fix: resolve infinite loop when using 'Modify with external editor' (#7669)

This fix addresses the infinite loop issue reported in #7669 where selecting
"Modify with external editor" would loop indefinitely when no editor was
configured or available.

Root cause: When getPreferredEditor() returned undefined, the code silently
returned without changing the outcome, causing the while loop to repeat.

Changes:
- Add detectFirstAvailableEditor() to auto-detect available editors
- Add resolveEditor() to handle editor resolution with proper error messages
- Update confirmation.ts to break the loop and show error when editor unavailable
- Update coreToolScheduler.ts to cancel operation with feedback when editor unavailable
- Add 11 new tests for the new editor resolution functions

The fix:
1. Properly validates editor availability before attempting to use it
2. Auto-detects an available editor if none is configured
3. Provides clear error messages explaining why the editor cannot be used
4. Breaks the loop gracefully instead of looping infinitely

packages/core/src/core/coreToolScheduler.ts

@@ -12,7 +12,8 @@ import {
   type ToolConfirmationPayload,
   ToolConfirmationOutcome,
 } from '../tools/tools.js';
-import type { EditorType } from '../utils/editor.js';
+import { resolveEditor, type EditorType } from '../utils/editor.js';
+import { coreEvents } from '../utils/events.js';
 import type { Config } from '../config/config.js';
 import { PolicyDecision, ApprovalMode } from '../policy/types.js';
 import { logToolCall } from '../telemetry/loggers.js';
@@ -758,8 +759,17 @@ export class CoreToolScheduler {
     } else if (outcome === ToolConfirmationOutcome.ModifyWithEditor) {
       const waitingToolCall = toolCall as WaitingToolCall;
 
-      const editorType = this.getPreferredEditor();
-      if (!editorType) {
+      // Use resolveEditor to check availability and auto-detect if needed
+      const preferredEditor = this.getPreferredEditor();
+      const resolution = resolveEditor(preferredEditor);
+
+      if (!resolution.editor) {
+        // No editor available - emit error feedback and cancel the operation
+        // This fixes the infinite loop issue reported in #7669
+        if (resolution.error) {
+          coreEvents.emitFeedback('error', resolution.error);
+        }
+        this.cancelAll(signal);
         return;
       }
 
@@ -770,7 +780,7 @@ export class CoreToolScheduler {
 
       const result = await this.toolModifier.handleModifyWithEditor(
         waitingToolCall,
-        editorType,
+        resolution.editor,
         signal,
       );
 

packages/core/src/scheduler/confirmation.ts

@@ -21,10 +21,11 @@ import type { ValidatingToolCall, WaitingToolCall } from './types.js';
 import type { Config } from '../config/config.js';
 import type { SchedulerStateManager } from './state-manager.js';
 import type { ToolModificationHandler } from './tool-modifier.js';
-import type { EditorType } from '../utils/editor.js';
+import { resolveEditor, type EditorType } from '../utils/editor.js';
 import type { DiffUpdateResult } from '../ide/ide-client.js';
 import { fireToolNotificationHook } from '../core/coreToolHookTriggers.js';
 import { debugLogger } from '../utils/debugLogger.js';
+import { coreEvents } from '../utils/events.js';
 
 export interface ConfirmationResult {
   outcome: ToolConfirmationOutcome;
@@ -151,7 +152,21 @@ export async function resolveConfirmation(
     outcome = response.outcome;
 
     if (outcome === ToolConfirmationOutcome.ModifyWithEditor) {
-      await handleExternalModification(deps, toolCall, signal);
+      const modResult = await handleExternalModification(
+        deps,
+        toolCall,
+        signal,
+      );
+      if (!modResult.success) {
+        // Editor is not available - emit error feedback and break the loop
+        // by cancelling the operation to prevent infinite loop
+        if (modResult.error) {
+          coreEvents.emitFeedback('error', modResult.error);
+        }
+        // Break the loop by changing outcome to Cancel
+        // This prevents the infinite loop issue reported in #7669
+        outcome = ToolConfirmationOutcome.Cancel;
+      }
     } else if (response.payload?.newContent) {
       await handleInlineModification(deps, toolCall, response.payload, signal);
       outcome = ToolConfirmationOutcome.ProceedOnce;
@@ -178,8 +193,19 @@ async function notifyHooks(
   }
 }
 
