diff --git a/app/src/lib/i18n/ar.ts b/app/src/lib/i18n/ar.ts
index b89e553db..0f44a0a31 100644
--- a/app/src/lib/i18n/ar.ts
+++ b/app/src/lib/i18n/ar.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'أُلغيت',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2817,10 +2818,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'يعمل…',
   'conversations.subagent.noOutputYet': 'لا يوجد ناتج بعد',
   'conversations.subagent.close': 'إغلاق',
+  'conversations.subagent.cancel': 'إلغاء المهمة',
+  'conversations.subagent.cancelling': 'جارٍ الإلغاء…',
+  'conversations.subagent.cancelFailed': 'تعذّر إلغاء المهمة. حاول مرة أخرى.',
   'conversations.subagent.statusRunning': 'قيد التشغيل',
   'conversations.subagent.statusCompleted': 'اكتمل',
   'conversations.subagent.statusFailed': 'فشل',
   'conversations.subagent.statusAwaitingUser': 'في انتظار المستخدم',
+  'conversations.subagent.statusCancelled': 'أُلغيت',
   'conversations.agentTaskInsights.title': 'رؤى مهام الوكيل',
   'conversations.agentTaskInsights.processSourceTitle': 'مصدر عملية الوكيل',
   'conversations.agentTaskInsights.stepsHeading': 'الخطوات',
diff --git a/app/src/lib/i18n/bn.ts b/app/src/lib/i18n/bn.ts
index f8e165fb6..ec9a54bf5 100644
--- a/app/src/lib/i18n/bn.ts
+++ b/app/src/lib/i18n/bn.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'বাতিল হয়েছে',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2876,10 +2877,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'কাজ করছে…',
   'conversations.subagent.noOutputYet': 'এখনও কোনো আউটপুট নেই',
   'conversations.subagent.close': 'বন্ধ করুন',
+  'conversations.subagent.cancel': 'কাজ বাতিল করুন',
+  'conversations.subagent.cancelling': 'বাতিল করা হচ্ছে…',
+  'conversations.subagent.cancelFailed': 'কাজটি বাতিল করা যায়নি। আবার চেষ্টা করুন।',
   'conversations.subagent.statusRunning': 'চলছে',
   'conversations.subagent.statusCompleted': 'সম্পন্ন',
   'conversations.subagent.statusFailed': 'ব্যর্থ',
   'conversations.subagent.statusAwaitingUser': 'ব্যবহারকারীর অপেক্ষায়',
+  'conversations.subagent.statusCancelled': 'বাতিল হয়েছে',
   'conversations.agentTaskInsights.title': 'এজেন্ট টাস্ক অন্তর্দৃষ্টি',
   'conversations.agentTaskInsights.processSourceTitle': 'এজেন্ট প্রক্রিয়া উৎস',
   'conversations.agentTaskInsights.stepsHeading': 'ধাপসমূহ',
diff --git a/app/src/lib/i18n/de.ts b/app/src/lib/i18n/de.ts
index 8526ec023..ca55e154a 100644
--- a/app/src/lib/i18n/de.ts
+++ b/app/src/lib/i18n/de.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Abgebrochen',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2946,10 +2947,15 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Arbeitet…',
   'conversations.subagent.noOutputYet': 'Noch keine Ausgabe',
   'conversations.subagent.close': 'Schließen',
+  'conversations.subagent.cancel': 'Aufgabe abbrechen',
+  'conversations.subagent.cancelling': 'Wird abgebrochen…',
+  'conversations.subagent.cancelFailed':
+    'Aufgabe konnte nicht abgebrochen werden. Bitte erneut versuchen.',
   'conversations.subagent.statusRunning': 'läuft',
   'conversations.subagent.statusCompleted': 'abgeschlossen',
   'conversations.subagent.statusFailed': 'fehlgeschlagen',
   'conversations.subagent.statusAwaitingUser': 'warte auf Benutzer',
+  'conversations.subagent.statusCancelled': 'abgebrochen',
   'conversations.agentTaskInsights.title': 'Agenten-Aufgabeneinblicke',
   'conversations.agentTaskInsights.processSourceTitle': 'Agentenprozess-Quelle',
   'conversations.agentTaskInsights.stepsHeading': 'Schritte',
diff --git a/app/src/lib/i18n/en.ts b/app/src/lib/i18n/en.ts
index 52baba9f5..07b1d6b33 100644
--- a/app/src/lib/i18n/en.ts
+++ b/app/src/lib/i18n/en.ts
@@ -3399,6 +3399,9 @@ const en: TranslationMap = {
   'conversations.subagent.working': 'Working…',
   'conversations.subagent.noOutputYet': 'No output yet',
   'conversations.subagent.close': 'Close',
+  'conversations.subagent.cancel': 'Cancel task',
+  'conversations.subagent.cancelling': 'Cancelling…',
+  'conversations.subagent.cancelFailed': "Couldn't cancel the task. Try again.",
   'conversations.backgroundTasks.title': 'Background tasks',
   'conversations.backgroundTasks.titleWithCount': 'Background tasks ({count})',
   'conversations.backgroundTasks.running': '{count} running',
@@ -3410,6 +3413,7 @@ const en: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Cancelled',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -3419,6 +3423,7 @@ const en: TranslationMap = {
   'conversations.subagent.statusCompleted': 'completed',
   'conversations.subagent.statusFailed': 'failed',
   'conversations.subagent.statusAwaitingUser': 'awaiting user',
+  'conversations.subagent.statusCancelled': 'cancelled',
   'conversations.agentTaskInsights.title': 'Agentic task insights',
   'conversations.agentTaskInsights.processSourceTitle': 'Agent Process Source',
   'conversations.agentTaskInsights.stepsHeading': 'Steps',
diff --git a/app/src/lib/i18n/es.ts b/app/src/lib/i18n/es.ts
index ff6c56c38..fb54de0b7 100644
--- a/app/src/lib/i18n/es.ts
+++ b/app/src/lib/i18n/es.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Cancelado',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2926,10 +2927,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Trabajando…',
   'conversations.subagent.noOutputYet': 'Aún no hay resultados',
   'conversations.subagent.close': 'Cerrar',
+  'conversations.subagent.cancel': 'Cancelar tarea',
+  'conversations.subagent.cancelling': 'Cancelando…',
+  'conversations.subagent.cancelFailed': 'No se pudo cancelar la tarea. Inténtalo de nuevo.',
   'conversations.subagent.statusRunning': 'en ejecución',
   'conversations.subagent.statusCompleted': 'completado',
   'conversations.subagent.statusFailed': 'fallido',
   'conversations.subagent.statusAwaitingUser': 'esperando al usuario',
+  'conversations.subagent.statusCancelled': 'cancelado',
   'conversations.agentTaskInsights.title': 'Información de tareas del agente',
   'conversations.agentTaskInsights.processSourceTitle': 'Fuente del proceso del agente',
   'conversations.agentTaskInsights.stepsHeading': 'Pasos',
diff --git a/app/src/lib/i18n/fr.ts b/app/src/lib/i18n/fr.ts
index 9684c92fc..111cb1e3d 100644
--- a/app/src/lib/i18n/fr.ts
+++ b/app/src/lib/i18n/fr.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Annulé',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2941,10 +2942,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'En cours…',
   'conversations.subagent.noOutputYet': 'Aucun résultat pour l’instant',
   'conversations.subagent.close': 'Fermer',
+  'conversations.subagent.cancel': 'Annuler la tâche',
+  'conversations.subagent.cancelling': 'Annulation…',
+  'conversations.subagent.cancelFailed': "Impossible d'annuler la tâche. Réessayez.",
   'conversations.subagent.statusRunning': 'en cours',
   'conversations.subagent.statusCompleted': 'terminé',
   'conversations.subagent.statusFailed': 'échoué',
   'conversations.subagent.statusAwaitingUser': "en attente de l'utilisateur",
+  'conversations.subagent.statusCancelled': 'annulé',
   'conversations.agentTaskInsights.title': "Aperçu des tâches de l'agent",
   'conversations.agentTaskInsights.processSourceTitle': "Source du processus de l'agent",
   'conversations.agentTaskInsights.stepsHeading': 'Étapes',
diff --git a/app/src/lib/i18n/hi.ts b/app/src/lib/i18n/hi.ts
index d55ef67df..b1eee3e76 100644
--- a/app/src/lib/i18n/hi.ts
+++ b/app/src/lib/i18n/hi.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'रद्द किया गया',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2877,10 +2878,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'काम कर रहा है…',
   'conversations.subagent.noOutputYet': 'अभी तक कोई आउटपुट नहीं',
   'conversations.subagent.close': 'बंद करें',
+  'conversations.subagent.cancel': 'कार्य रद्द करें',
+  'conversations.subagent.cancelling': 'रद्द किया जा रहा है…',
+  'conversations.subagent.cancelFailed': 'कार्य रद्द नहीं किया जा सका। पुनः प्रयास करें।',
   'conversations.subagent.statusRunning': 'चल रहा है',
   'conversations.subagent.statusCompleted': 'पूर्ण',
   'conversations.subagent.statusFailed': 'विफल',
   'conversations.subagent.statusAwaitingUser': 'उपयोगकर्ता की प्रतीक्षा',
+  'conversations.subagent.statusCancelled': 'रद्द किया गया',
   'conversations.agentTaskInsights.title': 'एजेंट कार्य अंतर्दृष्टि',
   'conversations.agentTaskInsights.processSourceTitle': 'एजेंट प्रक्रिया स्रोत',
   'conversations.agentTaskInsights.stepsHeading': 'चरण',
diff --git a/app/src/lib/i18n/id.ts b/app/src/lib/i18n/id.ts
index 9f69595d6..c15c09027 100644
--- a/app/src/lib/i18n/id.ts
+++ b/app/src/lib/i18n/id.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Dibatalkan',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2881,10 +2882,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Bekerja…',
   'conversations.subagent.noOutputYet': 'Belum ada keluaran',
   'conversations.subagent.close': 'Tutup',
+  'conversations.subagent.cancel': 'Batalkan tugas',
+  'conversations.subagent.cancelling': 'Membatalkan…',
+  'conversations.subagent.cancelFailed': 'Tidak dapat membatalkan tugas. Coba lagi.',
   'conversations.subagent.statusRunning': 'berjalan',
   'conversations.subagent.statusCompleted': 'selesai',
   'conversations.subagent.statusFailed': 'gagal',
   'conversations.subagent.statusAwaitingUser': 'menunggu pengguna',
+  'conversations.subagent.statusCancelled': 'dibatalkan',
   'conversations.agentTaskInsights.title': 'Wawasan tugas agen',
   'conversations.agentTaskInsights.processSourceTitle': 'Sumber proses agen',
   'conversations.agentTaskInsights.stepsHeading': 'Langkah',
diff --git a/app/src/lib/i18n/it.ts b/app/src/lib/i18n/it.ts
index a43a6b540..78576f457 100644
--- a/app/src/lib/i18n/it.ts
+++ b/app/src/lib/i18n/it.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Annullato',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2920,10 +2921,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'In corso…',
   'conversations.subagent.noOutputYet': 'Ancora nessun output',
   'conversations.subagent.close': 'Chiudi',
+  'conversations.subagent.cancel': 'Annulla attività',
+  'conversations.subagent.cancelling': 'Annullamento…',
+  'conversations.subagent.cancelFailed': "Impossibile annullare l'attività. Riprova.",
   'conversations.subagent.statusRunning': 'in esecuzione',
   'conversations.subagent.statusCompleted': 'completato',
   'conversations.subagent.statusFailed': 'non riuscito',
   'conversations.subagent.statusAwaitingUser': "in attesa dell'utente",
+  'conversations.subagent.statusCancelled': 'annullato',
   'conversations.agentTaskInsights.title': 'Approfondimenti attività agente',
   'conversations.agentTaskInsights.processSourceTitle': "Origine del processo dell'agente",
   'conversations.agentTaskInsights.stepsHeading': 'Passaggi',
diff --git a/app/src/lib/i18n/ko.ts b/app/src/lib/i18n/ko.ts
index 0a37938fd..75ed6a3d7 100644
--- a/app/src/lib/i18n/ko.ts
+++ b/app/src/lib/i18n/ko.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': '취소됨',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2852,10 +2853,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': '작업 중…',
   'conversations.subagent.noOutputYet': '아직 출력이 없습니다',
   'conversations.subagent.close': '닫기',
+  'conversations.subagent.cancel': '작업 취소',
+  'conversations.subagent.cancelling': '취소 중…',
+  'conversations.subagent.cancelFailed': '작업을 취소할 수 없습니다. 다시 시도하세요.',
   'conversations.subagent.statusRunning': '실행 중',
   'conversations.subagent.statusCompleted': '완료됨',
   'conversations.subagent.statusFailed': '실패',
   'conversations.subagent.statusAwaitingUser': '사용자 대기 중',
+  'conversations.subagent.statusCancelled': '취소됨',
   'conversations.agentTaskInsights.title': '에이전트 작업 인사이트',
   'conversations.agentTaskInsights.processSourceTitle': '에이전트 프로세스 소스',
   'conversations.agentTaskInsights.stepsHeading': '단계',
diff --git a/app/src/lib/i18n/pl.ts b/app/src/lib/i18n/pl.ts
index 0b6f268ea..caedc854a 100644
--- a/app/src/lib/i18n/pl.ts
+++ b/app/src/lib/i18n/pl.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Anulowano',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2909,10 +2910,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Pracuje…',
   'conversations.subagent.noOutputYet': 'Brak wyników',
   'conversations.subagent.close': 'Zamknij',
+  'conversations.subagent.cancel': 'Anuluj zadanie',
+  'conversations.subagent.cancelling': 'Anulowanie…',
+  'conversations.subagent.cancelFailed': 'Nie udało się anulować zadania. Spróbuj ponownie.',
   'conversations.subagent.statusRunning': 'w toku',
   'conversations.subagent.statusCompleted': 'zakończono',
   'conversations.subagent.statusFailed': 'niepowodzenie',
   'conversations.subagent.statusAwaitingUser': 'oczekiwanie na użytkownika',
+  'conversations.subagent.statusCancelled': 'anulowano',
   'conversations.agentTaskInsights.title': 'Wgląd w zadania agenta',
   'conversations.agentTaskInsights.processSourceTitle': 'Źródło procesu agenta',
   'conversations.agentTaskInsights.stepsHeading': 'Kroki',
diff --git a/app/src/lib/i18n/pt.ts b/app/src/lib/i18n/pt.ts
index ca1cd3867..75ca6e9b1 100644
--- a/app/src/lib/i18n/pt.ts
+++ b/app/src/lib/i18n/pt.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Cancelado',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2926,10 +2927,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Trabalhando…',
   'conversations.subagent.noOutputYet': 'Ainda sem resultado',
   'conversations.subagent.close': 'Fechar',
+  'conversations.subagent.cancel': 'Cancelar tarefa',
+  'conversations.subagent.cancelling': 'Cancelando…',
+  'conversations.subagent.cancelFailed': 'Não foi possível cancelar a tarefa. Tente novamente.',
   'conversations.subagent.statusRunning': 'em execução',
   'conversations.subagent.statusCompleted': 'concluído',
   'conversations.subagent.statusFailed': 'falhou',
   'conversations.subagent.statusAwaitingUser': 'aguardando usuário',
+  'conversations.subagent.statusCancelled': 'cancelado',
   'conversations.agentTaskInsights.title': 'Insights de tarefas do agente',
   'conversations.agentTaskInsights.processSourceTitle': 'Fonte do processo do agente',
   'conversations.agentTaskInsights.stepsHeading': 'Etapas',
diff --git a/app/src/lib/i18n/ru.ts b/app/src/lib/i18n/ru.ts
index cecc5a68e..1a8ad84e4 100644
--- a/app/src/lib/i18n/ru.ts
+++ b/app/src/lib/i18n/ru.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': 'Отменено',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2899,10 +2900,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': 'Выполняется…',
   'conversations.subagent.noOutputYet': 'Пока нет результата',
   'conversations.subagent.close': 'Закрыть',
+  'conversations.subagent.cancel': 'Отменить задачу',
+  'conversations.subagent.cancelling': 'Отмена…',
+  'conversations.subagent.cancelFailed': 'Не удалось отменить задачу. Попробуйте снова.',
   'conversations.subagent.statusRunning': 'выполняется',
   'conversations.subagent.statusCompleted': 'завершено',
   'conversations.subagent.statusFailed': 'ошибка',
   'conversations.subagent.statusAwaitingUser': 'ожидание пользователя',
+  'conversations.subagent.statusCancelled': 'отменено',
   'conversations.agentTaskInsights.title': 'Сведения о задачах агента',
   'conversations.agentTaskInsights.processSourceTitle': 'Источник процесса агента',
   'conversations.agentTaskInsights.stepsHeading': 'Шаги',
diff --git a/app/src/lib/i18n/zh-CN.ts b/app/src/lib/i18n/zh-CN.ts
index 3b8d21de1..e51241e59 100644
--- a/app/src/lib/i18n/zh-CN.ts
+++ b/app/src/lib/i18n/zh-CN.ts
@@ -14,6 +14,7 @@ const messages: TranslationMap = {
   'conversations.backgroundTasks.statusDone': 'Done',
   'conversations.backgroundTasks.statusFailed': 'Failed',
   'conversations.backgroundTasks.statusNeedsYou': 'Needs you',
+  'conversations.backgroundTasks.statusCancelled': '已取消',
   'conversations.backgroundTasks.toolCallOne': '{count} tool call',
   'conversations.backgroundTasks.toolCallOther': '{count} tool calls',
   'conversations.backgroundTasks.steps': '{count} steps',
@@ -2737,10 +2738,14 @@ const messages: TranslationMap = {
   'conversations.subagent.working': '处理中…',
   'conversations.subagent.noOutputYet': '暂无输出',
   'conversations.subagent.close': '关闭',
+  'conversations.subagent.cancel': '取消任务',
+  'conversations.subagent.cancelling': '正在取消…',
+  'conversations.subagent.cancelFailed': '无法取消任务。请重试。',
   'conversations.subagent.statusRunning': '运行中',
   'conversations.subagent.statusCompleted': '已完成',
   'conversations.subagent.statusFailed': '失败',
   'conversations.subagent.statusAwaitingUser': '等待用户',
+  'conversations.subagent.statusCancelled': '已取消',
   'conversations.agentTaskInsights.title': '智能体任务洞察',
   'conversations.agentTaskInsights.processSourceTitle': '智能体处理来源',
   'conversations.agentTaskInsights.stepsHeading': '步骤',
diff --git a/app/src/pages/Conversations.tsx b/app/src/pages/Conversations.tsx
index dbcc2d9f1..3ae97c273 100644
--- a/app/src/pages/Conversations.tsx
+++ b/app/src/pages/Conversations.tsx
@@ -30,6 +30,7 @@ import {
 import { useT } from '../lib/i18n/I18nContext';
 import { trackEvent } from '../services/analytics';
 import { applyOpenRouterFreeModels } from '../services/api/openrouterFreeModels';
+import { subagentApi } from '../services/api/subagentApi';
 import { threadApi } from '../services/api/threadApi';
 import { chatCancel, chatSend, useRustChat } from '../services/chatService';
 import { callCoreRpc } from '../services/coreRpcClient';
@@ -43,6 +44,7 @@ import {
   beginInferenceTurn,
   clearRuntimeForThread,
   fetchAndHydrateTurnState,
+  markSubagentCancelled,
   registerParallelRequest,
   setTaskBoardForThread,
   setToolTimelineForThread,
@@ -2591,8 +2593,28 @@ const Conversations = ({
         }}
       />
       <SubagentDrawer
+        key={openSubagentTaskId ?? 'none'}
         subagent={openSubagentEntry?.subagent ?? null}
         status={openSubagentEntry?.status}
+        onCancel={
+          openSubagentEntry?.subagent && selectedThreadId
+            ? async () => {
+                const taskId = openSubagentEntry.subagent!.taskId;
+                const result = await subagentApi.cancel(taskId);
+                // Only flip the row when something was actually aborted — a
+                // cancelled=false result means the run already finished/unknown,
+                // and overwriting its real terminal state would hide it. No
+                // terminal socket event arrives for an aborted run, so the
+                // optimistic mark is what surfaces the cancellation (the notice
+                // itself reaches chat via the idle-gated delivery path).
+                if (result.cancelled) {
+                  dispatch(
+                    markSubagentCancelled({ threadId: selectedThreadId, taskId: result.taskId })
+                  );
+                }
+              }
+            : undefined
+        }
         onClose={() => setOpenSubagentTaskId(null)}
       />
       <AgentProcessSourcePanel
diff --git a/app/src/pages/conversations/components/AgentTimelineRail.tsx b/app/src/pages/conversations/components/AgentTimelineRail.tsx
index d41b17cb6..389fb7f3a 100644
--- a/app/src/pages/conversations/components/AgentTimelineRail.tsx
+++ b/app/src/pages/conversations/components/AgentTimelineRail.tsx
@@ -43,6 +43,9 @@ export function agentNameTone(status: ToolTimelineEntryStatus | undefined): stri
       return 'text-coral-600 dark:text-coral-300';
     case 'awaiting_user':
       return 'animate-pulse text-amber-600 dark:text-amber-300';
+    case 'cancelled':
+      // Cancelled — terminal, so muted but NOT pulsing (it isn't in progress).
+      return 'text-stone-400 dark:text-neutral-500';
     default:
       // running / unknown — in progress: muted + blinking.
       return 'animate-pulse text-stone-400 dark:text-neutral-500';
diff --git a/app/src/pages/conversations/components/BackgroundProcessesPanel.tsx b/app/src/pages/conversations/components/BackgroundProcessesPanel.tsx
index 5a5c597e0..c43a391ec 100644
--- a/app/src/pages/conversations/components/BackgroundProcessesPanel.tsx
+++ b/app/src/pages/conversations/components/BackgroundProcessesPanel.tsx
@@ -58,7 +58,8 @@ type StatusLabelKey =
   | 'conversations.backgroundTasks.statusRunning'
   | 'conversations.backgroundTasks.statusDone'
   | 'conversations.backgroundTasks.statusFailed'
-  | 'conversations.backgroundTasks.statusNeedsYou';
+  | 'conversations.backgroundTasks.statusNeedsYou'
+  | 'conversations.backgroundTasks.statusCancelled';
 
