refactor(core): extract ToolModificationHandler from scheduler (#16118)

packages/core/src/core/coreToolScheduler.test.ts

@@ -750,6 +750,17 @@ describe('CoreToolScheduler with payload', () => {
       );
     }
 
+    // After internal update, the tool should be awaiting approval again with the NEW content.
+    const updatedAwaitingCall = (await waitForStatus(
+      onToolCallsUpdate,
+      'awaiting_approval',
+    )) as WaitingToolCall;
+
+    // Now confirm for real to execute.
+    await updatedAwaitingCall.confirmationDetails.onConfirm(
+      ToolConfirmationOutcome.ProceedOnce,
+    );
+
     // Wait for the tool execution to complete
     await vi.waitFor(() => {
       expect(onAllToolCallsComplete).toHaveBeenCalled();

packages/core/src/core/coreToolScheduler.ts

@@ -19,12 +19,7 @@ import { logToolCall } from '../telemetry/loggers.js';
 import { ToolErrorType } from '../tools/tool-error.js';
 import { ToolCallEvent } from '../telemetry/types.js';
 import { runInDevTraceSpan } from '../telemetry/trace.js';
-import type { ModifyContext } from '../tools/modifiable-tool.js';
-import {
-  isModifiableDeclarativeTool,
-  modifyWithEditor,
-} from '../tools/modifiable-tool.js';
-import * as Diff from 'diff';
+import { ToolModificationHandler } from '../scheduler/tool-modifier.js';
 import { getToolSuggestion } from '../utils/tool-utils.js';
 import type { ToolConfirmationRequest } from '../confirmation-bus/types.js';
 import { MessageBusType } from '../confirmation-bus/types.js';
@@ -124,6 +119,7 @@ export class CoreToolScheduler {
   private toolCallQueue: ToolCall[] = [];
   private completedToolCallsForBatch: CompletedToolCall[] = [];
   private toolExecutor: ToolExecutor;
+  private toolModifier: ToolModificationHandler;
 
   constructor(options: CoreToolSchedulerOptions) {
     this.config = options.config;
@@ -132,6 +128,7 @@ export class CoreToolScheduler {
     this.onToolCallsUpdate = options.onToolCallsUpdate;
     this.getPreferredEditor = options.getPreferredEditor;
     this.toolExecutor = new ToolExecutor(this.config);
+    this.toolModifier = new ToolModificationHandler();
 
     // Subscribe to message bus for ASK_USER policy decisions
     // Use a static WeakMap to ensure we only subscribe ONCE per MessageBus instance
@@ -749,105 +746,61 @@ export class CoreToolScheduler {
       return; // `cancelAll` calls `checkAndNotifyCompletion`, so we can exit here.
     } else if (outcome === ToolConfirmationOutcome.ModifyWithEditor) {
       const waitingToolCall = toolCall as WaitingToolCall;
-      if (isModifiableDeclarativeTool(waitingToolCall.tool)) {
-        const modifyContext = waitingToolCall.tool.getModifyContext(signal);
-        const editorType = this.getPreferredEditor();
-        if (!editorType) {
-          return;
-        }
 
