = mockOpenDiff.mock.calls[0];
+
+      expect(result).toBeDefined();
+      expect(result!.updatedParams).toEqual({
+        file_path: filePath,
+        edits: [{ 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,
+        edits: [{ 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,
+        ToolConfirmationOutcome.ModifyVSCode,
+      );
+      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,
+        ToolConfirmationOutcome.ModifyVSCode,
+      );
+      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);
+    });
+  });
 });

packages/core/src/tools/edit.ts

@@ -4,8 +4,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import fs from 'fs';
-import path from 'path';
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
 import * as Diff from 'diff';
 import {
   BaseTool,
@@ -22,6 +23,7 @@ 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';
 
 /**
@@ -75,6 +77,8 @@ export class EditTool extends BaseTool<EditToolParams, EditResult> {
   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
@@ -514,6 +518,142 @@ 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,
+    outcome: ToolConfirmationOutcome,
+  ): Promise<
+    { updatedParams: EditToolParams; updatedDiff: string } | undefined
+  > {
+    const { oldPath, newPath } = this.createTempFiles(params);
+    this.tempOldDiffPath = oldPath;
+    this.tempNewDiffPath = newPath;
+
+    await openDiff(
+      this.tempOldDiffPath,
+      this.tempNewDiffPath,
+      outcome === ToolConfirmationOutcome.ModifyVSCode ? 'vscode' : 'vim',
+    );
+    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,
+      edits: [
+        {
+          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;
+    for (const edit of params.edits) {
+      proposedContent = this._applyReplacement(
+        proposedContent,
+        edit.old_string,
+        edit.new_string,
+        edit.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
    */

packages/core/src/tools/tools.ts

@@ -202,6 +202,7 @@ export interface ToolEditConfirmationDetails {
   onConfirm: (outcome: ToolConfirmationOutcome) => Promise<void>;
   fileName: string;
   fileDiff: string;
+  isModifying?: boolean;
 }
 
 export interface ToolExecuteConfirmationDetails {
@@ -231,5 +232,7 @@ export enum ToolConfirmationOutcome {
   ProceedAlways = 'proceed_always',
   ProceedAlwaysServer = 'proceed_always_server',
   ProceedAlwaysTool = 'proceed_always_tool',
+  ModifyVSCode = 'modify_vscode',
+  ModifyVim = 'modify_vim',
   Cancel = 'cancel',
 }

packages/core/src/utils/editor.ts

@@ -0,0 +1,128 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { execSync, spawn } from 'child_process';
+
+type EditorType = 'vscode' | 'vim';
+
+interface DiffCommand {
+  command: string;
+  args: string[];
+}
+
+export function checkHasEditor(editor: EditorType): boolean {
+  const commandExists = (cmd: string): boolean => {
+    try {
+      execSync(`which ${cmd}`, { stdio: 'ignore' });
+      return true;
+    } catch {
+      return false;
+    }
+  };
+
+  if (editor === 'vscode') {
+    return commandExists('code');
+  } else if (editor === 'vim') {
+    return commandExists('vim');
+  }
+  return false;
+}
+
+/**
+ * Get the diff command for a specific editor.
+ */
+export function getDiffCommand(
+  oldPath: string,
+  newPath: string,
+  editor: EditorType,
+): DiffCommand | null {
+  switch (editor) {
+    case 'vscode':
+      return {
+        command: 'code',
+        args: ['--wait', '--diff', oldPath, newPath],
+      };
+    case 'vim':
+      return {
+        command: 'vim',
+        args: [
+          '-d',
+          // skip viminfo file to avoid E138 errors
+          '-i',
+          'NONE',
+          // make the left window read-only and the right window editable
+          '-c',
+          'wincmd h | set readonly | wincmd l',
+          // set up colors for diffs
+          '-c',
+          'highlight DiffAdd cterm=bold ctermbg=22 guibg=#005f00 | highlight DiffChange cterm=bold ctermbg=24 guibg=#005f87 | highlight DiffText ctermbg=21 guibg=#0000af | highlight DiffDelete ctermbg=52 guibg=#5f0000',
+          // Show helpful messages
+          '-c',
+          'set showtabline=2 | set tabline=[Instructions]\\ :wqa(save\\ &\\ quit)\\ \\|\\ i/esc(toggle\\ edit\\ mode)',
+          '-c',
+          'wincmd h | setlocal statusline=OLD\\ FILE',
+          '-c',
+          'wincmd l | setlocal statusline=%#StatusBold#NEW\\ FILE\\ :wqa(save\\ &\\ quit)\\ \\|\\ i/esc(toggle\\ edit\\ mode)',
+          // Auto close all windows when one is closed
+          '-c',
+          'autocmd WinClosed * wqa',
+          oldPath,
+          newPath,
+        ],
+      };
+    default:
+      return null;
+  }
+}
+
+/**
+ * Opens a diff tool to compare two files.
+ * Terminal-based editors by default blocks parent process until the editor exits.
+ * GUI-based editors requires args such as "--wait" to block parent process.
+ */
+export async function openDiff(
+  oldPath: string,
+  newPath: string,
+  editor: EditorType,
+): Promise<void> {
+  const diffCommand = getDiffCommand(oldPath, newPath, editor);
+  if (!diffCommand) {
+    console.error('No diff tool available. Install vim or vscode.');
+    return;
+  }
+
+  try {
+    if (editor === 'vscode') {
+      // Use spawn to avoid blocking the entire process, resolve this function when editor is closed.
+      return new Promise((resolve, reject) => {
+        const process = spawn(diffCommand.command, diffCommand.args, {
+          stdio: 'inherit',
+        });
+
+        process.on('close', (code) => {
+          if (code === 0) {
+            resolve();
+          } else {
+            reject(new Error(`VS Code exited with code ${code}`));
+          }
+        });
+
+        process.on('error', (error) => {
+          reject(error);
+        });
+      });
+    } else {
+      // Use execSync for terminal-based editors like vim
+      const command = `${diffCommand.command} ${diffCommand.args.map((arg) => `"${arg}"`).join(' ')}`;
+      execSync(command, {
+        stdio: 'inherit',
+        encoding: 'utf8',
+      });
+    }
+  } catch (error) {
+    console.error(error);
+  }
+}