 function statusStyle(status: ToolTimelineEntryStatus): {
   dot: string;
@@ -84,6 +85,12 @@ function statusStyle(status: ToolTimelineEntryStatus): {
         labelKey: 'conversations.backgroundTasks.statusNeedsYou',
         pill: 'text-blue-700 dark:text-blue-300',
       };
+    case 'cancelled':
+      return {
+        dot: 'bg-stone-400 dark:bg-neutral-500',
+        labelKey: 'conversations.backgroundTasks.statusCancelled',
+        pill: 'text-stone-600 dark:text-neutral-300',
+      };
     default:
       return {
         dot: 'bg-sage-500',
diff --git a/app/src/pages/conversations/components/SubagentDrawer.tsx b/app/src/pages/conversations/components/SubagentDrawer.tsx
index c94633dc4..25c69c17e 100644
--- a/app/src/pages/conversations/components/SubagentDrawer.tsx
+++ b/app/src/pages/conversations/components/SubagentDrawer.tsx
@@ -50,7 +50,12 @@ function transcriptFromMessages(messages: ThreadMessage[]): {
 function statusTone(status: ToolTimelineEntryStatus | undefined): {
   dot: string;
   pill: string;
-  label: 'statusRunning' | 'statusCompleted' | 'statusFailed' | 'statusAwaitingUser';
+  label:
+    | 'statusRunning'
+    | 'statusCompleted'
+    | 'statusFailed'
+    | 'statusAwaitingUser'
+    | 'statusCancelled';
 } {
   if (status === 'success') {
     return {
@@ -66,6 +71,13 @@ function statusTone(status: ToolTimelineEntryStatus | undefined): {
       label: 'statusFailed',
     };
   }
+  if (status === 'cancelled') {
+    return {
+      dot: 'bg-stone-400 dark:bg-neutral-500',
+      pill: 'bg-stone-100 dark:bg-neutral-700/40 text-stone-600 dark:text-neutral-300',
+      label: 'statusCancelled',
+    };
+  }
   if (status === 'awaiting_user') {
     return {
       dot: 'bg-amber-400 animate-pulse',
@@ -101,14 +113,28 @@ function formatElapsed(ms: number): string {
 export function SubagentDrawer({
   subagent,
   status,
+  onCancel,
   onClose,
 }: {
   subagent: SubagentActivity | null;
   /** Lifecycle status of the owning timeline row (running/success/error). */
   status?: ToolTimelineEntryStatus;
+  /**
+   * Cancel this still-running detached sub-agent. When provided and the run is
+   * running, a "Cancel task" affordance is shown. The parent owns the actual
+   * abort + chat delivery (via `subagentApi.cancel`); the drawer only manages
+   * the in-flight / error UI and closes on success. Rejecting surfaces an error.
+   */
+  onCancel?: () => Promise<void>;
   onClose: () => void;
 }) {
   const { t } = useT();
+  // Cancel-in-flight + last-error state for the "Cancel task" affordance.
+  // The parent keys this drawer by task id, so a different sub-agent remounts
+  // with fresh state — no effect-driven reset needed (which would trip the
+  // repo's `react-hooks/set-state-in-effect` rule).
+  const [cancelling, setCancelling] = useState(false);
+  const [cancelError, setCancelError] = useState(false);
 