+      const editorType = this.getPreferredEditor();
+      if (!editorType) {
+        return;
+      }
+
+      this.setStatusInternal(callId, 'awaiting_approval', signal, {
+        ...waitingToolCall.confirmationDetails,
+        isModifying: true,
+      } as ToolCallConfirmationDetails);
+
+      const result = await this.toolModifier.handleModifyWithEditor(
+        waitingToolCall,
+        editorType,
+        signal,
+      );
+
+      // Restore status (isModifying: false) and update diff if result exists
+      if (result) {
+        this.setArgsInternal(callId, result.updatedParams);
         this.setStatusInternal(callId, 'awaiting_approval', signal, {
           ...waitingToolCall.confirmationDetails,
-          isModifying: true,
+          fileDiff: result.updatedDiff,
+          isModifying: false,
         } as ToolCallConfirmationDetails);
-
-        const contentOverrides =
-          waitingToolCall.confirmationDetails.type === 'edit'
-            ? {
-                currentContent:
-                  waitingToolCall.confirmationDetails.originalContent,
-                proposedContent: waitingToolCall.confirmationDetails.newContent,
-              }
-            : undefined;
-
-        const { updatedParams, updatedDiff } = await modifyWithEditor<
-          typeof waitingToolCall.request.args
-        >(
-          waitingToolCall.request.args,
-          modifyContext as ModifyContext<typeof waitingToolCall.request.args>,
-          editorType,
-          signal,
-          contentOverrides,
-        );
-        this.setArgsInternal(callId, updatedParams);
+      } else {
         this.setStatusInternal(callId, 'awaiting_approval', signal, {
           ...waitingToolCall.confirmationDetails,
-          fileDiff: updatedDiff,
           isModifying: false,
         } as ToolCallConfirmationDetails);
       }
     } else {
-      // If the client provided new content, apply it before scheduling.
+      // If the client provided new content, apply it and wait for
+      // re-confirmation.
       if (payload?.newContent && toolCall) {
-        await this._applyInlineModify(
+        const result = await this.toolModifier.applyInlineModify(
           toolCall as WaitingToolCall,
           payload,
           signal,
         );
+        if (result) {
+          this.setArgsInternal(callId, result.updatedParams);
+          this.setStatusInternal(callId, 'awaiting_approval', signal, {
+            ...(toolCall as WaitingToolCall).confirmationDetails,
+            fileDiff: result.updatedDiff,
+          } as ToolCallConfirmationDetails);
+          // After an inline modification, wait for another user confirmation.
+          return;
+        }
       }
       this.setStatusInternal(callId, 'scheduled', signal);
     }
     await this.attemptExecutionOfScheduledCalls(signal);
   }
 
-  /**
-   * Applies user-provided content changes to a tool call that is awaiting confirmation.
-   * This method updates the tool's arguments and refreshes the confirmation prompt with a new diff
-   * before the tool is scheduled for execution.
-   * @private
-   */
-  private async _applyInlineModify(
-    toolCall: WaitingToolCall,
-    payload: ToolConfirmationPayload,
-    signal: AbortSignal,
-  ): Promise<void> {
-    if (
-      toolCall.confirmationDetails.type !== 'edit' ||
-      !isModifiableDeclarativeTool(toolCall.tool)
-    ) {
-      return;
-    }
-
-    const modifyContext = toolCall.tool.getModifyContext(signal);
-    const currentContent = await modifyContext.getCurrentContent(
-      toolCall.request.args,
-    );
-
-    const updatedParams = modifyContext.createUpdatedParams(
-      currentContent,
-      payload.newContent,
-      toolCall.request.args,
-    );
-    const updatedDiff = Diff.createPatch(
-      modifyContext.getFilePath(toolCall.request.args),
-      currentContent,
-      payload.newContent,
-      'Current',
-      'Proposed',
-    );
-
-    this.setArgsInternal(toolCall.request.callId, updatedParams);
-    this.setStatusInternal(
-      toolCall.request.callId,
-      'awaiting_approval',
-      signal,
-      {
-        ...toolCall.confirmationDetails,
-        fileDiff: updatedDiff,
-      },
-    );
-  }
-
   private async attemptExecutionOfScheduledCalls(
     signal: AbortSignal,
   ): Promise<void> {