+/**
+ * Result of attempting external modification.
+ */
+interface ExternalModificationResult {
+  /** Whether the modification was successful (editor was opened) */
+  success: boolean;
+  /** Error message if the modification failed */
+  error?: string;
+}
+
 /**
  * Handles modification via an external editor (e.g. Vim).
+ * Returns a result indicating success or failure with an error message.
  */
 async function handleExternalModification(
   deps: {
@@ -189,14 +215,29 @@ async function handleExternalModification(
   },
   toolCall: ValidatingToolCall,
   signal: AbortSignal,
-): Promise<void> {
+): Promise<ExternalModificationResult> {
   const { state, modifier, getPreferredEditor } = deps;
-  const editor = getPreferredEditor();
-  if (!editor) return;
+
+  // Use the new resolveEditor function which handles:
+  // 1. Checking if preferred editor is available
+  // 2. Auto-detecting an available editor if none is configured
+  // 3. Providing helpful error messages
+  const preferredEditor = getPreferredEditor();
+  const resolution = resolveEditor(preferredEditor);
+
+  if (!resolution.editor) {
+    // No editor available - return failure with error message
+    return {
+      success: false,
+      error:
+        resolution.error ||
+        'No external editor is available. Please run /editor to configure one.',
+    };
+  }
 
   const result = await modifier.handleModifyWithEditor(
     state.firstActiveCall as WaitingToolCall,
-    editor,
+    resolution.editor,
     signal,
   );
   if (result) {
@@ -207,6 +248,7 @@ async function handleExternalModification(
       newInvocation,
     );
   }
+  return { success: true };
 }
 
 /**

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

@@ -19,6 +19,8 @@ import {
   openDiff,
   allowEditorTypeInSandbox,
   isEditorAvailable,
+  detectFirstAvailableEditor,
+  resolveEditor,
   type EditorType,
 } from './editor.js';
 import { execSync, spawn, spawnSync } from 'node:child_process';
@@ -542,4 +544,130 @@ describe('editor utils', () => {
       expect(isEditorAvailable('neovim')).toBe(true);
     });
   });
+
+  describe('detectFirstAvailableEditor', () => {
+    it('should return undefined when no editors are installed', () => {
+      (execSync as Mock).mockImplementation(() => {
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      expect(detectFirstAvailableEditor()).toBeUndefined();
+    });
+
+    it('should prioritize terminal editors over GUI editors', () => {
+      // Mock vim as available
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('vim') && !cmd.includes('nvim')) {
+          return Buffer.from('/usr/bin/vim');
+        }
+        if (cmd.includes('code')) {
+          return Buffer.from('/usr/bin/code');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      expect(detectFirstAvailableEditor()).toBe('vim');
+    });
+
+    it('should return vim when vim is the only editor available in sandbox mode', () => {
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('vim') && !cmd.includes('nvim')) {
+          return Buffer.from('/usr/bin/vim');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', 'sandbox');
+      expect(detectFirstAvailableEditor()).toBe('vim');
+    });
+
+    it('should skip GUI editors in sandbox mode', () => {
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('code')) {
+          return Buffer.from('/usr/bin/code');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', 'sandbox');
+      // vscode is installed but not allowed in sandbox
+      expect(detectFirstAvailableEditor()).toBeUndefined();
+    });
+
+    it('should return first available terminal editor (neovim)', () => {
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('nvim')) {
+          return Buffer.from('/usr/bin/nvim');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      expect(detectFirstAvailableEditor()).toBe('neovim');
+    });
+  });
+
+  describe('resolveEditor', () => {
+    it('should return the preferred editor when available', () => {
+      (execSync as Mock).mockReturnValue(Buffer.from('/usr/bin/vim'));
+      vi.stubEnv('SANDBOX', '');
+      const result = resolveEditor('vim');
+      expect(result.editor).toBe('vim');
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return error when preferred editor is not installed', () => {
+      (execSync as Mock).mockImplementation(() => {
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      const result = resolveEditor('vim');
+      expect(result.editor).toBeUndefined();
+      expect(result.error).toContain('Vim');
+      expect(result.error).toContain('not installed');
+    });
+
+    it('should return error when preferred GUI editor cannot be used in sandbox mode', () => {
+      (execSync as Mock).mockReturnValue(Buffer.from('/usr/bin/code'));
+      vi.stubEnv('SANDBOX', 'sandbox');
+      const result = resolveEditor('vscode');
+      expect(result.editor).toBeUndefined();
+      expect(result.error).toContain('VS Code');
+      expect(result.error).toContain('sandbox mode');
+    });
+
+    it('should auto-detect editor when no preference is set', () => {
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('vim') && !cmd.includes('nvim')) {
+          return Buffer.from('/usr/bin/vim');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      const result = resolveEditor(undefined);
+      expect(result.editor).toBe('vim');
+      expect(result.error).toBeUndefined();
+    });
+
+    it('should return error when no preference is set and no editors are available', () => {
+      (execSync as Mock).mockImplementation(() => {
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', '');
+      const result = resolveEditor(undefined);
+      expect(result.editor).toBeUndefined();
+      expect(result.error).toContain('No external editor');
+      expect(result.error).toContain('/editor');
+    });
+
+    it('should work with terminal editors in sandbox mode when no preference is set', () => {
+      (execSync as Mock).mockImplementation((cmd: string) => {
+        if (cmd.includes('vim') && !cmd.includes('nvim')) {
+          return Buffer.from('/usr/bin/vim');
+        }
+        throw new Error('Command not found');
+      });
+      vi.stubEnv('SANDBOX', 'sandbox');
+      const result = resolveEditor(undefined);
+      expect(result.editor).toBe('vim');
+      expect(result.error).toBeUndefined();
+    });
+  });
 });