   // Close on Escape for keyboard parity with the backdrop click.
   useEffect(() => {
@@ -162,7 +188,25 @@ export function SubagentDrawer({
   if (!subagent) return null;
 
   const tone = statusTone(status);
-  const isRunning = status !== 'success' && status !== 'error';
+  const isRunning = status !== 'success' && status !== 'error' && status !== 'cancelled';
+  // The "Cancel task" CTA is only meaningful for a live, still-running run the
+  // parent gave us a cancel handler for.
+  const canCancel = status === 'running' && Boolean(onCancel);
+
+  const handleCancel = async () => {
+    if (!onCancel || cancelling) return;
+    setCancelling(true);
+    setCancelError(false);
+    try {
+      await onCancel();
+      // Success: the parent flips the row to cancelled and the notice rides the
+      // idle-delivery path into chat — close the drawer.
+      onClose();
+    } catch {
+      setCancelling(false);
+      setCancelError(true);
+    }
+  };
   // Only trust the fetched transcript when it belongs to the current worker.
   const fetchedForCurrent =
     fetched && workerThreadId && fetched.workerThreadId === workerThreadId ? fetched : null;
@@ -219,6 +263,18 @@ export function SubagentDrawer({
               {subagent.mode ? <span>{subagent.mode}</span> : null}
             </div>
           </div>
+          {canCancel ? (
+            <button
+              type="button"
+              onClick={handleCancel}
+              disabled={cancelling}
+              data-testid="subagent-cancel"
+              className="shrink-0 rounded-full border border-coral-200 px-2.5 py-1 text-xs font-medium text-coral-600 hover:bg-coral-50 disabled:cursor-not-allowed disabled:opacity-60 dark:border-coral-500/40 dark:text-coral-300 dark:hover:bg-coral-500/10">
+              {cancelling
+                ? t('conversations.subagent.cancelling')
+                : t('conversations.subagent.cancel')}
+            </button>
+          ) : null}
           <button
             type="button"
             onClick={onClose}
@@ -227,6 +283,14 @@ export function SubagentDrawer({
             ✕
           </button>
         </header>
+        {cancelError ? (
+          <div
+            role="alert"
+            data-testid="subagent-cancel-error"
+            className="border-b border-coral-200 bg-coral-50 px-4 py-2 text-xs text-coral-700 dark:border-coral-500/30 dark:bg-coral-500/10 dark:text-coral-300">
+            {t('conversations.subagent.cancelFailed')}
+          </div>
+        ) : null}
 
         {/* Body — a parent↔subagent conversation: the parent's delegation
             prompt opens it, then the sub-agent replies as one chronological
diff --git a/app/src/pages/conversations/components/__tests__/AgentTimelineRail.test.tsx b/app/src/pages/conversations/components/__tests__/AgentTimelineRail.test.tsx
index bac2e095f..212ea289a 100644
--- a/app/src/pages/conversations/components/__tests__/AgentTimelineRail.test.tsx
+++ b/app/src/pages/conversations/components/__tests__/AgentTimelineRail.test.tsx
@@ -27,6 +27,12 @@ describe('agentNameTone', () => {
   it('treats an unknown status as in-progress', () => {
     expect(agentNameTone(undefined)).toContain('animate-pulse');
   });
+
+  it('renders a cancelled agent muted and static (terminal, not pulsing)', () => {
+    const tone = agentNameTone('cancelled');
+    expect(tone).not.toContain('animate-pulse');
+    expect(tone).toContain('stone');
+  });
 });
 
 describe('AgentTimelineRail', () => {
diff --git a/app/src/pages/conversations/components/__tests__/BackgroundProcessesPanel.test.tsx b/app/src/pages/conversations/components/__tests__/BackgroundProcessesPanel.test.tsx
index d7491ceb2..230126498 100644
--- a/app/src/pages/conversations/components/__tests__/BackgroundProcessesPanel.test.tsx
+++ b/app/src/pages/conversations/components/__tests__/BackgroundProcessesPanel.test.tsx
@@ -128,12 +128,13 @@ describe('BackgroundProcessesPanel', () => {
     expect(screen.getByText(/No background tasks in this chat/i)).toBeInTheDocument();
   });
 
-  it('renders every status variant (running / done / failed / needs-you)', () => {
+  it('renders every status variant (running / done / failed / needs-you / cancelled)', () => {
     const all: BackgroundProcess[] = [
       { taskId: 'r', name: 'R', goal: 'g', status: 'running', toolCount: 2 },
       { taskId: 'd', name: 'D', goal: 'g', status: 'success', toolCount: 2 },
       { taskId: 'e', name: 'E', goal: 'g', status: 'error', toolCount: 2 },
       { taskId: 'a', name: 'A', goal: 'g', status: 'awaiting_user', toolCount: 2 },
+      { taskId: 'c', name: 'C', goal: 'g', status: 'cancelled', toolCount: 2 },
     ];
     render(
       <BackgroundProcessesPanel open processes={all} onClose={vi.fn()} onOpenProcess={vi.fn()} />
@@ -142,6 +143,7 @@ describe('BackgroundProcessesPanel', () => {
     expect(screen.getByText('Done')).toBeInTheDocument();
     expect(screen.getByText('Failed')).toBeInTheDocument();
     expect(screen.getByText('Needs you')).toBeInTheDocument();
+    expect(screen.getByText('Cancelled')).toBeInTheDocument();
   });
 