packages/core/src/scheduler/tool-modifier.test.ts

@@ -0,0 +1,252 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { ToolModificationHandler } from './tool-modifier.js';
+import type { WaitingToolCall, ToolCallRequestInfo } from './types.js';
+import * as modifiableToolModule from '../tools/modifiable-tool.js';
+import * as Diff from 'diff';
+import { MockModifiableTool, MockTool } from '../test-utils/mock-tool.js';
+import type {
+  ToolResult,
+  ToolInvocation,
+  ToolConfirmationPayload,
+} from '../tools/tools.js';
+import type { ModifyContext } from '../tools/modifiable-tool.js';
+import type { Mock } from 'vitest';
+
+// Mock the modules that export functions we need to control
+vi.mock('diff', () => ({
+  createPatch: vi.fn(),
+  diffLines: vi.fn(),
+}));
+
+vi.mock('../tools/modifiable-tool.js', () => ({
+  isModifiableDeclarativeTool: vi.fn(),
+  modifyWithEditor: vi.fn(),
+}));
+
+type MockModifyContext = {
+  [K in keyof ModifyContext<Record<string, unknown>>]: Mock;
+};
+
+function createMockWaitingToolCall(
+  overrides: Partial<WaitingToolCall> = {},
+): WaitingToolCall {
+  return {
+    status: 'awaiting_approval',
+    request: {
+      callId: 'test-call-id',
+      name: 'test-tool',
+      args: {},
+      isClientInitiated: false,
+      prompt_id: 'test-prompt-id',
+    } as ToolCallRequestInfo,
+    tool: new MockTool({ name: 'test-tool' }),
+    invocation: {} as ToolInvocation<Record<string, unknown>, ToolResult>, // We generally don't check invocation details in these tests
+    confirmationDetails: {
+      type: 'edit',
+      title: 'Test Confirmation',
+      fileName: 'test.txt',
+      filePath: '/path/to/test.txt',
+      fileDiff: 'diff',
+      originalContent: 'original',
+      newContent: 'new',
+      onConfirm: async () => {},
+    },
+    ...overrides,
+  };
+}
+
+describe('ToolModificationHandler', () => {
+  let handler: ToolModificationHandler;
+  let mockModifiableTool: MockModifiableTool;
+  let mockPlainTool: MockTool;
+  let mockModifyContext: MockModifyContext;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    handler = new ToolModificationHandler();
+    mockModifiableTool = new MockModifiableTool();
+    mockPlainTool = new MockTool({ name: 'plainTool' });
+
+    mockModifyContext = {
+      getCurrentContent: vi.fn(),
+      getFilePath: vi.fn(),
+      createUpdatedParams: vi.fn(),
+      getProposedContent: vi.fn(),
+    };
+
+    vi.spyOn(mockModifiableTool, 'getModifyContext').mockReturnValue(
+      mockModifyContext as unknown as ModifyContext<Record<string, unknown>>,
+    );
+  });
+
+  describe('handleModifyWithEditor', () => {
+    it('should return undefined if tool is not modifiable', async () => {
+      vi.mocked(
+        modifiableToolModule.isModifiableDeclarativeTool,
+      ).mockReturnValue(false);
+
+      const mockWaitingToolCall = createMockWaitingToolCall({
+        tool: mockPlainTool,
+        request: {
+          callId: 'call-1',
+          name: 'plainTool',
+          args: { path: 'foo.txt' },
+          isClientInitiated: false,
+          prompt_id: 'p1',
+        },
+      });
+
+      const result = await handler.handleModifyWithEditor(
+        mockWaitingToolCall,
+        'vscode',
+        new AbortController().signal,
+      );
+
+      expect(result).toBeUndefined();
+    });
+
+    it('should call modifyWithEditor and return updated params', async () => {
+      vi.mocked(
+        modifiableToolModule.isModifiableDeclarativeTool,
+      ).mockReturnValue(true);
+
+      vi.mocked(modifiableToolModule.modifyWithEditor).mockResolvedValue({
+        updatedParams: { path: 'foo.txt', content: 'new' },
+        updatedDiff: 'diff',
+      });
+
+      const mockWaitingToolCall = createMockWaitingToolCall({
