diff --git a/app/src/lib/i18n/ar.ts b/app/src/lib/i18n/ar.ts
index fbde7a5175..097a65f113 100644
--- a/app/src/lib/i18n/ar.ts
+++ b/app/src/lib/i18n/ar.ts
@@ -9,6 +9,34 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'الخزنة موجودة على مضيف النواة.',
   'crossHostVault.message':
     'يتم تخزين خزنة الذاكرة هذه على مضيف openhuman-core ({os}). لا يمكن فتحها أو عرضها إلا على ذلك الجهاز، وليس من هذا الجهاز.',
+  'conversations.toolFailure.whyLabel': 'لماذا',
+  'conversations.toolFailure.nextLabel': 'ما الذي يجب فعله بعد ذلك',
+  'conversations.toolFailure.missingPermission.cause': 'لا يملك OpenHuman الإذن للقيام بهذا بعد.',
+  'conversations.toolFailure.missingPermission.next': 'امنح الإذن المطلوب ثم حاول مرة أخرى.',
+  'conversations.toolFailure.missingApp.cause':
+    'التطبيق أو البرنامج اللازم لهذا الإجراء غير متوفر.',
+  'conversations.toolFailure.missingApp.next': 'ثبّت التطبيق أو افتحه ثم حاول مرة أخرى.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'إحدى الخدمات التي يحتاجها OpenHuman غير متوفرة مؤقتًا.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'سيحاول OpenHuman مرة أخرى قريبًا — لا حاجة لأي إجراء.',
+  'conversations.toolFailure.badCredentials.cause':
+    'بيانات تسجيل الدخول المحفوظة مفقودة أو لم تعد صالحة.',
+  'conversations.toolFailure.badCredentials.next':
+    'سجّل الدخول مجددًا أو حدّث بيانات الاعتماد ثم حاول مرة أخرى.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'هذا الإجراء محظور بواسطة إعدادات الأمان الخاصة بك.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'اسمح به من الإعدادات → وصول الوكيل إذا أردت تشغيله.',
+  'conversations.toolFailure.modelConnection.cause':
+    'تعذّر على OpenHuman الوصول إلى نموذج الذكاء الاصطناعي.',
+  'conversations.toolFailure.modelConnection.next':
+    'تحقّق من اتصالك أو إعدادات النموذج؛ سيعيد OpenHuman المحاولة.',
+  'conversations.toolFailure.timeout.cause': 'استغرق الإجراء وقتًا طويلاً وتم إيقافه.',
+  'conversations.toolFailure.timeout.next': 'سيعيد OpenHuman المحاولة، أو يمكنك إعادتها يدويًا.',
+  'conversations.toolFailure.unknown.cause': 'حدث خطأ ما في هذا الإجراء.',
+  'conversations.toolFailure.unknown.next':
+    'حاول مرة أخرى؛ وإذا استمر الفشل، شغّل التشخيص من الإعدادات.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'شارك ملاحظاتك',
   'feedback.board': 'لوحة الملاحظات',
diff --git a/app/src/lib/i18n/bn.ts b/app/src/lib/i18n/bn.ts
index 3f9ed538d5..74d3607cc8 100644
--- a/app/src/lib/i18n/bn.ts
+++ b/app/src/lib/i18n/bn.ts
@@ -10,6 +10,36 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'ভল্টটি কোর হোস্টে রয়েছে।',
   'crossHostVault.message':
     'এই মেমরি ভল্টটি openhuman-core হোস্টে ({os}) সংরক্ষিত আছে। এটি কেবল সেই মেশিনেই খোলা বা দেখানো যায়, এই ডিভাইস থেকে নয়।',
+  'conversations.toolFailure.whyLabel': 'কেন',
+  'conversations.toolFailure.nextLabel': 'এরপর কী করবেন',
+  'conversations.toolFailure.missingPermission.cause': 'OpenHuman-এর এখনও এটি করার অনুমতি নেই।',
+  'conversations.toolFailure.missingPermission.next':
+    'প্রয়োজনীয় অনুমতি দিন, তারপর আবার চেষ্টা করুন।',
+  'conversations.toolFailure.missingApp.cause':
+    'এই কাজের জন্য প্রয়োজনীয় অ্যাপ বা প্রোগ্রামটি উপলব্ধ নেই।',
+  'conversations.toolFailure.missingApp.next':
+    'অ্যাপটি ইনস্টল করুন বা খুলুন, তারপর আবার চেষ্টা করুন।',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'OpenHuman-এর প্রয়োজনীয় একটি পরিষেবা সাময়িকভাবে অনুপলব্ধ।',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman শীঘ্রই আবার চেষ্টা করবে — কোনো পদক্ষেপের প্রয়োজন নেই।',
+  'conversations.toolFailure.badCredentials.cause':
+    'সংরক্ষিত সাইন-ইন তথ্য অনুপস্থিত বা আর বৈধ নয়।',
+  'conversations.toolFailure.badCredentials.next':
+    'আবার সাইন ইন করুন বা শংসাপত্র আপডেট করুন, তারপর আবার চেষ্টা করুন।',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'এই কাজটি আপনার নিরাপত্তা সেটিংস দ্বারা অবরুদ্ধ।',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'আপনি চালাতে চাইলে সেটিংস → এজেন্ট অ্যাক্সেস-এ এটি অনুমতি দিন।',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman AI মডেলে পৌঁছাতে পারেনি।',
+  'conversations.toolFailure.modelConnection.next':
+    'আপনার সংযোগ বা মডেল সেটিংস পরীক্ষা করুন; OpenHuman আবার চেষ্টা করবে।',
+  'conversations.toolFailure.timeout.cause': 'কাজটি অনেক বেশি সময় নেওয়ায় থামিয়ে দেওয়া হয়েছে।',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman আবার চেষ্টা করবে, অথবা আপনি নিজে আবার চেষ্টা করতে পারেন।',
+  'conversations.toolFailure.unknown.cause': 'এই কাজটিতে কিছু ভুল হয়েছে।',
+  'conversations.toolFailure.unknown.next':
+    'আবার চেষ্টা করুন; বারবার ব্যর্থ হলে সেটিংস থেকে ডায়াগনস্টিকস চালান।',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'মতামত দিন',
   'feedback.board': 'মতামত বোর্ড',
diff --git a/app/src/lib/i18n/de.ts b/app/src/lib/i18n/de.ts
index 8d2ba158d3..35b81c61bc 100644
--- a/app/src/lib/i18n/de.ts
+++ b/app/src/lib/i18n/de.ts
@@ -9,6 +9,38 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'Der Vault liegt auf dem Core-Host.',
   'crossHostVault.message':
     'Dieser Memory-Vault wird auf dem openhuman-core-Host ({os}) gespeichert. Er kann nur auf diesem Rechner geöffnet oder angezeigt werden, nicht von diesem Gerät.',
+  'conversations.toolFailure.whyLabel': 'Warum',
+  'conversations.toolFailure.nextLabel': 'Nächste Schritte',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman hat noch keine Berechtigung, dies zu tun.',
+  'conversations.toolFailure.missingPermission.next':
+    'Erteile die benötigte Berechtigung und versuche es erneut.',
+  'conversations.toolFailure.missingApp.cause':
+    'Die für diese Aktion benötigte App oder das Programm ist nicht verfügbar.',
+  'conversations.toolFailure.missingApp.next':
+    'Installiere oder öffne die App und versuche es erneut.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Ein von OpenHuman benötigter Dienst ist vorübergehend nicht verfügbar.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman versucht es in Kürze erneut – nichts weiter nötig.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Die gespeicherten Anmeldedaten fehlen oder sind nicht mehr gültig.',
+  'conversations.toolFailure.badCredentials.next':
