diff --git a/src/commands/delete.test.ts b/src/commands/delete.test.ts
index 50a200c..bdfd4ac 100644
--- a/src/commands/delete.test.ts
+++ b/src/commands/delete.test.ts
@@ -181,4 +181,64 @@ describe('DeleteCommand', () => {
       expect(result.isOk()).toBe(true);
     });
   });
+
+  describe('non-interactive execution guard (issue #4)', () => {
+    let originalIsTTY: boolean | undefined;
+
+    beforeEach(() => {
+      originalIsTTY = process.stdout.isTTY;
+    });
+
+    afterEach(() => {
+      process.stdout.isTTY = originalIsTTY;
+    });
+
+    function createCommand(flags: { recursive: boolean; yes: boolean }): DeleteCommand {
+      const cmd = new DeleteCommand([], {} as Config);
+      vi.spyOn(cmd as any, 'parse').mockResolvedValue({
+        args: { path: 'users/doc1' },
+        flags,
+      });
+      return cmd;
+    }
+
+    it('fails loudly and performs no deletion when non-TTY and --yes is omitted', async () => {
+      process.stdout.isTTY = false;
+      const cmd = createCommand({ recursive: false, yes: false });
+      const initializeSpy = vi.spyOn(cmd as any, 'initialize');
+
+      await expect(cmd.run()).rejects.toMatchObject({ oclif: { exit: 1 } });
+
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringContaining('非対話実行では -y が必要です')
+      );
+      expect(initializeSpy).not.toHaveBeenCalled();
+    });
+
+    it('proceeds past the guard when non-TTY and --yes is specified', async () => {
+      process.stdout.isTTY = false;
+      const cmd = createCommand({ recursive: false, yes: true });
+      const initializeSpy = vi
+        .spyOn(cmd as any, 'initialize')
+        .mockResolvedValue(err({ type: 'CONFIG_ERROR', message: 'stop after guard' }));
+      vi.spyOn(cmd as any, 'handleError').mockImplementation(() => {});
+
+      await cmd.run();
+
+      expect(initializeSpy).toHaveBeenCalled();
+    });
+
+    it('proceeds past the guard on a TTY even when --yes is omitted', async () => {
+      process.stdout.isTTY = true;
+      const cmd = createCommand({ recursive: false, yes: false });
+      const initializeSpy = vi
+        .spyOn(cmd as any, 'initialize')
+        .mockResolvedValue(err({ type: 'CONFIG_ERROR', message: 'stop after guard' }));
+      vi.spyOn(cmd as any, 'handleError').mockImplementation(() => {});
+
+      await cmd.run();
+
+      expect(initializeSpy).toHaveBeenCalled();
+    });
+  });
 });
diff --git a/src/commands/delete.ts b/src/commands/delete.ts
index daeb8bc..89f9621 100644
--- a/src/commands/delete.ts
+++ b/src/commands/delete.ts
@@ -67,6 +67,13 @@ export class DeleteCommand extends BaseCommand {
     const recursive = flags.recursive;
     const skipConfirm = flags.yes;
 
+    // A confirmation prompt cannot be shown without a TTY; fail loudly
+    // instead of silently no-op'ing (see #4).
+    if (!skipConfirm && !process.stdout.isTTY) {
+      this.handleError(t('err.nonInteractiveRequiresYes'), 'validation');
+      return;
+    }
+
     // Initialize config
     const initResult = await this.initialize();
     if (initResult.isErr()) {
diff --git a/src/shared/i18n.ts b/src/shared/i18n.ts
index c4ebd13..a7be967 100644
--- a/src/shared/i18n.ts
+++ b/src/shared/i18n.ts
@@ -77,6 +77,7 @@ export interface Messages {
   'err.invalidJson': string;
   'err.authFailed': string;
   'err.permissionDenied': string;
+  'err.nonInteractiveRequiresYes': string;
 
   // Prompts
   'prompt.confirmDelete': string;
@@ -379,6 +380,7 @@ const jaMessages: Messages = {
   'err.invalidJson': 'JSONの形式が不正です',
   'err.authFailed': '認証に失敗しました',
   'err.permissionDenied': '権限がありません',
+  'err.nonInteractiveRequiresYes': '非対話実行では -y が必要です（確認プロンプトを表示できません）',
 
   // Prompts
   'prompt.confirmDelete': 'を削除しますか？',
@@ -681,6 +683,7 @@ const enMessages: Messages = {
   'err.invalidJson': 'Invalid JSON format',
   'err.authFailed': 'Authentication failed',
   'err.permissionDenied': 'Permission denied',
+  'err.nonInteractiveRequiresYes': '-y is required for non-interactive execution (confirmation prompt cannot be shown)',
 
   // Prompts
   'prompt.confirmDelete': 'Are you sure you want to delete',