   it('renders singular tool-call wording, step count, and suppresses an empty goal', () => {
diff --git a/app/src/pages/conversations/components/__tests__/SubagentDrawer.test.tsx b/app/src/pages/conversations/components/__tests__/SubagentDrawer.test.tsx
index 11f48108d..d74fb7324 100644
--- a/app/src/pages/conversations/components/__tests__/SubagentDrawer.test.tsx
+++ b/app/src/pages/conversations/components/__tests__/SubagentDrawer.test.tsx
@@ -173,4 +173,73 @@ describe('SubagentDrawer', () => {
     await userEvent.click(screen.getByText('✕'));
     expect(onClose).toHaveBeenCalledTimes(1);
   });
+
+  it('shows the Cancel task CTA only while running and with an onCancel handler', () => {
+    // No handler → no CTA.
+    const { rerender } = render(
+      <SubagentDrawer subagent={activity()} status="running" onClose={() => {}} />
+    );
+    expect(screen.queryByTestId('subagent-cancel')).toBeNull();
+
+    // Handler present but already finished → no CTA.
+    rerender(
+      <SubagentDrawer
+        subagent={activity()}
+        status="success"
+        onCancel={vi.fn()}
+        onClose={() => {}}
+      />
+    );
+    expect(screen.queryByTestId('subagent-cancel')).toBeNull();
+
+    // Running + handler → CTA shown.
+    rerender(
+      <SubagentDrawer
+        subagent={activity()}
+        status="running"
+        onCancel={vi.fn()}
+        onClose={() => {}}
+      />
+    );
+    expect(screen.getByTestId('subagent-cancel')).toBeTruthy();
+  });
+
+  it('cancels via onCancel, then closes on success', async () => {
+    const onCancel = vi.fn().mockResolvedValue(undefined);
+    const onClose = vi.fn();
+    render(
+      <SubagentDrawer
+        subagent={activity()}
+        status="running"
+        onCancel={onCancel}
+        onClose={onClose}
+      />
+    );
+    await userEvent.click(screen.getByTestId('subagent-cancel'));
+    expect(onCancel).toHaveBeenCalledTimes(1);
+    await waitFor(() => expect(onClose).toHaveBeenCalledTimes(1));
+    expect(screen.queryByTestId('subagent-cancel-error')).toBeNull();
+  });
+
+  it('surfaces an error and stays open when cancel fails', async () => {
+    const onCancel = vi.fn().mockRejectedValue(new Error('boom'));
+    const onClose = vi.fn();
+    render(
+      <SubagentDrawer
+        subagent={activity()}
+        status="running"
+        onCancel={onCancel}
+        onClose={onClose}
+      />
+    );
+    await userEvent.click(screen.getByTestId('subagent-cancel'));
+    await waitFor(() => expect(screen.getByTestId('subagent-cancel-error')).toBeTruthy());
+    expect(onClose).not.toHaveBeenCalled();
+  });
+
+  it('renders the cancelled status label', () => {
+    render(<SubagentDrawer subagent={activity()} status="cancelled" onClose={() => {}} />);
+    // Case-robust: the label may be rendered as "Cancelled" or "cancelled".
+    expect(screen.getByTestId('subagent-drawer').textContent?.toLowerCase()).toContain('cancelled');
+  });
 });
diff --git a/app/src/services/api/subagentApi.test.ts b/app/src/services/api/subagentApi.test.ts
new file mode 100644
index 000000000..84ebd0a52
--- /dev/null
+++ b/app/src/services/api/subagentApi.test.ts
@@ -0,0 +1,51 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { callCoreRpc } from '../coreRpcClient';
+import { subagentApi } from './subagentApi';
+
+vi.mock('../coreRpcClient', () => ({ callCoreRpc: vi.fn() }));
+
+const mockCall = vi.mocked(callCoreRpc);
+
+describe('subagentApi.cancel', () => {
+  beforeEach(() => {
+    mockCall.mockReset();
+  });
+
+  it('calls openhuman.subagent_cancel with the trimmed taskId', async () => {
+    mockCall.mockResolvedValue({ cancelled: true, taskId: 'sub-1' });
+    const result = await subagentApi.cancel('  sub-1  ');
+    expect(mockCall).toHaveBeenCalledWith({
+      method: 'openhuman.subagent_cancel',
+      params: { taskId: 'sub-1' },
+    });
+    expect(result).toEqual({ cancelled: true, taskId: 'sub-1' });
+  });
+
+  it('forwards a non-empty reason and omits a blank one', async () => {
+    mockCall.mockResolvedValue({ cancelled: true, taskId: 'sub-1' });
+    await subagentApi.cancel('sub-1', '  user changed their mind  ');
+    expect(mockCall).toHaveBeenCalledWith({
+      method: 'openhuman.subagent_cancel',
+      params: { taskId: 'sub-1', reason: 'user changed their mind' },
+    });
+
+    mockCall.mockClear();
+    await subagentApi.cancel('sub-1', '   ');
+    expect(mockCall).toHaveBeenCalledWith({
+      method: 'openhuman.subagent_cancel',
+      params: { taskId: 'sub-1' },
+    });
+  });
+
+  it('rejects an empty taskId without calling core', async () => {
+    await expect(subagentApi.cancel('   ')).rejects.toThrow('taskId is required');
+    expect(mockCall).not.toHaveBeenCalled();
+  });
+
+  it('passes through cancelled=false (already finished / unknown)', async () => {
+    mockCall.mockResolvedValue({ cancelled: false, taskId: 'sub-x' });
+    const result = await subagentApi.cancel('sub-x');
+    expect(result.cancelled).toBe(false);
+  });
+});
diff --git a/app/src/services/api/subagentApi.ts b/app/src/services/api/subagentApi.ts
new file mode 100644
index 000000000..cfd780f25
--- /dev/null
+++ b/app/src/services/api/subagentApi.ts
@@ -0,0 +1,43 @@
+/**
+ * Frontend client for user-driven control of detached background sub-agents
+ * (`openhuman.subagent_cancel`). Backs the "Cancel task" affordance in the
+ * background-tasks drawer: aborting a still-running detached sub-agent and
+ * surfacing a "cancelled" notice back in the parent chat.
+ *
+ * The wire payload is camelCase on both directions, so this client only types
+ * the shape and forwards `taskId` (+ optional `reason`).
+ */
+import debug from 'debug';
+
+import { callCoreRpc } from '../coreRpcClient';
+
+const log = debug('subagentApi');
+
+/** Result of a cancel request. Mirrors the Rust handler payload. */
+export interface SubagentCancelResult {
+  /** True if a running sub-agent was aborted; false if it was already done/unknown. */
+  cancelled: boolean;
+  taskId: string;
+}
+
+export const subagentApi = {
+  /**
+   * Cancel a still-running detached background sub-agent by its spawn task id.
+   * Resolves with `cancelled: false` (not an error) when the sub-agent already
+   * finished or the id is unknown.
+   */
+  cancel: async (taskId: string, reason?: string): Promise<SubagentCancelResult> => {
+    const id = taskId.trim();
+    if (!id) throw new Error('subagentApi.cancel: taskId is required');
+    const params: Record<string, unknown> = { taskId: id };
+    const trimmedReason = reason?.trim();
+    if (trimmedReason) params.reason = trimmedReason;
+    log('cancel taskId=%s', id);
+    const result = await callCoreRpc<SubagentCancelResult>({
+      method: 'openhuman.subagent_cancel',
+      params,
+    });
+    log('cancel received cancelled=%s', result.cancelled);
+    return result;
+  },
+};
diff --git a/app/src/store/__tests__/chatRuntimeSlice.subagentStream.test.ts b/app/src/store/__tests__/chatRuntimeSlice.subagentStream.test.ts
index 236359011..0dd40bde4 100644
--- a/app/src/store/__tests__/chatRuntimeSlice.subagentStream.test.ts
+++ b/app/src/store/__tests__/chatRuntimeSlice.subagentStream.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from 'vitest';
 
 import reducer, {
   appendSubagentStreamDelta,
+  markSubagentCancelled,
   recordSubagentTranscriptTool,
   resolveSubagentTranscriptTool,
   setToolTimelineForThread,
@@ -190,3 +191,25 @@ describe('subagent transcript reducers', () => {
     expect(transcriptOf(unknownRow)).toHaveLength(0);
   });
 });
+
+describe('markSubagentCancelled', () => {
+  it('flips the matching row (and its subagent) to cancelled, located by taskId', () => {
+    const state = reducer(
+      withSubagentRow(),
+      markSubagentCancelled({ threadId: THREAD, taskId: 'sub-1' })
+    );
+    const entry = state.toolTimelineByThread[THREAD][0];
+    expect(entry.status).toBe('cancelled');
+    expect(entry.subagent?.status).toBe('cancelled');
+  });
+
+  it('is a no-op for an unknown taskId or thread', () => {
+    const base = withSubagentRow();
+    expect(reducer(base, markSubagentCancelled({ threadId: THREAD, taskId: 'nope' }))).toEqual(
+      base
+    );
+    expect(
+      reducer(base, markSubagentCancelled({ threadId: 'other-thread', taskId: 'sub-1' }))
+    ).toEqual(base);
+  });
+});
diff --git a/app/src/store/chatRuntimeSlice.ts b/app/src/store/chatRuntimeSlice.ts
index fd08a2f81..9a173d39c 100644
--- a/app/src/store/chatRuntimeSlice.ts
+++ b/app/src/store/chatRuntimeSlice.ts
@@ -14,7 +14,12 @@ import { resetUserScopedState } from './resetActions';
 
 const turnStateLog = debug('chatRuntime.turnState');
 
-export type ToolTimelineEntryStatus = 'running' | 'success' | 'error' | 'awaiting_user';
+export type ToolTimelineEntryStatus =
+  | 'running'
+  | 'success'
+  | 'error'
+  | 'awaiting_user'
+  | 'cancelled';
 
 export interface InferenceStatus {
   phase: 'thinking' | 'tool_use' | 'subagent';
@@ -400,8 +405,9 @@ function timelineStatusFromRun(status: AgentRun['status']): ToolTimelineEntrySta
   switch (status) {
     case 'completed':
       return 'success';
-    case 'failed':
     case 'cancelled':
+      return 'cancelled';
+    case 'failed':
     case 'interrupted':
       return 'error';
     case 'awaiting_user':
@@ -514,6 +520,19 @@ const chatRuntimeSlice = createSlice({
     clearToolTimelineForThread: (state, action: PayloadAction<{ threadId: string }>) => {
       delete state.toolTimelineByThread[action.payload.threadId];
     },
+    /**
+     * Optimistically mark a detached background sub-agent as cancelled after the
+     * user confirms a cancel via `openhuman.subagent_cancel`. The aborted run
+     * emits no terminal socket event, so without this the row would keep showing
+     * "running" forever. Located by the subagent's stable `taskId`.
+     */
+    markSubagentCancelled: (state, action: PayloadAction<{ threadId: string; taskId: string }>) => {
+      const { threadId, taskId } = action.payload;
+      const entry = state.toolTimelineByThread[threadId]?.find(e => e.subagent?.taskId === taskId);
+      if (!entry) return;
+      entry.status = 'cancelled';
+      if (entry.subagent) entry.subagent.status = 'cancelled';
+    },
     /**
      * Append a streamed `subagent_text_delta` / `subagent_thinking_delta`
      * chunk to the ordered transcript of the matching subagent row. The row
@@ -909,6 +928,7 @@ export const {
   clearParallelRequest,
   setToolTimelineForThread,
   clearToolTimelineForThread,
+  markSubagentCancelled,
   appendSubagentStreamDelta,
   recordSubagentTranscriptTool,
   resolveSubagentTranscriptTool,
diff --git a/src/core/all.rs b/src/core/all.rs
index 1c6b13290..b12997a19 100644
--- a/src/core/all.rs
+++ b/src/core/all.rs
@@ -309,6 +309,10 @@ fn build_registered_controllers() -> Vec<RegisteredController> {
     // Git-worktree isolation manager — list / status / diff / remove worker worktrees (#3376)
     controllers
         .extend(crate::openhuman::agent_orchestration::all_worktree_registered_controllers());
+    // User-driven cancel of detached background sub-agents (#3711)
+    controllers.extend(
+        crate::openhuman::agent_orchestration::all_subagent_control_registered_controllers(),
+    );
     controllers
 }
 