+    'Melde dich erneut an oder aktualisiere die Anmeldedaten und versuche es erneut.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Diese Aktion wird durch deine Sicherheitseinstellungen blockiert.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Erlaube sie unter Einstellungen → Agentenzugriff, wenn sie ausgeführt werden soll.',
+  'conversations.toolFailure.modelConnection.cause':
+    'OpenHuman konnte das KI-Modell nicht erreichen.',
+  'conversations.toolFailure.modelConnection.next':
+    'Prüfe deine Verbindung oder Modelleinstellungen; OpenHuman versucht es erneut.',
+  'conversations.toolFailure.timeout.cause': 'Die Aktion hat zu lange gedauert und wurde gestoppt.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman versucht es erneut, oder du kannst es manuell wiederholen.',
+  'conversations.toolFailure.unknown.cause': 'Bei dieser Aktion ist etwas schiefgelaufen.',
+  'conversations.toolFailure.unknown.next':
+    'Versuche es erneut; wenn es weiterhin fehlschlägt, führe die Diagnose in den Einstellungen aus.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Feedback geben',
   'feedback.board': 'Feedback-Board',
diff --git a/app/src/lib/i18n/en.ts b/app/src/lib/i18n/en.ts
index 7111c751bc..7fd766e50d 100644
--- a/app/src/lib/i18n/en.ts
+++ b/app/src/lib/i18n/en.ts
@@ -3786,6 +3786,37 @@ const en: TranslationMap = {
   'conversations.subagent.cancel': 'Cancel task',
   'conversations.subagent.cancelling': 'Cancelling…',
   'conversations.subagent.cancelFailed': "Couldn't cancel the task. Try again.",
+  // Tool-failure explanation surfaced under a failed step in "View processing" (#4254).
+  'conversations.toolFailure.whyLabel': 'Why',
+  'conversations.toolFailure.nextLabel': 'What to do next',
+  'conversations.toolFailure.missingPermission.cause':
+    "OpenHuman doesn't have permission to do this yet.",
+  'conversations.toolFailure.missingPermission.next':
+    'Grant the permission it needs, then try again.',
+  'conversations.toolFailure.missingApp.cause':
+    "The app or program needed for this action isn't available.",
+  'conversations.toolFailure.missingApp.next': 'Install or open the app, then try again.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'A service OpenHuman needs is temporarily unavailable.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman will try again shortly — no action needed.',
+  'conversations.toolFailure.badCredentials.cause':
+    'The saved sign-in details are missing or no longer valid.',
+  'conversations.toolFailure.badCredentials.next':
+    'Sign in again or update the credentials, then try again.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'This action is blocked by your safety settings.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Allow it in Settings → Agent access if you want it to run.',
+  'conversations.toolFailure.modelConnection.cause': "OpenHuman couldn't reach the AI model.",
+  'conversations.toolFailure.modelConnection.next':
+    'Check your connection or model settings; OpenHuman will retry.',
+  'conversations.toolFailure.timeout.cause': 'The action took too long and was stopped.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman will try again, or you can retry it manually.',
+  'conversations.toolFailure.unknown.cause': 'Something went wrong with this action.',
+  'conversations.toolFailure.unknown.next':
+    'Try again; if it keeps failing, run diagnostics from Settings.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'conversations.backgroundTasks.titleWithCount': 'Background tasks ({count})',
   'conversations.backgroundTasks.running': '{count} running',
diff --git a/app/src/lib/i18n/es.ts b/app/src/lib/i18n/es.ts
index ca549b36cd..6d7333c055 100644
--- a/app/src/lib/i18n/es.ts
+++ b/app/src/lib/i18n/es.ts
@@ -10,6 +10,37 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'El vault está en el host del core.',
   'crossHostVault.message':
     'Este vault de memoria se almacena en el host de openhuman-core ({os}). Solo se puede abrir o mostrar en esa máquina, no desde este dispositivo.',
+  'conversations.toolFailure.whyLabel': 'Por qué',
+  'conversations.toolFailure.nextLabel': 'Qué hacer a continuación',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman aún no tiene permiso para hacer esto.',
+  'conversations.toolFailure.missingPermission.next':
+    'Concede el permiso que necesita y vuelve a intentarlo.',
+  'conversations.toolFailure.missingApp.cause':
+    'La app o el programa necesario para esta acción no está disponible.',
+  'conversations.toolFailure.missingApp.next': 'Instala o abre la app y vuelve a intentarlo.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Un servicio que OpenHuman necesita no está disponible temporalmente.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman lo intentará de nuevo en breve; no hace falta hacer nada.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Los datos de inicio de sesión guardados faltan o ya no son válidos.',
+  'conversations.toolFailure.badCredentials.next':
+    'Inicia sesión de nuevo o actualiza las credenciales y vuelve a intentarlo.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Esta acción está bloqueada por tu configuración de seguridad.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Permítela en Configuración → Acceso del agente si quieres que se ejecute.',
+  'conversations.toolFailure.modelConnection.cause':
+    'OpenHuman no pudo conectarse con el modelo de IA.',
+  'conversations.toolFailure.modelConnection.next':
+    'Comprueba tu conexión o la configuración del modelo; OpenHuman lo reintentará.',
+  'conversations.toolFailure.timeout.cause': 'La acción tardó demasiado y se detuvo.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman lo intentará de nuevo, o puedes reintentarlo manualmente.',
+  'conversations.toolFailure.unknown.cause': 'Algo salió mal con esta acción.',
+  'conversations.toolFailure.unknown.next':
+    'Inténtalo de nuevo; si sigue fallando, ejecuta el diagnóstico desde Configuración.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Compartir opiniones',
   'feedback.board': 'Tablero de opiniones',
diff --git a/app/src/lib/i18n/fr.ts b/app/src/lib/i18n/fr.ts
index 5caf845b29..55cfe4fe12 100644
--- a/app/src/lib/i18n/fr.ts
+++ b/app/src/lib/i18n/fr.ts