+        tool: mockModifiableTool,
+        request: {
+          callId: 'call-1',
+          name: 'mockModifiableTool',
+          args: { path: 'foo.txt' },
+          isClientInitiated: false,
+          prompt_id: 'p1',
+        },
+        confirmationDetails: {
+          type: 'edit',
+          title: 'Confirm',
+          fileName: 'foo.txt',
+          filePath: 'foo.txt',
+          fileDiff: 'diff',
+          originalContent: 'old',
+          newContent: 'new',
+          onConfirm: async () => {},
+        },
+      });
+
+      const result = await handler.handleModifyWithEditor(
+        mockWaitingToolCall,
+        'vscode',
+        new AbortController().signal,
+      );
+
+      expect(modifiableToolModule.modifyWithEditor).toHaveBeenCalledWith(
+        mockWaitingToolCall.request.args,
+        mockModifyContext,
+        'vscode',
+        expect.any(AbortSignal),
+        { currentContent: 'old', proposedContent: 'new' },
+      );
+
+      expect(result).toEqual({
+        updatedParams: { path: 'foo.txt', content: 'new' },
+        updatedDiff: 'diff',
+      });
+    });
+  });
+
+  describe('applyInlineModify', () => {
+    it('should return undefined if tool is not modifiable', async () => {
+      vi.mocked(
+        modifiableToolModule.isModifiableDeclarativeTool,
+      ).mockReturnValue(false);
+
+      const mockWaitingToolCall = createMockWaitingToolCall({
+        tool: mockPlainTool,
+      });
+
+      const result = await handler.applyInlineModify(
+        mockWaitingToolCall,
+        { newContent: 'foo' },
+        new AbortController().signal,
+      );
+
+      expect(result).toBeUndefined();
+    });
+
+    it('should return undefined if payload has no new content', async () => {
+      vi.mocked(
+        modifiableToolModule.isModifiableDeclarativeTool,
+      ).mockReturnValue(true);
+
+      const mockWaitingToolCall = createMockWaitingToolCall({
+        tool: mockModifiableTool,
+      });
+
+      const result = await handler.applyInlineModify(
+        mockWaitingToolCall,
+        { newContent: undefined } as unknown as ToolConfirmationPayload,
+        new AbortController().signal,
+      );
+
+      expect(result).toBeUndefined();
+    });
+
+    it('should calculate diff and return updated params', async () => {
+      vi.mocked(
+        modifiableToolModule.isModifiableDeclarativeTool,
+      ).mockReturnValue(true);
+      (Diff.createPatch as unknown as Mock).mockReturnValue('mock-diff');
+
+      mockModifyContext.getCurrentContent.mockResolvedValue('old content');
+      mockModifyContext.getFilePath.mockReturnValue('test.txt');
+      mockModifyContext.createUpdatedParams.mockReturnValue({
+        content: 'new content',
+      });
+
+      const mockWaitingToolCall = createMockWaitingToolCall({
+        tool: mockModifiableTool,
+        request: {
+          callId: 'call-1',
+          name: 'mockModifiableTool',
+          args: { content: 'original' },
+          isClientInitiated: false,
+          prompt_id: 'p1',
+        },
+      });
+
+      const result = await handler.applyInlineModify(
+        mockWaitingToolCall,
+        { newContent: 'new content' },
+        new AbortController().signal,
+      );
+
+      expect(mockModifyContext.getCurrentContent).toHaveBeenCalled();
+      expect(mockModifyContext.createUpdatedParams).toHaveBeenCalledWith(
+        'old content',
+        'new content',
+        { content: 'original' },
+      );
+      expect(Diff.createPatch).toHaveBeenCalledWith(
+        'test.txt',
+        'old content',
+        'new content',
+        'Current',
+        'Proposed',
+      );
+
+      expect(result).toEqual({
+        updatedParams: { content: 'new content' },
+        updatedDiff: 'mock-diff',
+      });
+    });
+  });
+});