@@ -445,6 +449,9 @@ fn build_declared_controller_schemas() -> Vec<ControllerSchema> {
     schemas.extend(crate::openhuman::agent_orchestration::all_agent_team_controller_schemas());
     // Git-worktree isolation manager (#3376)
     schemas.extend(crate::openhuman::agent_orchestration::all_worktree_controller_schemas());
+    // User-driven cancel of detached background sub-agents (#3711)
+    schemas
+        .extend(crate::openhuman::agent_orchestration::all_subagent_control_controller_schemas());
     schemas
 }
 
diff --git a/src/openhuman/agent_orchestration/background_completions.rs b/src/openhuman/agent_orchestration/background_completions.rs
index c31f54659..2b71e2bcd 100644
--- a/src/openhuman/agent_orchestration/background_completions.rs
+++ b/src/openhuman/agent_orchestration/background_completions.rs
@@ -9,7 +9,7 @@
 //! tagged by its sub-agent process id. This module owns only the queue + the
 //! notice formatting.
 
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet, VecDeque};
 use std::sync::{Mutex, OnceLock};
 
 /// One finished background sub-agent's deliverable result.
@@ -26,14 +26,57 @@ pub struct CompletedBackgroundAgent {
     pub parent_thread_id: Option<String>,
 }
 
-static QUEUE: OnceLock<Mutex<HashMap<String, Vec<CompletedBackgroundAgent>>>> = OnceLock::new();
+/// Upper bound on the cancelled-thread tombstone set. A thread id is a one-shot
+/// UUID, so only the *recently* cancelled threads can still be racing a late
+/// completion; older tombstones are evicted in insertion order. 512 is far more
+/// than the number of sub-agents that could realistically be mid-flight when a
+/// batch of threads is deleted.
+const CANCELLED_TOMBSTONE_CAP: usize = 512;
 
-fn queue() -> &'static Mutex<HashMap<String, Vec<CompletedBackgroundAgent>>> {
-    QUEUE.get_or_init(|| Mutex::new(HashMap::new()))
+/// Shared state behind a single mutex so the cancellation check in
+/// [`record_completion`] is atomic against the tombstone+sweep in
+/// [`discard_for_thread`] — otherwise the cooperative-abort race could enqueue a
+/// completion for a thread that was just deleted (see issue #3711 review).
+#[derive(Default)]
+struct QueueState {
+    /// Finished results awaiting idle delivery, keyed by `parent_session`.
+    pending: HashMap<String, Vec<CompletedBackgroundAgent>>,
+    /// Threads whose sub-agents were cancelled because the thread was
+    /// deleted/purged. A completion that lands here *after* the discard sweep
+    /// (Tokio `abort()` is cooperative — a task already past its last `.await`
+    /// still runs to `record_completion`) is dropped instead of delivered into
+    /// a thread that no longer exists.
+    cancelled_threads: HashSet<String>,
+    /// Insertion order for `cancelled_threads`, used to bound the set.
+    cancelled_order: VecDeque<String>,
+}
+
+impl QueueState {
+    /// Tombstone `thread_id` so any straggler completion for it is dropped.
+    fn tombstone(&mut self, thread_id: &str) {
+        if self.cancelled_threads.insert(thread_id.to_string()) {
+            self.cancelled_order.push_back(thread_id.to_string());
+            while self.cancelled_order.len() > CANCELLED_TOMBSTONE_CAP {
+                if let Some(evicted) = self.cancelled_order.pop_front() {
+                    self.cancelled_threads.remove(&evicted);
+                }
+            }
+        }
+    }
+}
+
+static QUEUE: OnceLock<Mutex<QueueState>> = OnceLock::new();
+
+fn queue() -> &'static Mutex<QueueState> {
+    QUEUE.get_or_init(|| Mutex::new(QueueState::default()))
 }
 
 /// Record a finished background sub-agent for later idle delivery, keyed by
 /// `parent_session`. Idempotent on `task_id` within a session.
+///
+/// Drops the result outright if its parent thread has been tombstoned by
+/// [`discard_for_thread`] — closing the race where a detached sub-agent finishes
+/// (and records) concurrently with its parent thread being deleted.
 pub fn record_completion(
     parent_session: impl Into<String>,
     task_id: impl Into<String>,
@@ -48,10 +91,20 @@ pub fn record_completion(
         summary: summary.into(),
         parent_thread_id,
     };
-    let mut map = queue()
+    let mut state = queue()
         .lock()
         .expect("background_completions queue poisoned");
-    let pending = map.entry(parent_session).or_default();
+    if let Some(thread_id) = entry.parent_thread_id.as_deref() {
+        if state.cancelled_threads.contains(thread_id) {
+            log::debug!(
+                "[background_completions] dropping completion task_id={} for cancelled thread_id={}",
+                entry.task_id,
+                thread_id
+            );
+            return;
+        }
+    }
+    let pending = state.pending.entry(parent_session).or_default();
     if pending.iter().any(|c| c.task_id == entry.task_id) {
         return;
     }
@@ -63,6 +116,7 @@ pub fn has_pending(parent_session: &str) -> bool {
     queue()
         .lock()
         .expect("background_completions queue poisoned")
+        .pending
         .get(parent_session)
         .is_some_and(|v| !v.is_empty())
 }
@@ -72,6 +126,7 @@ pub fn pending_count(parent_session: &str) -> usize {
     queue()
         .lock()
         .expect("background_completions queue poisoned")
+        .pending
         .get(parent_session)
         .map_or(0, Vec::len)
 }
@@ -83,10 +138,55 @@ pub fn take_pending(parent_session: &str) -> Vec<CompletedBackgroundAgent> {
     queue()
         .lock()
         .expect("background_completions queue poisoned")
+        .pending
         .remove(parent_session)
         .unwrap_or_default()
 }
 
+/// Drop every queued completion whose `parent_thread_id` is `thread_id`, across
+/// **all** sessions, and **tombstone** the thread so any straggler completion
+/// that records *after* this sweep (the cooperative-abort race) is dropped by
+/// [`record_completion`] rather than delivered into a thread that no longer
+/// exists. Called when that thread is deleted. Returns the number of queued
+/// completions removed.
+pub fn discard_for_thread(thread_id: &str) -> usize {
+    let mut state = queue()
+        .lock()
+        .expect("background_completions queue poisoned");
+    state.tombstone(thread_id);
+    let mut removed = 0;
+    for pending in state.pending.values_mut() {
+        let before = pending.len();
+        pending.retain(|c| c.parent_thread_id.as_deref() != Some(thread_id));
+        removed += before - pending.len();
+    }
+    // Drop now-empty session buckets so the map doesn't accumulate keys.
+    state.pending.retain(|_, v| !v.is_empty());
+    let sessions_left = state.pending.len();
+    log::debug!(
+        "[background_completions] discard_for_thread thread_id={} removed={} sessions_left={}",
+        thread_id,
+        removed,
+        sessions_left
+    );
+    removed
+}
+
+/// Wipe every queued completion across all sessions. Called on a full thread
+/// purge. Tombstones are left intact (the per-thread protection set by
+/// [`discard_for_thread`]); the purge path tombstones each in-flight sub-agent's
+/// thread before calling this, so stragglers are still dropped. Returns the
+/// number of queued completions removed.
+pub fn clear_all() -> usize {
+    let mut state = queue()
+        .lock()
+        .expect("background_completions queue poisoned");
+    let removed: usize = state.pending.values().map(Vec::len).sum();
+    state.pending.clear();
+    log::debug!("[background_completions] clear_all removed={}", removed);
+    removed
+}
+
 /// The thread id to deliver a batch into — the first record that carries one.
 pub fn batch_thread_id(completed: &[CompletedBackgroundAgent]) -> Option<String> {
     completed.iter().find_map(|c| c.parent_thread_id.clone())
@@ -126,6 +226,17 @@ pub fn build_batched_notice(completed: &[CompletedBackgroundAgent]) -> Option<St
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::sync::MutexGuard;
+
+    /// Serializes every test that touches the global [`QUEUE`]. We reuse the
+    /// crate-wide `TEST_ENV_LOCK` because `clear_all` is also reachable from the
+    /// `threads::ops` purge test (which holds the same lock); a module-local
+    /// mutex wouldn't prevent that cross-module race.
+    fn test_guard() -> MutexGuard<'static, ()> {
+        crate::openhuman::config::TEST_ENV_LOCK
+            .lock()
+            .unwrap_or_else(|e| e.into_inner())
+    }
 
     fn c(task: &str, agent: &str, summary: &str) -> CompletedBackgroundAgent {
         CompletedBackgroundAgent {
@@ -138,6 +249,7 @@ mod tests {
 
     #[test]
     fn record_and_drain_is_session_scoped_and_batches() {
+        let _guard = test_guard();
         let s = "sess-batch-A";
         record_completion(s, "sub-1", "researcher", "eiffel", Some("thread-A".into()));
         record_completion(s, "sub-2", "researcher", "liberty", Some("thread-A".into()));
@@ -163,6 +275,7 @@ mod tests {
 