@@ -10,6 +10,36 @@ const messages: TranslationMap = {
   'crossHostVault.title': "Le coffre se trouve sur l'hôte du cœur.",
   'crossHostVault.message':
     "Ce coffre de mémoire est stocké sur l'hôte openhuman-core ({os}). Il ne peut être ouvert ou affiché que sur cette machine, pas depuis cet appareil.",
+  'conversations.toolFailure.whyLabel': 'Pourquoi',
+  'conversations.toolFailure.nextLabel': 'Que faire ensuite',
+  'conversations.toolFailure.missingPermission.cause':
+    "OpenHuman n'a pas encore l'autorisation de faire cela.",
+  'conversations.toolFailure.missingPermission.next':
+    "Accordez l'autorisation nécessaire, puis réessayez.",
+  'conversations.toolFailure.missingApp.cause':
+    "L'application ou le programme requis pour cette action n'est pas disponible.",
+  'conversations.toolFailure.missingApp.next': "Installez ou ouvrez l'application, puis réessayez.",
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Un service dont OpenHuman a besoin est temporairement indisponible.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman réessaiera bientôt — aucune action nécessaire.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Les identifiants enregistrés sont manquants ou ne sont plus valides.',
+  'conversations.toolFailure.badCredentials.next':
+    'Reconnectez-vous ou mettez à jour les identifiants, puis réessayez.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Cette action est bloquée par vos paramètres de sécurité.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    "Autorisez-la dans Paramètres → Accès de l'agent si vous voulez qu'elle s'exécute.",
+  'conversations.toolFailure.modelConnection.cause': "OpenHuman n'a pas pu joindre le modèle d'IA.",
+  'conversations.toolFailure.modelConnection.next':
+    'Vérifiez votre connexion ou les paramètres du modèle ; OpenHuman réessaiera.',
+  'conversations.toolFailure.timeout.cause': "L'action a pris trop de temps et a été arrêtée.",
+  'conversations.toolFailure.timeout.next':
+    "OpenHuman réessaiera, ou vous pouvez relancer l'action manuellement.",
+  'conversations.toolFailure.unknown.cause': 'Un problème est survenu avec cette action.',
+  'conversations.toolFailure.unknown.next':
+    "Réessayez ; si l'échec persiste, lancez le diagnostic depuis les Paramètres.",
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Donner mon avis',
   'feedback.board': 'Tableau des suggestions',
diff --git a/app/src/lib/i18n/hi.ts b/app/src/lib/i18n/hi.ts
index 7db36e6675..a325ecdab0 100644
--- a/app/src/lib/i18n/hi.ts
+++ b/app/src/lib/i18n/hi.ts
@@ -9,6 +9,35 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'वॉल्ट कोर होस्ट पर है।',
   'crossHostVault.message':
     'यह मेमोरी वॉल्ट openhuman-core होस्ट ({os}) पर संग्रहीत है। इसे केवल उसी मशीन पर खोला या दिखाया जा सकता है, इस डिवाइस से नहीं।',
+  'conversations.toolFailure.whyLabel': 'क्यों',
+  'conversations.toolFailure.nextLabel': 'आगे क्या करें',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman के पास अभी यह करने की अनुमति नहीं है।',
+  'conversations.toolFailure.missingPermission.next': 'आवश्यक अनुमति दें, फिर दोबारा प्रयास करें।',
+  'conversations.toolFailure.missingApp.cause':
+    'इस कार्य के लिए आवश्यक ऐप या प्रोग्राम उपलब्ध नहीं है।',
+  'conversations.toolFailure.missingApp.next': 'ऐप इंस्टॉल करें या खोलें, फिर दोबारा प्रयास करें।',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'OpenHuman को जिस सेवा की आवश्यकता है वह अस्थायी रूप से अनुपलब्ध है।',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman शीघ्र ही फिर से प्रयास करेगा — कोई कार्रवाई आवश्यक नहीं।',
+  'conversations.toolFailure.badCredentials.cause':
+    'सहेजी गई साइन-इन जानकारी गायब है या अब मान्य नहीं है।',
+  'conversations.toolFailure.badCredentials.next':
+    'फिर से साइन इन करें या क्रेडेंशियल अपडेट करें, फिर दोबारा प्रयास करें।',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'यह कार्य आपकी सुरक्षा सेटिंग्स द्वारा अवरुद्ध है।',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'यदि आप इसे चलाना चाहते हैं तो इसे सेटिंग्स → एजेंट एक्सेस में अनुमति दें।',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman AI मॉडल तक नहीं पहुँच सका।',
+  'conversations.toolFailure.modelConnection.next':
+    'अपना कनेक्शन या मॉडल सेटिंग्स जाँचें; OpenHuman फिर से प्रयास करेगा।',
+  'conversations.toolFailure.timeout.cause': 'कार्य में बहुत अधिक समय लगा और इसे रोक दिया गया।',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman फिर से प्रयास करेगा, या आप इसे मैन्युअल रूप से दोबारा कर सकते हैं।',
+  'conversations.toolFailure.unknown.cause': 'इस कार्य में कुछ गड़बड़ हो गई।',
+  'conversations.toolFailure.unknown.next':
+    'दोबारा प्रयास करें; यदि यह बार-बार विफल हो, तो सेटिंग्स से डायग्नोस्टिक्स चलाएँ।',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'फ़ीडबैक साझा करें',
   'feedback.board': 'फ़ीडबैक बोर्ड',
diff --git a/app/src/lib/i18n/id.ts b/app/src/lib/i18n/id.ts
index 76c8941be3..7913894dc2 100644
--- a/app/src/lib/i18n/id.ts
+++ b/app/src/lib/i18n/id.ts
@@ -10,6 +10,36 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'Vault berada di host core.',
   'crossHostVault.message':
     'Vault memori ini disimpan di host openhuman-core ({os}). Hanya dapat dibuka atau ditampilkan di mesin tersebut, bukan dari perangkat ini.',
+  'conversations.toolFailure.whyLabel': 'Mengapa',
+  'conversations.toolFailure.nextLabel': 'Yang harus dilakukan',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman belum memiliki izin untuk melakukan ini.',
+  'conversations.toolFailure.missingPermission.next':
+    'Berikan izin yang diperlukan, lalu coba lagi.',
+  'conversations.toolFailure.missingApp.cause':
+    'Aplikasi atau program yang diperlukan untuk tindakan ini tidak tersedia.',
+  'conversations.toolFailure.missingApp.next': 'Instal atau buka aplikasinya, lalu coba lagi.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Layanan yang dibutuhkan OpenHuman sedang tidak tersedia untuk sementara.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman akan mencoba lagi sebentar lagi — tidak perlu tindakan apa pun.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Detail masuk yang tersimpan tidak ada atau tidak lagi valid.',
+  'conversations.toolFailure.badCredentials.next':
