diff --git a/apps/desktop/renderer-architecture.json b/apps/desktop/renderer-architecture.json
index 50490edac0..51b23b7387 100644
--- a/apps/desktop/renderer-architecture.json
+++ b/apps/desktop/renderer-architecture.json
@@ -715,7 +715,7 @@
           "react": 1
         },
         "importSpecifiers": 11,
-        "nonTriviaTokens": 1399
+        "nonTriviaTokens": 1395
       },
       "src/renderer/app-shell.tsx": {
         "importDeclarations": 79,
diff --git a/apps/desktop/src/main/__tests__/session-error-presentation.test.ts b/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
index 59c568c674..f9dec247ed 100644
--- a/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
+++ b/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
@@ -21,21 +21,13 @@ import assert from 'node:assert/strict';
 import { describe, it } from 'node:test';
 
 import { describeSessionErrorReason } from '../../renderer/session-error-presentation.js';
-import { sessionEventErrorMessage } from '../../renderer/model-connection-errors.js';
 import { describeTurnErrorClass } from '../../renderer/session-status-presentation.js';
 
 describe('provider capacity presentation', () => {
   it('uses capacity-specific copy instead of the unknown error fallback', () => {
     assert.match(describeSessionErrorReason('provider_capacity', 'zh-CN') ?? '', /满载/);
     assert.match(describeSessionErrorReason('provider_capacity', 'en') ?? '', /at capacity/);
-    assert.match(describeTurnErrorClass('provider_capacity', 'zh-CN'), /满载/);
-    assert.match(describeTurnErrorClass('provider_capacity', 'en'), /at capacity/);
-  });
-
-  it('does not recommend an immediate direct retry', () => {
-    const label = describeTurnErrorClass('provider_capacity', 'zh-CN');
-    assert.match(label, /等几分钟|换一个模型/);
-    assert.doesNotMatch(label, /直接重试/);
+    assert.equal(describeTurnErrorClass('provider_capacity', 'zh-CN'), '模型服务暂时满载。');
+    assert.equal(describeTurnErrorClass('provider_capacity', 'en'), 'The model service is temporarily at capacity.');
   });
 });
-
diff --git a/apps/desktop/src/main/__tests__/session-status-presentation.test.ts b/apps/desktop/src/main/__tests__/session-status-presentation.test.ts
index 111c5e707f..0119fb04c6 100644
--- a/apps/desktop/src/main/__tests__/session-status-presentation.test.ts
+++ b/apps/desktop/src/main/__tests__/session-status-presentation.test.ts
@@ -19,6 +19,8 @@
 
 import assert from 'node:assert/strict';
 import { describe, it } from 'node:test';
+import type { TurnViewModel } from '@maka/ui';
+import { deriveAppShellTurnPresentation } from '../../renderer/app-shell-turn-view-model.js';
 import {
   describeFailedTurnExecutionState,
   describeTurnErrorClass,
@@ -39,13 +41,20 @@ describe('failed turn presentation', () => {
     assert.equal(describeTurnErrorClass('ECONNRESET', 'en'), describeTurnErrorClass('network', 'en'));
   });
 
-  it('states what to do without promising a resume the UI cannot offer', () => {
-    for (const errorClass of ['rate_limit', 'network', 'timeout']) {
-      assert.match(describeTurnErrorClass(errorClass, 'zh-CN'), /重新发消息|再发消息|发消息/);
-    }
-    assert.doesNotMatch(describeTurnErrorClass('unknown_failure', 'zh-CN'), /重试|重发/);
-    assert.match(describeTurnErrorClass('stream_truncated', 'zh-CN'), /中途断开/);
-    assert.match(describeFailedTurnExecutionState({ ...NOTHING_RAN, retry: { decision: 'declined', because: 'side_effects' } }, 'zh-CN')!, /未自动重试/);
+  it('shows the failure cause alongside the recorded retry refusal', () => {
+    const turn: TurnViewModel = {
+      turnId: 't1', status: 'failed', errorClass: 'network',
+      retry: { decision: 'declined', because: 'side_effects' },
+      tools: [], timeline: [], notes: [], startedAt: 1,
+    };
+    const presentation = deriveAppShellTurnPresentation([turn], {
+      activeId: 'session-1', pendingTurnActions: new Set<string>(), uiLocale: 'zh-CN',
+    });
+    assert.equal(presentation.failedReasonLabels.t1, '网络连接失败，请检查网络。');
+    assert.equal(presentation.failedExecutionStateLabels.t1,
+      '本次已有工具活动，为避免重复操作，未自动重试。请先检查工具结果。');
+    assert.equal(describeTurnErrorClass('rate_limit', 'zh-CN'), '模型请求太频繁被限流了。');
+    assert.equal(describeTurnErrorClass('timeout', 'zh-CN'), '模型请求超时。');
   });
 
   it('grades continuable outcomes below outcomes the user must act on', () => {
@@ -93,3 +102,10 @@ describe('failed turn execution state', () => {
   });
 