     #[test]
     fn record_is_idempotent_on_task_id() {
+        let _guard = test_guard();
         let s = "sess-dupe";
         record_completion(s, "sub-1", "researcher", "first", None);
         record_completion(s, "sub-1", "researcher", "second", None);
@@ -196,4 +309,96 @@ mod tests {
     fn empty_batch_is_none() {
         assert_eq!(build_batched_notice(&[]), None);
     }
+
+    #[test]
+    fn discard_for_thread_removes_matching_across_sessions() {
+        let _guard = test_guard();
+        // Two sessions, each with a completion for the doomed thread plus one
+        // for a thread that must survive.
+        record_completion(
+            "sess-d1",
+            "sub-a",
+            "researcher",
+            "x",
+            Some("thread-DEL".into()),
+        );
+        record_completion(
+            "sess-d1",
+            "sub-b",
+            "researcher",
+            "y",
+            Some("thread-KEEP".into()),
+        );
+        record_completion(
+            "sess-d2",
+            "sub-c",
+            "researcher",
+            "z",
+            Some("thread-DEL".into()),
+        );
+        // Headless completion (no parent thread) must survive.
+        record_completion("sess-d2", "sub-d", "researcher", "w", None);
+
+        let removed = discard_for_thread("thread-DEL");
+        assert_eq!(removed, 2, "both thread-DEL completions removed");
+
+        // thread-KEEP survives in sess-d1; sess-d2 keeps only the headless one.
+        assert_eq!(pending_count("sess-d1"), 1);
+        let d1 = take_pending("sess-d1");
+        assert_eq!(d1[0].task_id, "sub-b");
+
+        assert_eq!(pending_count("sess-d2"), 1);
+        let d2 = take_pending("sess-d2");
+        assert_eq!(d2[0].task_id, "sub-d");
+
+        // Idempotent: nothing left to discard.
+        assert_eq!(discard_for_thread("thread-DEL"), 0);
+    }
+
+    #[test]
+    fn record_after_discard_is_dropped_by_tombstone() {
+        let _guard = test_guard();
+        // Deleting the thread tombstones it...
+        discard_for_thread("thread-race");
+        // ...so a straggler completion that records *after* the sweep (the
+        // cooperative-abort race) is dropped rather than queued.
+        record_completion(
+            "sess-race",
+            "sub-late",
+            "researcher",
+            "stale",
+            Some("thread-race".into()),
+        );
+        assert_eq!(
+            pending_count("sess-race"),
+            0,
+            "late completion for a cancelled thread must be dropped"
+        );
+        // A completion for a different, live thread still records normally.
+        record_completion(
+            "sess-race",
+            "sub-ok",
+            "researcher",
+            "fresh",
+            Some("thread-live-race".into()),
+        );
+        assert_eq!(pending_count("sess-race"), 1);
+        take_pending("sess-race");
+    }
+
+    #[test]
+    fn clear_all_empties_the_queue() {
+        let _guard = test_guard();
+        record_completion("sess-c1", "sub-1", "researcher", "a", Some("t1".into()));
+        record_completion("sess-c2", "sub-2", "researcher", "b", None);
+
+        let removed = clear_all();
+        assert!(
+            removed >= 2,
+            "clear_all should report at least the two just queued, got {removed}"
+        );
+        assert!(!has_pending("sess-c1"));
+        assert!(!has_pending("sess-c2"));
+        assert_eq!(clear_all(), 0);
+    }
 }
diff --git a/src/openhuman/agent_orchestration/mod.rs b/src/openhuman/agent_orchestration/mod.rs
index f2fcedc43..4c5ebc456 100644
--- a/src/openhuman/agent_orchestration/mod.rs
+++ b/src/openhuman/agent_orchestration/mod.rs
@@ -12,6 +12,7 @@ pub mod command_center;
 mod ops;
 pub(crate) mod parent_context;
 pub mod running_subagents;
+pub mod subagent_control;
 pub mod tools;
 pub mod types;
 pub mod workflow_runs;
@@ -26,6 +27,10 @@ pub use command_center::{
     all_command_center_controller_schemas, all_command_center_registered_controllers,
 };
 pub use ops::{AgentOrchestrationSession, OrchestrationError};
+pub use subagent_control::{
+    all_controller_schemas as all_subagent_control_controller_schemas,
+    all_registered_controllers as all_subagent_control_registered_controllers,
+};
 pub use types::{
     AgentMessage, AgentOrchestrationEvent, AgentSnapshot, AgentStatus, CloseAgentRequest,
     FollowUpRequest, MessageAgentRequest, ResumeAgentRequest, SpawnAgentRequest,
diff --git a/src/openhuman/agent_orchestration/running_subagents.rs b/src/openhuman/agent_orchestration/running_subagents.rs
index f805bf4e4..c9584b00a 100644
--- a/src/openhuman/agent_orchestration/running_subagents.rs
+++ b/src/openhuman/agent_orchestration/running_subagents.rs
@@ -18,7 +18,7 @@
 //! grow unbounded if a parent never waits (the Codex "spawn-slot leak" failure
 //! mode — openai/codex#18335).
 
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::sync::{Arc, Mutex, OnceLock};
 use std::time::Duration;
 
@@ -48,9 +48,12 @@ impl SubagentStatus {
 }
 
 struct RunningSubagentEntry {
-    #[allow(dead_code)]
     agent_id: String,
     parent_session: String,
+    /// Parent chat thread that spawned this sub-agent, captured at registration.
+    /// `None` for a headless spawn with no originating thread. Used to abort the
+    /// sub-agent when its parent thread is deleted (see [`cancel_for_thread`]).
+    parent_thread_id: Option<String>,
     run_queue: Arc<RunQueue>,
     abort: AbortHandle,
     status: watch::Receiver<SubagentStatus>,
@@ -90,6 +93,7 @@ pub fn register(
     task_id: String,
     agent_id: String,
     parent_session: String,
+    parent_thread_id: Option<String>,
     run_queue: Arc<RunQueue>,
     abort: AbortHandle,
     status: watch::Receiver<SubagentStatus>,
@@ -97,6 +101,7 @@ pub fn register(
     let entry = RunningSubagentEntry {
         agent_id,
         parent_session,
+        parent_thread_id,
         run_queue,
         abort,
         status,
@@ -231,6 +236,41 @@ pub async fn wait(
     }
 }
 
+/// Metadata captured when a sub-agent is cancelled, so the caller can surface
+/// the cancellation back in the parent chat (record a "cancelled" completion
+/// for idle-gated delivery).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CancelledSubagent {
+    pub agent_id: String,
+    pub parent_session: String,
+    pub parent_thread_id: Option<String>,
+}
+
+/// Abort and drop the sub-agent with `task_id`, returning its metadata so the
+/// caller can deliver a "cancelled" notice into the parent chat. Returns `None`
+/// if no such sub-agent is registered (already finished, or unknown id).
+///
+/// Unlike [`close`], this is keyed by `task_id` alone with no parent-session
+/// ownership check — it backs the user-facing "Cancel" affordance, and the
+/// desktop user owns every sub-agent in their own core.
+pub fn cancel_by_task(task_id: &str) -> Option<CancelledSubagent> {
+    let mut map = registry().lock().expect("running_subagents mutex poisoned");
+    let entry = map.remove(task_id)?;
+    entry.abort.abort();
+    log::debug!(
+        "[running_subagents] cancel_by_task task_id={} agent_id={} parent_thread_id={:?} live_entries={}",
+        task_id,
+        entry.agent_id,
+        entry.parent_thread_id,
+        map.len()
+    );
+    Some(CancelledSubagent {
+        agent_id: entry.agent_id,
+        parent_session: entry.parent_session,
+        parent_thread_id: entry.parent_thread_id,
+    })
+}
+
 /// Abort a running sub-agent and drop its registry entry. Kept for a future
 /// `close_agent` tool; the abort handle is stored at spawn time.
 pub fn close(task_id: &str, parent_session: &str) -> bool {
@@ -245,6 +285,59 @@ pub fn close(task_id: &str, parent_session: &str) -> bool {
     }
 }
 
+/// Abort and drop every running sub-agent whose parent chat thread is
+/// `thread_id`. Called when that thread is deleted so detached children don't
+/// keep running (and later try to deliver) against a thread that no longer
+/// exists. Returns the number of sub-agents cancelled.
+pub fn cancel_for_thread(thread_id: &str) -> usize {
+    let mut map = registry().lock().expect("running_subagents mutex poisoned");
+    let to_cancel: Vec<String> = map
+        .iter()
+        .filter(|(_, e)| e.parent_thread_id.as_deref() == Some(thread_id))
+        .map(|(id, _)| id.clone())
+        .collect();
+    for id in &to_cancel {
+        if let Some(entry) = map.remove(id) {
+            entry.abort.abort();
+        }
+    }
+    let count = to_cancel.len();
+    log::debug!(
+        "[running_subagents] cancel_for_thread thread_id={} cancelled={} live_entries={}",
+        thread_id,
+        count,
+        map.len()
+    );
+    count
+}
+
+/// Abort and drop **every** registered sub-agent. Called on a full thread purge
+/// where no parent thread survives. Returns the **distinct parent thread ids**
+/// that had sub-agents, so the purge path can tombstone them in
+/// [`super::background_completions`] and drop any straggler completion that wins
+/// the cooperative-abort race. Headless sub-agents (no parent thread) are still
+/// aborted but contribute no id.
+pub fn cancel_all() -> Vec<String> {
+    let mut map = registry().lock().expect("running_subagents mutex poisoned");
+    let count = map.len();
+    let mut thread_ids: Vec<String> = Vec::new();
+    let mut seen: HashSet<String> = HashSet::new();
+    for (_, entry) in map.drain() {
+        entry.abort.abort();
+        if let Some(thread_id) = entry.parent_thread_id {
+            if seen.insert(thread_id.clone()) {
+                thread_ids.push(thread_id);
+            }
+        }
+    }
+    log::debug!(
+        "[running_subagents] cancel_all cancelled={} distinct_threads={}",
+        count,
+        thread_ids.len()
+    );
+    thread_ids
+}
+
 fn prune(task_id: &str) {
     registry()
         .lock()
@@ -262,6 +355,19 @@ fn now_ms() -> u64 {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::sync::MutexGuard;
+
+    /// Serializes every test that touches the global [`REGISTRY`]. We reuse the
+    /// crate-wide `TEST_ENV_LOCK` (rather than a module-local mutex) because the
+    /// destructive `cancel_all` path is also reachable from the `threads::ops`
+    /// tests — those hold the same lock, so this prevents a purge there from
+    /// wiping entries a test here is mid-way through.
+    fn test_guard() -> MutexGuard<'static, ()> {
+        // Recover from a poisoned guard so one panicking test doesn't cascade.
+        crate::openhuman::config::TEST_ENV_LOCK
+            .lock()
+            .unwrap_or_else(|e| e.into_inner())
+    }
 
     fn dummy_abort() -> AbortHandle {
         tokio::spawn(async {}).abort_handle()
@@ -273,12 +379,24 @@ mod tests {
         task_id: &str,
         parent_session: &str,
         rq: Arc<RunQueue>,
+    ) -> watch::Sender<SubagentStatus> {
+        register_test_with_thread(task_id, parent_session, None, rq)
+    }
+
+    /// Like [`register_test`] but lets a test set the parent thread id so it can
+    /// exercise [`cancel_for_thread`].
+    fn register_test_with_thread(
+        task_id: &str,
+        parent_session: &str,
+        parent_thread_id: Option<&str>,
+        rq: Arc<RunQueue>,
     ) -> watch::Sender<SubagentStatus> {
         let (tx, rx) = status_channel();
         register(
             task_id.into(),
             "researcher".into(),
             parent_session.into(),
+            parent_thread_id.map(Into::into),
             rq,
             dummy_abort(),
             rx,
@@ -288,6 +406,7 @@ mod tests {
 