+    'Masuk kembali atau perbarui kredensial, lalu coba lagi.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Tindakan ini diblokir oleh pengaturan keamanan Anda.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Izinkan di Pengaturan → Akses agen jika Anda ingin menjalankannya.',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman tidak dapat menjangkau model AI.',
+  'conversations.toolFailure.modelConnection.next':
+    'Periksa koneksi atau pengaturan model Anda; OpenHuman akan mencoba lagi.',
+  'conversations.toolFailure.timeout.cause': 'Tindakan memakan waktu terlalu lama dan dihentikan.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman akan mencoba lagi, atau Anda dapat mengulanginya secara manual.',
+  'conversations.toolFailure.unknown.cause': 'Terjadi kesalahan pada tindakan ini.',
+  'conversations.toolFailure.unknown.next':
+    'Coba lagi; jika terus gagal, jalankan diagnostik dari Pengaturan.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Bagikan masukan',
   'feedback.board': 'Papan masukan',
diff --git a/app/src/lib/i18n/it.ts b/app/src/lib/i18n/it.ts
index 613a8cbca8..4b71e774a6 100644
--- a/app/src/lib/i18n/it.ts
+++ b/app/src/lib/i18n/it.ts
@@ -10,6 +10,38 @@ const messages: TranslationMap = {
   'crossHostVault.title': "Il vault è sull'host del core.",
   'crossHostVault.message':
     "Questo vault di memoria è archiviato sull'host openhuman-core ({os}). Può essere aperto o mostrato solo su quella macchina, non da questo dispositivo.",
+  'conversations.toolFailure.whyLabel': 'Perché',
+  'conversations.toolFailure.nextLabel': 'Cosa fare ora',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman non ha ancora il permesso di farlo.',
+  'conversations.toolFailure.missingPermission.next':
+    'Concedi il permesso necessario, poi riprova.',
+  'conversations.toolFailure.missingApp.cause':
+    "L'app o il programma necessario per questa azione non è disponibile.",
+  'conversations.toolFailure.missingApp.next': "Installa o apri l'app, poi riprova.",
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Un servizio necessario a OpenHuman è temporaneamente non disponibile.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman riproverà a breve — non serve fare nulla.',
+  'conversations.toolFailure.badCredentials.cause':
+    'I dati di accesso salvati mancano o non sono più validi.',
+  'conversations.toolFailure.badCredentials.next':
+    'Accedi di nuovo o aggiorna le credenziali, poi riprova.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Questa azione è bloccata dalle tue impostazioni di sicurezza.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    "Consentila in Impostazioni → Accesso dell'agente se vuoi che venga eseguita.",
+  'conversations.toolFailure.modelConnection.cause':
+    'OpenHuman non è riuscito a raggiungere il modello di IA.',
+  'conversations.toolFailure.modelConnection.next':
+    'Controlla la connessione o le impostazioni del modello; OpenHuman riproverà.',
+  'conversations.toolFailure.timeout.cause':
+    "L'azione ha richiesto troppo tempo ed è stata interrotta.",
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman riproverà, oppure puoi riprovare manualmente.',
+  'conversations.toolFailure.unknown.cause': 'Qualcosa è andato storto con questa azione.',
+  'conversations.toolFailure.unknown.next':
+    'Riprova; se continua a fallire, esegui la diagnostica dalle Impostazioni.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Condividi feedback',
   'feedback.board': 'Bacheca dei feedback',
diff --git a/app/src/lib/i18n/ko.ts b/app/src/lib/i18n/ko.ts
index 0a5c175f42..80fc2ffe03 100644
--- a/app/src/lib/i18n/ko.ts
+++ b/app/src/lib/i18n/ko.ts
@@ -9,6 +9,34 @@ const messages: TranslationMap = {
   'crossHostVault.title': '보관소가 코어 호스트에 있습니다.',
   'crossHostVault.message':
     '이 메모리 보관소는 openhuman-core 호스트({os})에 저장되어 있습니다. 해당 컴퓨터에서만 열거나 표시할 수 있으며 이 기기에서는 불가능합니다.',
+  'conversations.toolFailure.whyLabel': '이유',
+  'conversations.toolFailure.nextLabel': '다음 할 일',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman에 아직 이 작업을 수행할 권한이 없습니다.',
+  'conversations.toolFailure.missingPermission.next': '필요한 권한을 부여한 후 다시 시도하세요.',
+  'conversations.toolFailure.missingApp.cause':
+    '이 작업에 필요한 앱 또는 프로그램을 사용할 수 없습니다.',
+  'conversations.toolFailure.missingApp.next': '앱을 설치하거나 연 후 다시 시도하세요.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'OpenHuman에 필요한 서비스를 일시적으로 사용할 수 없습니다.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman이 곧 다시 시도합니다 — 별도의 조치가 필요하지 않습니다.',
+  'conversations.toolFailure.badCredentials.cause':
+    '저장된 로그인 정보가 없거나 더 이상 유효하지 않습니다.',
+  'conversations.toolFailure.badCredentials.next':
+    '다시 로그인하거나 자격 증명을 업데이트한 후 다시 시도하세요.',
+  'conversations.toolFailure.blockedByPolicy.cause': '이 작업은 보안 설정에 의해 차단되었습니다.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    '실행하려면 설정 → 에이전트 액세스에서 허용하세요.',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman이 AI 모델에 연결하지 못했습니다.',
+  'conversations.toolFailure.modelConnection.next':
+    '연결 또는 모델 설정을 확인하세요. OpenHuman이 다시 시도합니다.',
+  'conversations.toolFailure.timeout.cause': '작업이 너무 오래 걸려 중지되었습니다.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman이 다시 시도하거나 수동으로 다시 실행할 수 있습니다.',
+  'conversations.toolFailure.unknown.cause': '이 작업에서 문제가 발생했습니다.',
+  'conversations.toolFailure.unknown.next':
+    '다시 시도하세요. 계속 실패하면 설정에서 진단을 실행하세요.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': '피드백 보내기',
   'feedback.board': '피드백 보드',
diff --git a/app/src/lib/i18n/pl.ts b/app/src/lib/i18n/pl.ts
index 971aa31720..d9b6d628e5 100644
--- a/app/src/lib/i18n/pl.ts
+++ b/app/src/lib/i18n/pl.ts
@@ -10,6 +10,38 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'Skarbiec znajduje się na hoście rdzenia.',
   'crossHostVault.message':
     'Ten skarbiec pamięci jest przechowywany na hoście openhuman-core ({os}). Można go otworzyć lub pokazać tylko na tym komputerze, a nie z tego urządzenia.',
+  'conversations.toolFailure.whyLabel': 'Dlaczego',
+  'conversations.toolFailure.nextLabel': 'Co dalej',
+  'conversations.toolFailure.missingPermission.cause':
+    'OpenHuman nie ma jeszcze uprawnień, aby to zrobić.',
+  'conversations.toolFailure.missingPermission.next':
+    'Nadaj wymagane uprawnienie i spróbuj ponownie.',