packages/core/src/utils/editor.ts

@@ -142,6 +142,79 @@ export function isEditorAvailable(editor: string | undefined): boolean {
   return false;
 }
 
+/**
+ * Detects the first available editor from the supported list.
+ * Prioritizes terminal editors (vim, neovim, emacs, hx) as they work in all environments
+ * including sandboxed mode, then falls back to GUI editors.
+ * Returns undefined if no supported editor is found.
+ */
+export function detectFirstAvailableEditor(): EditorType | undefined {
+  // Prioritize terminal editors as they work in sandbox mode
+  for (const editor of TERMINAL_EDITORS) {
+    if (isEditorAvailable(editor)) {
+      return editor;
+    }
+  }
+  // Fall back to GUI editors (won't work in sandbox mode but checked above)
+  for (const editor of GUI_EDITORS) {
+    if (isEditorAvailable(editor)) {
+      return editor;
+    }
+  }
+  return undefined;
+}
+
+/**
+ * Result of attempting to resolve an editor for use.
+ */
+export interface EditorResolutionResult {
+  /** The editor to use, if available */
+  editor?: EditorType;
+  /** Error message if no editor is available */
+  error?: string;
+}
+
+/**
+ * Resolves an editor to use for external editing.
+ * 1. If a preferred editor is set and available, uses it.
+ * 2. If a preferred editor is set but not available, returns an error.
+ * 3. If no preferred editor is set, attempts to auto-detect an available editor.
+ * 4. If no editor can be found, returns an error with instructions.
+ */
+export function resolveEditor(
+  preferredEditor: EditorType | undefined,
+): EditorResolutionResult {
+  // Case 1: Preferred editor is set
+  if (preferredEditor) {
+    if (isEditorAvailable(preferredEditor)) {
+      return { editor: preferredEditor };
+    }
+    // Preferred editor is set but not available
+    const displayName = getEditorDisplayName(preferredEditor);
+    if (!checkHasEditorType(preferredEditor)) {
+      return {
+        error: `${displayName} is configured as your preferred editor but is not installed. Please install it or run /editor to choose a different editor.`,
+      };
+    }
+    // If the editor is installed but not available, it must be due to sandbox restrictions.
+    return {
+      error: `${displayName} cannot be used in sandbox mode. Please run /editor to choose a terminal-based editor (vim, neovim, emacs, or helix).`,
+    };
+  }
+
+  // Case 2: No preferred editor set, try to auto-detect
+  const detectedEditor = detectFirstAvailableEditor();
+  if (detectedEditor) {
+    return { editor: detectedEditor };
+  }
+
+  // Case 3: No editor available at all
+  return {
+    error:
+      'No external editor is configured or available. Please run /editor to set your preferred editor, or install one of the supported editors: vim, neovim, emacs, helix, VS Code, Cursor, Zed, or Windsurf.',
+  };
+}
+
 /**
  * Get the diff command for a specific editor.
  */