     #[tokio::test]
     async fn steer_pushes_into_the_subagent_queue() {
+        let _guard = test_guard();
         let rq = RunQueue::new();
         let tx = register_test("task-steer", "session-A", rq.clone());
 
@@ -323,6 +442,7 @@ mod tests {
 
     #[tokio::test]
     async fn steer_rejects_cross_parent_and_unknown() {
+        let _guard = test_guard();
         let rq = RunQueue::new();
         let _tx = register_test("task-owned", "session-owner", rq);
 
@@ -351,6 +471,7 @@ mod tests {
 
     #[tokio::test]
     async fn steer_after_terminal_is_rejected() {
+        let _guard = test_guard();
         let rq = RunQueue::new();
         let tx = register_test("task-term", "session-A", rq);
         let _ = tx.send(SubagentStatus::Failed {
@@ -366,6 +487,7 @@ mod tests {
 
     #[tokio::test]
     async fn wait_returns_completion_once_published() {
+        let _guard = test_guard();
         let rq = RunQueue::new();
         let tx = register_test("task-wait", "session-A", rq);
 
@@ -398,6 +520,7 @@ mod tests {
 
     #[tokio::test]
     async fn wait_times_out_and_leaves_entry_intact() {
+        let _guard = test_guard();
         let rq = RunQueue::new();
         let _tx = register_test("task-slow", "session-A", rq);
 
@@ -420,4 +543,97 @@ mod tests {
         .is_ok());
         prune("task-slow");
     }
+
+    #[tokio::test]
+    async fn cancel_for_thread_aborts_only_matching_entries() {
+        let _guard = test_guard();
+        let rq = RunQueue::new();
+        let _a = register_test_with_thread("task-tA-1", "session-A", Some("thread-X"), rq.clone());
+        let _b = register_test_with_thread("task-tA-2", "session-A", Some("thread-X"), rq.clone());
+        // Different thread — must survive.
+        let _c = register_test_with_thread("task-tB", "session-A", Some("thread-Y"), rq.clone());
+        // Headless (no parent thread) — must survive.
+        let _d = register_test_with_thread("task-headless", "session-A", None, rq);
+
+        let cancelled = cancel_for_thread("thread-X");
+        assert_eq!(cancelled, 2, "both thread-X entries should be cancelled");
+
+        // The two cancelled entries are gone (steer can't find them).
+        assert_eq!(
+            steer("task-tA-1", "session-A", "x".into(), QueueMode::Steer).await,
+            Err(SteerError::Unknown)
+        );
+        assert_eq!(
+            steer("task-tA-2", "session-A", "x".into(), QueueMode::Steer).await,
+            Err(SteerError::Unknown)
+        );
+
+        // Non-matching entries stay live and steerable.
+        assert!(steer("task-tB", "session-A", "x".into(), QueueMode::Steer)
+            .await
+            .is_ok());
+        assert!(
+            steer("task-headless", "session-A", "x".into(), QueueMode::Steer)
+                .await
+                .is_ok()
+        );
+
+        // Idempotent: a second pass cancels nothing.
+        assert_eq!(cancel_for_thread("thread-X"), 0);
+
+        prune("task-tB");
+        prune("task-headless");
+    }
+
+    #[tokio::test]
+    async fn cancel_by_task_returns_metadata_and_removes_entry() {
+        let _guard = test_guard();
+        let rq = RunQueue::new();
+        let _tx =
+            register_test_with_thread("task-cbt", "session-Z", Some("thread-cbt"), rq.clone());
+
+        let meta = cancel_by_task("task-cbt").expect("known task should cancel");
+        assert_eq!(meta.agent_id, "researcher");
+        assert_eq!(meta.parent_session, "session-Z");
+        assert_eq!(meta.parent_thread_id.as_deref(), Some("thread-cbt"));
+
+        // Entry is gone — steer can no longer find it, and a second cancel is a no-op.
+        assert_eq!(
+            steer("task-cbt", "session-Z", "x".into(), QueueMode::Steer).await,
+            Err(SteerError::Unknown)
+        );
+        assert!(cancel_by_task("task-cbt").is_none());
+        // Unknown ids are simply None.
+        assert!(cancel_by_task("never-existed").is_none());
+    }
+
+    #[tokio::test]
+    async fn cancel_all_clears_everything() {
+        let _guard = test_guard();
+        let rq = RunQueue::new();
+        let _a = register_test_with_thread("task-all-1", "session-A", Some("thread-1"), rq.clone());
+        // Headless (no parent thread) — aborted, but contributes no thread id.
+        let _b = register_test_with_thread("task-all-2", "session-B", None, rq);
+
+        let cancelled_threads = cancel_all();
+        assert!(
+            cancelled_threads.contains(&"thread-1".to_string()),
+            "cancel_all should report the parent thread of the cancelled sub-agent"
+        );
+        assert!(
+            !cancelled_threads.iter().any(|t| t.is_empty()),
+            "headless sub-agents must not contribute an id"
+        );
+
+        assert_eq!(
+            steer("task-all-1", "session-A", "x".into(), QueueMode::Steer).await,
+            Err(SteerError::Unknown)
+        );
+        assert_eq!(
+            steer("task-all-2", "session-B", "x".into(), QueueMode::Steer).await,
+            Err(SteerError::Unknown)
+        );
+        // Registry is empty now.
+        assert!(cancel_all().is_empty());
+    }
 }
diff --git a/src/openhuman/agent_orchestration/subagent_control.rs b/src/openhuman/agent_orchestration/subagent_control.rs
new file mode 100644
index 000000000..8ef5391e7
--- /dev/null
+++ b/src/openhuman/agent_orchestration/subagent_control.rs
@@ -0,0 +1,214 @@
+//! Controller schema + JSON-RPC dispatcher for user-driven control of detached
+//! background sub-agents (`spawn_async_subagent`).
+//!
+//! Exposes `openhuman.subagent_cancel`: the frontend "Cancel" affordance in the
+//! background-tasks drawer calls this to abort a still-running detached
+//! sub-agent. Cancellation aborts the in-flight task via the
+//! [`super::running_subagents`] registry and records a "cancelled" pseudo-
+//! completion so the existing idle-gated delivery path
+//! ([`super::background_delivery`]) surfaces it back in the parent chat.
+//!
+//! This is the *manual* counterpart to the *automatic* thread-close
+//! cancellation in [`crate::openhuman::threads`]: there the thread is being
+//! deleted (so nothing is delivered and the thread is tombstoned), whereas here
+//! the thread stays alive and the user expects to see that their sub-agent was
+//! cancelled.
+
+use serde_json::{json, Map, Value};
+
+use crate::core::all::{ControllerFuture, RegisteredController};
+use crate::core::{ControllerSchema, FieldSchema, TypeSchema};
+use crate::openhuman::agent_orchestration::{background_completions, running_subagents};
+use crate::rpc::RpcOutcome;
+
+/// Controller schemas exposed for detached sub-agent control.
+pub fn all_controller_schemas() -> Vec<ControllerSchema> {
+    vec![schema_for("subagent_cancel")]
+}
+
+/// Registered controllers (schema + handler) for detached sub-agent control.
+pub fn all_registered_controllers() -> Vec<RegisteredController> {
+    vec![RegisteredController {
+        schema: schema_for("subagent_cancel"),
+        handler: handle_subagent_cancel,
+    }]
+}
+
+fn schema_for(function: &str) -> ControllerSchema {
+    match function {
+        "subagent_cancel" => ControllerSchema {
+            namespace: "subagent",
+            function: "cancel",
+            description: "Cancel a still-running detached background sub-agent by its spawn task \
+                          id. Aborts the in-flight run and posts a 'cancelled' notice back into \
+                          the parent chat thread. No-op (cancelled=false) if the sub-agent already \
+                          finished or the id is unknown.",
+            inputs: vec![
+                required_str(
+                    "taskId",
+                    "Spawn task id (`sub-…`) of the background sub-agent.",
+                ),
+                optional_str(
+                    "reason",
+                    "Optional reason, included in the cancelled notice shown in chat.",
+                ),
+            ],
+            outputs: vec![json_output(
+                "result",
+                "{ cancelled: bool, taskId: string } — cancelled=false if nothing was running.",
+            )],
+        },
+        _ => ControllerSchema {
+            namespace: "subagent",
+            function: "unknown",
+            description: "unknown subagent control function",
+            inputs: vec![],
+            outputs: vec![],
+        },
+    }
+}
+
+fn handle_subagent_cancel(params: Map<String, Value>) -> ControllerFuture {
+    Box::pin(async move {
+        let cid = new_correlation_id();
+        let task_id = require_str(&params, "taskId")?;
+        let reason = opt_str(&params, "reason");
+        log::debug!(
+            target: "subagent_control_rpc",
+            "[subagent_control_rpc][{cid}] cancel.entry task_id={task_id}"
+        );
+
+        let cancelled = match running_subagents::cancel_by_task(&task_id) {
+            Some(meta) => {
+                let summary = match reason.as_deref().map(str::trim).filter(|r| !r.is_empty()) {
+                    Some(r) => format!(
+                        "You cancelled this background sub-agent before it finished. Reason: {r}"
+                    ),
+                    None => {
+                        "You cancelled this background sub-agent before it finished.".to_string()
+                    }
+                };
+                // The thread is still alive (unlike the delete path), so we
+                // record a completion that flows through the same idle-gated
+                // delivery and surfaces the cancellation in chat.
+                background_completions::record_completion(
+                    meta.parent_session,
+                    &task_id,
+                    meta.agent_id,
+                    summary,
+                    meta.parent_thread_id,
+                );
+                true
+            }
+            None => false,
+        };
+
+        log::debug!(
+            target: "subagent_control_rpc",
+            "[subagent_control_rpc][{cid}] cancel.done task_id={task_id} cancelled={cancelled}"
+        );
+        to_json(json!({ "cancelled": cancelled, "taskId": task_id }))
+    })
+}
+
+fn to_json<T: serde::Serialize>(value: T) -> Result<Value, String> {
+    RpcOutcome::new(value, vec![]).into_cli_compatible_json()
+}
+
+fn new_correlation_id() -> String {
+    uuid::Uuid::new_v4().simple().to_string()[..8].to_string()
+}
+
+fn required_str(name: &'static str, comment: &'static str) -> FieldSchema {
+    FieldSchema {
+        name,
+        ty: TypeSchema::String,
+        comment,
+        required: true,
+    }
+}
+
+fn optional_str(name: &'static str, comment: &'static str) -> FieldSchema {
+    FieldSchema {
+        name,
+        ty: TypeSchema::Option(Box::new(TypeSchema::String)),
+        comment,
+        required: false,
+    }
+}
+
+fn json_output(name: &'static str, comment: &'static str) -> FieldSchema {
+    FieldSchema {
+        name,
+        ty: TypeSchema::Json,
+        comment,
+        required: true,
+    }
+}
+
+/// Extract a required non-empty string param, **trimmed**, or an RPC-facing
+/// error. Trimming matters for `taskId`: a whitespace-padded id would otherwise
+/// pass validation yet never match the registry key in `cancel_by_task`.
+fn require_str(params: &Map<String, Value>, key: &str) -> Result<String, String> {
+    params
+        .get(key)
+        .and_then(Value::as_str)
+        .map(str::trim)
+        .filter(|s| !s.is_empty())
+        .map(str::to_string)
+        .ok_or_else(|| format!("missing required param: {key}"))
+}
+
+/// Extract an optional non-empty string param.
+fn opt_str(params: &Map<String, Value>, key: &str) -> Option<String> {
+    params
+        .get(key)
+        .and_then(Value::as_str)
+        .filter(|s| !s.trim().is_empty())
+        .map(str::to_string)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn registered_controllers_match_schemas() {
+        let schemas = all_controller_schemas();
+        let registered = all_registered_controllers();
+        assert_eq!(schemas.len(), registered.len());
+        assert_eq!(schemas.len(), 1);
+        assert_eq!(schema_for("subagent_cancel").namespace, "subagent");
+        assert_eq!(schema_for("subagent_cancel").function, "cancel");
+    }
+
+    #[test]
+    fn require_str_rejects_blank_and_missing() {
+        let mut params = Map::new();
+        assert!(require_str(&params, "taskId").is_err());
+        params.insert("taskId".into(), json!("   "));
+        assert!(require_str(&params, "taskId").is_err());
+        params.insert("taskId".into(), json!("sub-1"));
+        assert_eq!(require_str(&params, "taskId").unwrap(), "sub-1");
+        // Whitespace-padded ids are trimmed so they match the registry key.
+        params.insert("taskId".into(), json!("  sub-1  "));
+        assert_eq!(require_str(&params, "taskId").unwrap(), "sub-1");
+    }
+
+    #[tokio::test]
+    async fn cancel_unknown_task_is_a_noop_false() {
+        let _lock = crate::openhuman::config::TEST_ENV_LOCK
+            .lock()
+            .unwrap_or_else(|e| e.into_inner());
+        let mut params = Map::new();
+        params.insert("taskId".into(), json!("sub-does-not-exist"));
+        let out = handle_subagent_cancel(params).await.expect("handler ok");
+        // RpcOutcome wraps the payload under `data`.
+        let cancelled = out
+            .get("data")
+            .and_then(|d| d.get("cancelled"))
+            .or_else(|| out.get("cancelled"))
+            .and_then(Value::as_bool);
+        assert_eq!(cancelled, Some(false));
+    }
+}
diff --git a/src/openhuman/agent_orchestration/tools/spawn_async_subagent.rs b/src/openhuman/agent_orchestration/tools/spawn_async_subagent.rs
index a0d9cb4bf..592af786a 100644
--- a/src/openhuman/agent_orchestration/tools/spawn_async_subagent.rs
+++ b/src/openhuman/agent_orchestration/tools/spawn_async_subagent.rs
@@ -246,6 +246,18 @@ impl Tool for SpawnAsyncSubagentTool {
         // finished result can be delivered back into it as a system turn.
         let background_parent_thread_id =
             crate::openhuman::inference::provider::thread_context::current_thread_id();
+        // Kept on this side (the closure moves its own clone) so the registry
+        // entry knows which parent thread owns this sub-agent — that's how
+        // `cancel_for_thread` aborts it when the thread is deleted.
+        let register_parent_thread_id = background_parent_thread_id.clone();
+        // Lifecycle-critical wiring: log the parent-thread binding so the
+        // thread-close cancellation path (`cancel_for_thread`) is grep-friendly.
+        log::debug!(
+            "[spawn_async_subagent] register task_id={} parent_session={} parent_thread_id={}",
+            task_id,
+            parent_session,
+            register_parent_thread_id.as_deref().unwrap_or("none")
+        );
         let background_prompt = add_background_contract(&prompt);
 