packages/core/src/scheduler/tool-modifier.ts

@@ -0,0 +1,105 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as Diff from 'diff';
+import type { EditorType } from '../utils/editor.js';
+import {
+  isModifiableDeclarativeTool,
+  modifyWithEditor,
+  type ModifyContext,
+} from '../tools/modifiable-tool.js';
+import type { ToolConfirmationPayload } from '../tools/tools.js';
+import type { WaitingToolCall } from './types.js';
+
+export interface ModificationResult {
+  updatedParams: Record<string, unknown>;
+  updatedDiff?: string;
+}
+
+export class ToolModificationHandler {
+  /**
+   * Handles the "Modify with Editor" flow where an external editor is launched
+   * to modify the tool's parameters.
+   */
+  async handleModifyWithEditor(
+    toolCall: WaitingToolCall,
+    editorType: EditorType,
+    signal: AbortSignal,
+  ): Promise<ModificationResult | undefined> {
+    if (!isModifiableDeclarativeTool(toolCall.tool)) {
+      return undefined;
+    }
+
+    const confirmationDetails = toolCall.confirmationDetails;
+    const modifyContext = toolCall.tool.getModifyContext(signal);
+
+    const contentOverrides =
+      confirmationDetails.type === 'edit'
+        ? {
+            currentContent: confirmationDetails.originalContent,
+            proposedContent: confirmationDetails.newContent,
+          }
+        : undefined;
+
+    const { updatedParams, updatedDiff } = await modifyWithEditor<
+      typeof toolCall.request.args
+    >(
+      toolCall.request.args,
+      modifyContext as ModifyContext<typeof toolCall.request.args>,
+      editorType,
+      signal,
+      contentOverrides,
+    );
+
+    return {
+      updatedParams,
+      updatedDiff,
+    };
+  }
+
+  /**
+   * Applies user-provided inline content updates (e.g. from the chat UI).
+   */
+  async applyInlineModify(
+    toolCall: WaitingToolCall,
+    payload: ToolConfirmationPayload,
+    signal: AbortSignal,
+  ): Promise<ModificationResult | undefined> {
+    if (
+      toolCall.confirmationDetails.type !== 'edit' ||
+      !payload.newContent ||
+      !isModifiableDeclarativeTool(toolCall.tool)
+    ) {
+      return undefined;
+    }
+
+    const modifyContext = toolCall.tool.getModifyContext(
+      signal,
+    ) as ModifyContext<typeof toolCall.request.args>;
+    const currentContent = await modifyContext.getCurrentContent(
+      toolCall.request.args,
+    );
+
+    const updatedParams = modifyContext.createUpdatedParams(
+      currentContent,
+      payload.newContent,
+      toolCall.request.args,
+    );
+
+    const updatedDiff = Diff.createPatch(
+      modifyContext.getFilePath(toolCall.request.args),
+      currentContent,
+      payload.newContent,
+      'Current',
+      'Proposed',
+    );
+
+    return {
+      updatedParams,
+      updatedDiff,
+    };
+  }
+}

packages/core/src/utils/editor.test.ts

@@ -311,11 +311,18 @@ describe('editor utils', () => {
       });
     }
 
-    it('should return the correct command for emacs', () => {
-      const command = getDiffCommand('old.txt', 'new.txt', 'emacs');
+    it('should return the correct command for emacs with escaped paths', () => {
+      const command = getDiffCommand(
+        'old file "quote".txt',
+        'new file \\back\\slash.txt',
+        'emacs',
+      );
       expect(command).toEqual({
         command: 'emacs',
-        args: ['--eval', '(ediff "old.txt" "new.txt")'],
+        args: [
+          '--eval',
+          '(ediff "old file \\"quote\\".txt" "new file \\\\back\\\\slash.txt")',
+        ],
       });
     });
 

packages/core/src/utils/editor.ts

@@ -60,6 +60,14 @@ function isValidEditorType(editor: string): editor is EditorType {
   return EDITORS_SET.has(editor);
 }
 
+/**
+ * Escapes a string for use in an Emacs Lisp string literal.
+ * Wraps in double quotes and escapes backslashes and double quotes.
+ */
+function escapeELispString(str: string): string {
+  return `"${str.replace(/\\/g, '\\\\').replace(/"/g, '\\"')}"`;
+}
+
 interface DiffCommand {
   command: string;
   args: string[];
@@ -182,7 +190,10 @@ export function getDiffCommand(
     case 'emacs':
       return {
         command: 'emacs',
-        args: ['--eval', `(ediff "${oldPath}" "${newPath}")`],
+        args: [
+          '--eval',
+          `(ediff ${escapeELispString(oldPath)} ${escapeELispString(newPath)})`,
+        ],
       };
     case 'hx':
       return {