+  'conversations.toolFailure.missingApp.cause':
+    'Aplikacja lub program potrzebny do tej czynności jest niedostępny.',
+  'conversations.toolFailure.missingApp.next':
+    'Zainstaluj lub otwórz aplikację i spróbuj ponownie.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Usługa, której potrzebuje OpenHuman, jest chwilowo niedostępna.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman spróbuje ponownie za chwilę — nie musisz nic robić.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Zapisane dane logowania są niedostępne lub już nieważne.',
+  'conversations.toolFailure.badCredentials.next':
+    'Zaloguj się ponownie lub zaktualizuj dane logowania, a potem spróbuj jeszcze raz.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Ta czynność jest zablokowana przez Twoje ustawienia bezpieczeństwa.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Zezwól na nią w Ustawienia → Dostęp agenta, jeśli ma zostać wykonana.',
+  'conversations.toolFailure.modelConnection.cause':
+    'OpenHuman nie mógł połączyć się z modelem AI.',
+  'conversations.toolFailure.modelConnection.next':
+    'Sprawdź połączenie lub ustawienia modelu; OpenHuman spróbuje ponownie.',
+  'conversations.toolFailure.timeout.cause': 'Czynność trwała zbyt długo i została zatrzymana.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman spróbuje ponownie lub możesz powtórzyć ją ręcznie.',
+  'conversations.toolFailure.unknown.cause': 'Coś poszło nie tak z tą czynnością.',
+  'conversations.toolFailure.unknown.next':
+    'Spróbuj ponownie; jeśli nadal się nie udaje, uruchom diagnostykę w Ustawieniach.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Podziel się opinią',
   'feedback.board': 'Tablica opinii',
diff --git a/app/src/lib/i18n/pt.ts b/app/src/lib/i18n/pt.ts
index 15755004db..4a7a4a66db 100644
--- a/app/src/lib/i18n/pt.ts
+++ b/app/src/lib/i18n/pt.ts
@@ -10,6 +10,37 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'O vault está no host do core.',
   'crossHostVault.message':
     'Este vault de memória fica armazenado no host openhuman-core ({os}). Só pode ser aberto ou exibido nessa máquina, não a partir deste dispositivo.',
+  'conversations.toolFailure.whyLabel': 'Por quê',
+  'conversations.toolFailure.nextLabel': 'O que fazer a seguir',
+  'conversations.toolFailure.missingPermission.cause':
+    'O OpenHuman ainda não tem permissão para fazer isso.',
+  'conversations.toolFailure.missingPermission.next':
+    'Conceda a permissão necessária e tente novamente.',
+  'conversations.toolFailure.missingApp.cause':
+    'O app ou programa necessário para esta ação não está disponível.',
+  'conversations.toolFailure.missingApp.next': 'Instale ou abra o app e tente novamente.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Um serviço de que o OpenHuman precisa está temporariamente indisponível.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'O OpenHuman tentará de novo em breve — nenhuma ação necessária.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Os dados de login salvos estão ausentes ou não são mais válidos.',
+  'conversations.toolFailure.badCredentials.next':
+    'Faça login novamente ou atualize as credenciais e tente de novo.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Esta ação está bloqueada pelas suas configurações de segurança.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Permita-a em Configurações → Acesso do agente se quiser que ela seja executada.',
+  'conversations.toolFailure.modelConnection.cause':
+    'O OpenHuman não conseguiu acessar o modelo de IA.',
+  'conversations.toolFailure.modelConnection.next':
+    'Verifique sua conexão ou as configurações do modelo; o OpenHuman tentará novamente.',
+  'conversations.toolFailure.timeout.cause': 'A ação demorou demais e foi interrompida.',
+  'conversations.toolFailure.timeout.next':
+    'O OpenHuman tentará novamente, ou você pode tentar manualmente.',
+  'conversations.toolFailure.unknown.cause': 'Algo deu errado com esta ação.',
+  'conversations.toolFailure.unknown.next':
+    'Tente novamente; se continuar falhando, execute o diagnóstico nas Configurações.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Enviar feedback',
   'feedback.board': 'Quadro de feedback',
diff --git a/app/src/lib/i18n/ru.ts b/app/src/lib/i18n/ru.ts
index b7a17b96e8..1119d5ac5c 100644
--- a/app/src/lib/i18n/ru.ts
+++ b/app/src/lib/i18n/ru.ts
@@ -10,6 +10,38 @@ const messages: TranslationMap = {
   'crossHostVault.title': 'Хранилище находится на хосте ядра.',
   'crossHostVault.message':
     'Это хранилище памяти размещено на хосте openhuman-core ({os}). Его можно открыть или показать только на той машине, но не с этого устройства.',
+  'conversations.toolFailure.whyLabel': 'Почему',
+  'conversations.toolFailure.nextLabel': 'Что делать дальше',
+  'conversations.toolFailure.missingPermission.cause':
+    'У OpenHuman пока нет разрешения на это действие.',
+  'conversations.toolFailure.missingPermission.next':
+    'Предоставьте нужное разрешение и повторите попытку.',
+  'conversations.toolFailure.missingApp.cause':
+    'Приложение или программа, необходимые для этого действия, недоступны.',
+  'conversations.toolFailure.missingApp.next':
+    'Установите или откройте приложение и повторите попытку.',
+  'conversations.toolFailure.serviceUnavailable.cause':
+    'Служба, необходимая OpenHuman, временно недоступна.',
+  'conversations.toolFailure.serviceUnavailable.next':
+    'OpenHuman скоро повторит попытку — ничего делать не нужно.',
+  'conversations.toolFailure.badCredentials.cause':
+    'Сохранённые данные для входа отсутствуют или больше недействительны.',
+  'conversations.toolFailure.badCredentials.next':
+    'Войдите заново или обновите учётные данные, затем повторите попытку.',
+  'conversations.toolFailure.blockedByPolicy.cause':
+    'Это действие заблокировано вашими настройками безопасности.',
+  'conversations.toolFailure.blockedByPolicy.next':
+    'Разрешите его в разделе Настройки → Доступ агента, если хотите, чтобы оно выполнилось.',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman не удалось связаться с ИИ-моделью.',
+  'conversations.toolFailure.modelConnection.next':