         let join = tokio::spawn(async move {
@@ -362,6 +374,7 @@ impl Tool for SpawnAsyncSubagentTool {
             task_id.clone(),
             definition.id.clone(),
             parent_session.clone(),
+            register_parent_thread_id,
             steer_queue,
             join.abort_handle(),
             status_rx,
diff --git a/src/openhuman/threads/ops.rs b/src/openhuman/threads/ops.rs
index 3c149ba6a..efe55a3bf 100644
--- a/src/openhuman/threads/ops.rs
+++ b/src/openhuman/threads/ops.rs
@@ -514,6 +514,24 @@ pub async fn thread_delete(
     // session for the now-deleted thread could linger and try to
     // append to a thread index row that no longer exists.
     web_channel::invalidate_thread_sessions(&request.thread_id).await;
+    // Cancel any detached sub-agents this thread spawned BEFORE clearing their
+    // queued results: abort the in-flight ones first so a child can't record a
+    // completion in the gap between the two calls, then discard anything already
+    // queued for delivery. Both target a thread that's being deleted, so there's
+    // nowhere left to deliver to — abort + cleanup is the whole behavior.
+    let cancelled = crate::openhuman::agent_orchestration::running_subagents::cancel_for_thread(
+        &request.thread_id,
+    );
+    let discarded =
+        crate::openhuman::agent_orchestration::background_completions::discard_for_thread(
+            &request.thread_id,
+        );
+    log::debug!(
+        "[threads] thread_delete thread_id={} cancelled_subagents={} discarded_completions={}",
+        request.thread_id,
+        cancelled,
+        discarded
+    );
     // Drop any persisted in-flight turn snapshot for this thread —
     // otherwise `threads_turn_state_list` keeps surfacing it (as
     // `Interrupted` on next restart) for a thread that no longer
@@ -541,6 +559,24 @@ pub async fn threads_purge(
 ) -> Result<RpcOutcome<ApiEnvelope<PurgeConversationThreadsResponse>>, String> {
     let dir = workspace_dir().await?;
     let stats = conversations::purge_threads(dir.clone())?;
+    // No parent thread survives a purge, so cancel every detached sub-agent and
+    // wipe every queued result. Same ordering as `thread_delete`: abort the
+    // in-flight runs first, then clear the delivery queue. Tombstone each
+    // cancelled sub-agent's thread BEFORE the final wipe so a straggler that
+    // wins the cooperative-abort race (records after the wipe) is still dropped
+    // by `record_completion` rather than delivered into a purged thread.
+    use crate::openhuman::agent_orchestration::{background_completions, running_subagents};
+    let cancelled_threads = running_subagents::cancel_all();
+    let mut discarded = 0;
+    for thread_id in &cancelled_threads {
+        discarded += background_completions::discard_for_thread(thread_id);
+    }
+    discarded += background_completions::clear_all();
+    log::debug!(
+        "[threads] threads_purge cancelled_threads={} discarded_completions={}",
+        cancelled_threads.len(),
+        discarded
+    );
     // Threads are gone, so any orphan turn snapshots can never be
     // reattached to a live thread. Wipe them in the same call so
     // `turn_state_list` returns an empty set after a purge. Use the
diff --git a/src/openhuman/threads/ops_tests.rs b/src/openhuman/threads/ops_tests.rs
index facb0666a..eb99a7725 100644
--- a/src/openhuman/threads/ops_tests.rs
+++ b/src/openhuman/threads/ops_tests.rs
@@ -623,6 +623,18 @@ async fn thread_delete_removes_persisted_turn_state_snapshot() {
     turn_state::store::put(dir.clone(), &snapshot).expect("put snapshot");
     assert!(turn_state::store::get(dir, thread_id).unwrap().is_some());
 
+    // Queue a finished background sub-agent result for this thread; deleting the
+    // thread must discard it so it's never delivered into a dead thread.
+    use crate::openhuman::agent_orchestration::background_completions as bg;
+    bg::record_completion(
+        "sess-del",
+        "sub-del-1",
+        "researcher",
+        "result",
+        Some(thread_id.to_string()),
+    );
+    assert_eq!(bg::pending_count("sess-del"), 1);
+
     thread_delete(DeleteConversationThreadRequest {
         thread_id: thread_id.to_string(),
         deleted_at: "2026-01-01T00:02:00Z".into(),
@@ -630,6 +642,12 @@ async fn thread_delete_removes_persisted_turn_state_snapshot() {
     .await
     .expect("delete thread");
 
+    assert_eq!(
+        bg::pending_count("sess-del"),
+        0,
+        "queued completion for the deleted thread should be discarded"
+    );
+
     let turn_state = turn_state_get(GetTurnStateRequest {
         thread_id: thread_id.to_string(),
     })
@@ -663,10 +681,31 @@ async fn threads_purge_removes_valid_and_corrupted_turn_state_files() {
     )
     .unwrap();
 
+    // Queue background sub-agent results across sessions; a full purge must wipe
+    // them all since no parent thread survives.
+    use crate::openhuman::agent_orchestration::background_completions as bg;
+    bg::record_completion(
+        "sess-p1",
+        "sub-p1",
+        "researcher",
+        "x",
+        Some("thread-a".into()),
+    );
+    bg::record_completion(
+        "sess-p2",
+        "sub-p2",
+        "researcher",
+        "y",
+        Some("thread-b".into()),
+    );
+
     threads_purge(EmptyRequest {})
         .await
         .expect("purge threads should also clear snapshots");
 
+    assert!(!bg::has_pending("sess-p1"));
+    assert!(!bg::has_pending("sess-p2"));
+
     if turn_state_dir.exists() {
         let remaining_json: Vec<_> = std::fs::read_dir(&turn_state_dir)
             .unwrap()