+    'Проверьте подключение или настройки модели; OpenHuman повторит попытку.',
+  'conversations.toolFailure.timeout.cause':
+    'Действие заняло слишком много времени и было остановлено.',
+  'conversations.toolFailure.timeout.next':
+    'OpenHuman повторит попытку, или вы можете повторить её вручную.',
+  'conversations.toolFailure.unknown.cause': 'С этим действием что-то пошло не так.',
+  'conversations.toolFailure.unknown.next':
+    'Повторите попытку; если ошибка повторяется, запустите диагностику в Настройках.',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': 'Поделиться отзывом',
   'feedback.board': 'Доска отзывов',
diff --git a/app/src/lib/i18n/zh-CN.ts b/app/src/lib/i18n/zh-CN.ts
index 78453ad354..fcc569122b 100644
--- a/app/src/lib/i18n/zh-CN.ts
+++ b/app/src/lib/i18n/zh-CN.ts
@@ -9,6 +9,25 @@ const messages: TranslationMap = {
   'crossHostVault.title': '记忆库位于核心主机上。',
   'crossHostVault.message':
     '此记忆库存储在 openhuman-core 主机（{os}）上。只能在该机器上打开或显示，无法从本设备访问。',
+  'conversations.toolFailure.whyLabel': '原因',
+  'conversations.toolFailure.nextLabel': '接下来该怎么做',
+  'conversations.toolFailure.missingPermission.cause': 'OpenHuman 目前还没有执行此操作的权限。',
+  'conversations.toolFailure.missingPermission.next': '授予所需权限，然后重试。',
+  'conversations.toolFailure.missingApp.cause': '此操作所需的应用或程序不可用。',
+  'conversations.toolFailure.missingApp.next': '安装或打开该应用，然后重试。',
+  'conversations.toolFailure.serviceUnavailable.cause': 'OpenHuman 需要的某项服务暂时不可用。',
+  'conversations.toolFailure.serviceUnavailable.next': 'OpenHuman 稍后会重试——无需任何操作。',
+  'conversations.toolFailure.badCredentials.cause': '保存的登录信息缺失或已失效。',
+  'conversations.toolFailure.badCredentials.next': '请重新登录或更新凭据，然后重试。',
+  'conversations.toolFailure.blockedByPolicy.cause': '此操作已被你的安全设置阻止。',
+  'conversations.toolFailure.blockedByPolicy.next': '如果希望执行，请在“设置 → 代理访问”中允许它。',
+  'conversations.toolFailure.modelConnection.cause': 'OpenHuman 无法连接到 AI 模型。',
+  'conversations.toolFailure.modelConnection.next':
+    '请检查你的网络连接或模型设置；OpenHuman 将会重试。',
+  'conversations.toolFailure.timeout.cause': '操作耗时过长，已被停止。',
+  'conversations.toolFailure.timeout.next': 'OpenHuman 会重试，你也可以手动重试。',
+  'conversations.toolFailure.unknown.cause': '此操作出现了问题。',
+  'conversations.toolFailure.unknown.next': '请重试；如果持续失败，请在设置中运行诊断。',
   'conversations.backgroundTasks.title': 'Background tasks',
   'nav.feedback': '分享反馈',
   'feedback.board': '反馈板',
diff --git a/app/src/pages/conversations/components/ProcessingTranscriptView.test.tsx b/app/src/pages/conversations/components/ProcessingTranscriptView.test.tsx
new file mode 100644
index 0000000000..94a6926b77
--- /dev/null
+++ b/app/src/pages/conversations/components/ProcessingTranscriptView.test.tsx
@@ -0,0 +1,74 @@
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+
+import type { ToolTimelineEntry } from '../../../store/chatRuntimeSlice';
+import { ProcessingTranscriptView } from './ProcessingTranscriptView';
+
+// Echo i18n: return the fallback when one is provided (so localized copy keys
+// resolve to the English source we pass), otherwise the key itself. This lets
+// us assert on the human strings carried on the failure payload.
+vi.mock('../../../lib/i18n/I18nContext', () => ({
+  useT: () => ({ t: (key: string, fallback?: string) => fallback ?? key, locale: 'en' }),
+}));
+
+function failedEntry(overrides: Partial<ToolTimelineEntry> = {}): ToolTimelineEntry {
+  return {
+    id: 'call-1',
+    name: 'read_file',
+    round: 1,
+    status: 'error',
+    failure: {
+      class: 'MissingPermission',
+      category: 'NeedsUserConfirmation',
+      recoverable: false,
+      causePlain: 'No permission yet.',
+      nextAction: 'Grant it, then retry.',
+    },
+    ...overrides,
+  };
+}
+
+describe('ProcessingTranscriptView tool failure explanation', () => {
+  it('renders the cause + next-action under a failed tool row', () => {
+    // Empty transcript → single tool group over all entries (legacy path).
+    render(<ProcessingTranscriptView transcript={[]} entries={[failedEntry()]} />);
+
+    const failure = screen.getByTestId('processing-tool-failure');
+    expect(failure).toBeTruthy();
+    expect(failure.textContent).toContain('No permission yet.');
+    expect(failure.textContent).toContain('Grant it, then retry.');
+  });
+
+  it('falls back to the plain English copy for an unrecognized class', () => {
+    render(
+      <ProcessingTranscriptView
+        transcript={[]}
+        entries={[
+          failedEntry({
+            failure: {
+              class: 'SomethingBrandNew',
+              category: 'Recoverable',
+              recoverable: true,
+              causePlain: 'Mystery cause.',
+              nextAction: 'Mystery next.',
+            },
+          }),
+        ]}
+      />
+    );
+
+    const failure = screen.getByTestId('processing-tool-failure');
+    expect(failure.textContent).toContain('Mystery cause.');
+    expect(failure.textContent).toContain('Mystery next.');
+  });
+
+  it('does not render the failure block for a successful entry', () => {
+    render(
+      <ProcessingTranscriptView
+        transcript={[]}
+        entries={[failedEntry({ status: 'success', failure: undefined })]}
+      />
+    );
+    expect(screen.queryByTestId('processing-tool-failure')).toBeNull();
+  });
+});
diff --git a/app/src/pages/conversations/components/ProcessingTranscriptView.tsx b/app/src/pages/conversations/components/ProcessingTranscriptView.tsx
index 5180c83033..d82ba2f1f7 100644
--- a/app/src/pages/conversations/components/ProcessingTranscriptView.tsx
+++ b/app/src/pages/conversations/components/ProcessingTranscriptView.tsx
@@ -1,6 +1,7 @@
 import { useT } from '../../../lib/i18n/I18nContext';
 import type {
   ProcessingTranscriptItem,
+  ToolFailureExplanation,
   ToolTimelineEntry,
   ToolTimelineEntryStatus,
 } from '../../../store/chatRuntimeSlice';
@@ -112,6 +113,27 @@ function ToolGroupBlock({ summary, entries }: { summary: string; entries: ToolTi
   );
 }
 
+/**
+ * The 8 failure classes the UI has localized copy for (#4254), keyed by the
+ * camelCase form of the wire's PascalCase `class`. Any class not in this set
+ * falls back to the English `causePlain` / `nextAction` carried on the payload.
+ */
+const LOCALIZED_FAILURE_CLASSES: ReadonlySet<string> = new Set([
+  'missingPermission',
+  'missingApp',
+  'serviceUnavailable',
+  'badCredentials',
+  'blockedByPolicy',
+  'modelConnection',
+  'timeout',
+  'unknown',
+]);
+
+/** Lowercase the first character: `MissingPermission` → `missingPermission`. */
+function toCamelClass(cls: string): string {
+  return cls.length > 0 ? cls[0].toLowerCase() + cls.slice(1) : cls;
+}
+
 /** One tool step: type icon + human sentence + contextual detail chip. */
 function ToolRow({ entry }: { entry: ToolTimelineEntry }) {
   const { title, detail } = formatTimelineEntry(entry);
@@ -127,11 +149,44 @@ function ToolRow({ entry }: { entry: ToolTimelineEntry }) {
             {detail}
           </span>
         ) : null}
+        {entry.status === 'error' && entry.failure ? (
+          <ToolFailureLines failure={entry.failure} />
+        ) : null}
       </span>
     </li>
   );
 }
 
+/**
+ * The "why + what to do next" pair rendered under a failed tool row (#4254).
+ * Copy is resolved by failure class from i18n, falling back to the English
+ * `causePlain` / `nextAction` carried on the wire when the class is one the UI
+ * hasn't localized. The failure line is coral to match the error glyph.
+ */
+function ToolFailureLines({ failure }: { failure: ToolFailureExplanation }) {
+  const { t } = useT();
+  const camel = toCamelClass(failure.class);
+  const known = LOCALIZED_FAILURE_CLASSES.has(camel);
+  const cause = known
+    ? t(`conversations.toolFailure.${camel}.cause`, failure.causePlain)
+    : failure.causePlain;
+  const next = known
+    ? t(`conversations.toolFailure.${camel}.next`, failure.nextAction)
+    : failure.nextAction;
+  return (
+    <span
+      data-testid="processing-tool-failure"
+      className="mt-1 flex flex-col gap-0.5 text-[11px] leading-snug">
+      <span className="text-coral-600 dark:text-coral-300">
+        <span className="font-semibold">{t('conversations.toolFailure.whyLabel')}:</span> {cause}
+      </span>
+      <span className="text-content-muted">
+        <span className="font-semibold">{t('conversations.toolFailure.nextLabel')}:</span> {next}
+      </span>
+    </span>
+  );
+}
+
 /** Compact terminal status glyph for the group's "Done" line. */
 function StatusGlyph({ status }: { status: ToolTimelineEntryStatus }) {
   if (status === 'error') {
diff --git a/app/src/providers/ChatRuntimeProvider.tsx b/app/src/providers/ChatRuntimeProvider.tsx
index aadab9151a..0143c405d7 100644
--- a/app/src/providers/ChatRuntimeProvider.tsx
+++ b/app/src/providers/ChatRuntimeProvider.tsx
@@ -39,6 +39,7 @@ import {
   clearStreamingAssistantForThread,
   endInferenceTurn,
   markInferenceTurnStreaming,
+  parseToolFailure,
   recordChatTurnUsage,
   recordProcessingTool,
   recordSubagentTranscriptTool,
@@ -586,6 +587,11 @@ const ChatRuntimeProvider = ({ children }: { children: React.ReactNode }) => {
         )
           return;
 
+        // On failure, parse the optional structured explanation (#4254) once so
+        // both the id-match and name/round-fallback paths can attach it. A
+        // successful result clears any stale failure carried on the row.
+        const failure = event.success ? undefined : parseToolFailure(event.failure);
+
         const existing = store.getState().chatRuntime.toolTimelineByThread[event.thread_id] ?? [];
         if (existing.length > 0) {
           const nextEntries = [...existing];
@@ -597,6 +603,7 @@ const ChatRuntimeProvider = ({ children }: { children: React.ReactNode }) => {
               nextEntries[idx] = {
                 ...nextEntries[idx],
                 status: event.success ? 'success' : 'error',
+                failure,
               };
               changed = true;
             }
@@ -610,7 +617,7 @@ const ChatRuntimeProvider = ({ children }: { children: React.ReactNode }) => {
                 entry.name === event.tool_name &&
                 entry.round === event.round
               ) {
-                nextEntries[i] = { ...entry, status: event.success ? 'success' : 'error' };
+                nextEntries[i] = { ...entry, status: event.success ? 'success' : 'error', failure };
                 changed = true;
                 break;
               }
diff --git a/app/src/services/chatService.ts b/app/src/services/chatService.ts
index 94c0257a10..ddb1e55674 100644
--- a/app/src/services/chatService.ts
+++ b/app/src/services/chatService.ts
@@ -49,6 +49,13 @@ export interface ChatToolResultEvent {
   round: number;
   /** Matches the id on the corresponding {@link ChatToolCallEvent}. */
   tool_call_id?: string;
+  /**
+   * Optional structured failure explanation, present only when `success` is
+   * false (#4254). Raw snake_case wire object (`class`, `category`,
+   * `recoverable`, `cause_plain`, `next_action`); parsed defensively via
+   * `parseToolFailure` before it reaches the store.
+   */
+  failure?: unknown;
 }
 
 /** One sub-agent's token/cost contribution within a turn (hover breakdown). */
@@ -1168,9 +1175,8 @@ export interface ChatSendParams {
   /**
    * Queue mode for concurrent messages. When a turn is already in
    * flight: `steer` injects at the next iteration boundary, `followup`
-   * queues for after the turn, `collect` adds as context, and `parallel`
-   * starts a forked same-thread turn. `interrupt` (default) aborts the
-   * running turn.
+   * queues for after the turn, `collect` adds as context. `interrupt`
+   * (default) aborts the running turn.
    */
   queueMode?: QueueMode | null;
 }
diff --git a/app/src/store/chatRuntimeSlice.toolFailure.test.ts b/app/src/store/chatRuntimeSlice.toolFailure.test.ts
new file mode 100644
index 0000000000..41640ce8cb
--- /dev/null
+++ b/app/src/store/chatRuntimeSlice.toolFailure.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it } from 'vitest';
+
+import { parseToolFailure } from './chatRuntimeSlice';
+
+describe('parseToolFailure', () => {
+  it('parses a well-formed snake_case wire object into camelCase', () => {
+    const parsed = parseToolFailure({
+      class: 'MissingPermission',
+      category: 'NeedsUserConfirmation',
+      recoverable: false,
+      cause_plain: "OpenHuman doesn't have permission to do this yet.",
+      next_action: 'Grant the permission it needs, then try again.',
+    });
+    expect(parsed).toEqual({
+      class: 'MissingPermission',
+      category: 'NeedsUserConfirmation',
+      recoverable: false,
+      causePlain: "OpenHuman doesn't have permission to do this yet.",
+      nextAction: 'Grant the permission it needs, then try again.',
+    });
+  });
+
+  it('accepts camelCase keys (persisted round-trip)', () => {
+    const parsed = parseToolFailure({
+      class: 'Timeout',
+      category: 'Recoverable',
+      recoverable: true,
+      causePlain: 'Took too long.',
+      nextAction: 'Retry.',
+    });
+    expect(parsed?.causePlain).toBe('Took too long.');
+    expect(parsed?.nextAction).toBe('Retry.');
+    expect(parsed?.recoverable).toBe(true);
+  });
+
+  it('defaults recoverable to false when absent or non-boolean', () => {
+    const parsed = parseToolFailure({
+      class: 'Unknown',
+      category: 'Recoverable',
+      cause_plain: 'Oops.',
+      next_action: 'Try again.',
+    });
+    expect(parsed?.recoverable).toBe(false);
+  });
+
+  it.each([
+    ['null', null],
+    ['undefined', undefined],
+    ['a string', 'MissingPermission'],
+    ['a number', 42],
+    ['an empty object', {}],
+    ['missing next_action', { class: 'X', category: 'Y', cause_plain: 'c' }],
+    ['non-string class', { class: 1, category: 'Y', cause_plain: 'c', next_action: 'n' }],
+  ])('returns undefined for garbage input (%s)', (_label, input) => {
+    expect(parseToolFailure(input)).toBeUndefined();
+  });
+});
diff --git a/app/src/store/chatRuntimeSlice.ts b/app/src/store/chatRuntimeSlice.ts
index deedd6b641..f921108092 100644
--- a/app/src/store/chatRuntimeSlice.ts
+++ b/app/src/store/chatRuntimeSlice.ts
@@ -176,6 +176,59 @@ export interface SubagentToolCallEntry {
   detail?: string;
 }
 
+/**
+ * Human-readable explanation for a FAILED tool call (#4254). Carried on the
+ * tool-completion socket event (optional `failure` object, snake_case on the
+ * wire) and surfaced in the "View processing" timeline as a "why" + "what to
+ * do next" pair. `class`/`category` come from the core's failure taxonomy;
+ * `causePlain`/`nextAction` are English fallbacks used when the class is not
+ * one the UI has localized copy for.
+ */
+export interface ToolFailureExplanation {
+  /** PascalCase failure class, e.g. `MissingPermission`, `Timeout`, `Unknown`. */
+  class: string;
+  /** `Recoverable` | `BlockedByPolicy` | `NeedsUserConfirmation`. */
+  category: string;
+  /** Whether the core considers the failure automatically recoverable. */
+  recoverable: boolean;
+  /** English fallback cause copy (used when `class` is unrecognized). */
+  causePlain: string;
+  /** English fallback next-action copy (used when `class` is unrecognized). */
+  nextAction: string;
+}
+
+/**
+ * Defensively parse an incoming `failure` object from a tool-completion socket
+ * payload (snake_case wire) or a persisted entry (camelCase) into a
+ * {@link ToolFailureExplanation}. Returns `undefined` for anything that is not
+ * a well-formed failure object so a malformed/partial payload never corrupts a
+ * timeline entry.
+ */
+export function parseToolFailure(raw: unknown): ToolFailureExplanation | undefined {
+  if (!raw || typeof raw !== 'object') return undefined;
+  const obj = raw as Record<string, unknown>;
+  const cls = obj.class;
+  const category = obj.category;
+  // Accept both wire (snake_case) and persisted (camelCase) key spellings.
+  const causePlain = obj.cause_plain ?? obj.causePlain;
+  const nextAction = obj.next_action ?? obj.nextAction;
+  if (
+    typeof cls !== 'string' ||
+    typeof category !== 'string' ||
+    typeof causePlain !== 'string' ||
+    typeof nextAction !== 'string'
+  ) {
+    return undefined;
+  }
+  return {
+    class: cls,
+    category,
+    recoverable: typeof obj.recoverable === 'boolean' ? obj.recoverable : false,
+    causePlain,
+    nextAction,
+  };
+}
+
 export interface ToolTimelineEntry {
   id: string;
   name: string;
@@ -192,6 +245,13 @@ export interface ToolTimelineEntry {
    * rows and for legacy snapshots emitted by older cores.
    */
   subagent?: SubagentActivity;
+  /**
+   * Human-readable failure explanation for an `error` row (#4254). Parsed from
+   * the tool-completion socket event's optional `failure` object; absent on
+   * successful/running rows and on legacy snapshots. Preserved through the
+   * persisted round-trip so a reloaded failed turn keeps its explanation.
+   */
+  failure?: ToolFailureExplanation;
 }
 
 export interface StreamingAssistantState {
@@ -669,6 +729,9 @@ function toolTimelineFromPersisted(entry: PersistedToolTimelineEntry): ToolTimel
     detail: entry.detail,
     sourceToolName: entry.sourceToolName,
     subagent: entry.subagent ? subagentActivityFromPersisted(entry.subagent) : undefined,
+    // Carry a persisted failure explanation across the round-trip (#4254). The
+    // shared parser tolerates both camelCase (persisted) and snake_case (wire).
+    failure: parseToolFailure(entry.failure),
   };
 }
 
diff --git a/app/src/types/turnState.ts b/app/src/types/turnState.ts
index 544a803fda..8cb02c7e8b 100644
--- a/app/src/types/turnState.ts
+++ b/app/src/types/turnState.ts
@@ -118,6 +118,20 @@ export interface PersistedSubagentActivity {
   transcript?: PersistedSubagentTranscriptItem[];
 }
 
+/**
+ * Human-readable failure explanation persisted alongside a FAILED tool row
+ * (#4254). Mirrors the socket `failure` object; carried in the snapshot so a
+ * settled/reloaded turn keeps its "why + what to do next" explanation. Absent
+ * on successful rows and on snapshots written before this field.
+ */
+export interface PersistedToolFailure {
+  class: string;
+  category: string;
+  recoverable: boolean;
+  causePlain: string;
+  nextAction: string;
+}
+
 export interface PersistedToolTimelineEntry {
   id: string;
   name: string;
@@ -128,6 +142,7 @@ export interface PersistedToolTimelineEntry {
   detail?: string;
   sourceToolName?: string;
   subagent?: PersistedSubagentActivity;
+  failure?: PersistedToolFailure;
 }
 
 export interface PersistedTurnState {