 });
+
+it('does not hide a terminal diagnostic behind a sandbox tool failure or promote a tool failure to a failed turn', () => {
+  const turn: TurnViewModel = { turnId: 't1', status: 'failed', errorClass: 'unknown', failureMessage: 'Provider request failed after the tool result', tools: [{ toolUseId: 'tool-1', toolName: 'Bash', status: 'errored', args: {}, result: { kind: 'text', text: 'Operation not permitted', sandboxDenial: { likely: true } } }], timeline: [], notes: [], startedAt: 1 };
+  const context = { activeId: 'session-1', pendingTurnActions: new Set<string>(), uiLocale: 'en' as const };
+  assert.ok(deriveAppShellTurnPresentation([turn], context).failedReasonLabels.t1);
+  assert.equal(deriveAppShellTurnPresentation([{ ...turn, status: 'completed' }], context).failedReasonLabels.t1, undefined);
+});
diff --git a/apps/desktop/src/renderer/app-shell-turn-view-model.ts b/apps/desktop/src/renderer/app-shell-turn-view-model.ts
index 570de2bc48..7c8f920576 100644
--- a/apps/desktop/src/renderer/app-shell-turn-view-model.ts
+++ b/apps/desktop/src/renderer/app-shell-turn-view-model.ts
@@ -66,14 +66,7 @@ function isSandboxOnlyToolFailure(turn: TurnViewModel): boolean {
   const erroredTools = turn.tools.filter((tool) => tool.status === 'errored');
   if (erroredTools.length === 0 || !erroredTools.every(isSandboxDeniedTool)) return false;
 
-  const errorClass = turn.errorClass?.toLowerCase();
-  return (
-    errorClass === undefined
-    || errorClass === 'unknown'
-    || errorClass === 'tool_failed'
-    || errorClass === 'sandbox_denial'
-    || errorClass === 'sandbox_denied'
-  );
+  return [undefined, 'unknown', 'tool_failed', 'sandbox_denial', 'sandbox_denied'].includes(turn.errorClass?.toLowerCase());
 }
 
 /**
@@ -219,7 +212,7 @@ function deriveTurnPresentationEntry(input: {
 
   const entry: TurnPresentationEntry = { footerActions };
 
-  if (turn.status === 'failed' && !isSandboxOnlyToolFailure(turn)) {
+  if (turn.status === 'failed' && (turn.failureMessage || !isSandboxOnlyToolFailure(turn))) {
     entry.failedReasonLabel = describeTurnErrorClass(turn.errorClass, uiLocale);
     entry.failedSeverity = deriveFailedTurnSeverity(turn.errorClass);
     entry.failedExecutionStateLabel = describeFailedTurnExecutionState({
diff --git a/apps/desktop/src/renderer/locales/conversation-copy.ts b/apps/desktop/src/renderer/locales/conversation-copy.ts
index b538c0ea67..2c92b47f64 100644
--- a/apps/desktop/src/renderer/locales/conversation-copy.ts
+++ b/apps/desktop/src/renderer/locales/conversation-copy.ts
@@ -683,7 +683,7 @@ const COPY = {
       reauth: { label: '上次连接测试鉴权失败', tooltip: '最近一次连接测试返回鉴权失败（401 / 403），密钥可能已过期或被吊销。这不会拦截发送，但若发送失败请到 设置 · 模型 重新登录。' },
       testError: { label: '上次连接测试失败', tooltip: '最近一次连接测试因网络 / 超时 / 5xx 失败。这不会拦截发送，但若问题持续请到 设置 · 模型 检查 Base URL / 代理。' },
     },
-    turnError: { streamTruncated: '响应中途断开。', requestRejected: '模型服务拒绝了请求，请检查模型与请求配置。', retryExhausted: '已达到自动重试次数上限。', retryDeclined: { side_effects: '本次已有工具活动，为避免重复操作，未自动重试。请先检查工具结果。', observable_output: '本次已有部分输出，未自动重试。请先检查已保留的内容。', policy: '按当前重试规则，本次未自动重试。', budget: '本次执行预算已用尽，未自动重试。' }, unknown: '出错了，暂时无法确定原因。', contextOverflow: '上下文超出模型窗口限制，减少附件或开启新任务。', timeout: '模型请求超时，重新发消息重试。', auth: '模型鉴权失败，请到设置里重新连接或登录。', providerBilling: '模型服务计费受限，请检查账号余额或订阅状态。', providerCapacity: '模型服务暂时满载，等几分钟重试，或换一个模型。', rateLimit: '模型请求太频繁被限流了，等一会儿再发消息重试。', network: '网络连接失败，检查网络后重新发消息。', provider: '模型服务返回错误，稍后重试或换一个模型。', stepCap: '达到工具调用步数上限，任务可能没做完。发消息让它继续。', tool: '工具调用失败，看一下上面的工具结果再决定要不要重试。', permission: '这一轮在等权限确认时结束了，重新发消息会再问一次。', restarted: '本地应用重启，上一轮没有完成', sandboxBoundaryClosed: '本地应用重启时，等待确认的「允许访问工作区以外的内容」请求已按拒绝关闭。重新发消息可以再决定一次。', executionState: { erroredTool: '这一轮有工具执行出错，先看它的结果，再决定要不要重发。', toolRan: '这一轮已经执行过工具，可能已经产生实际改动，重发前先看工具结果。' } },
+    turnError: { streamTruncated: '响应中途断开。', requestRejected: '模型服务拒绝了请求，请检查模型与请求配置。', retryExhausted: '已达到自动重试次数上限。', retryDeclined: { side_effects: '本次已有工具活动，为避免重复操作，未自动重试。请先检查工具结果。', observable_output: '本次已有部分输出，未自动重试。请先检查已保留的内容。', policy: '按当前重试规则，本次未自动重试。', budget: '本次执行预算已用尽，未自动重试。' }, unknown: '出错了，暂时无法确定原因。', contextOverflow: '上下文超出模型窗口限制，减少附件或开启新任务。', timeout: '模型请求超时。', auth: '模型鉴权失败，请到设置里重新连接或登录。', providerBilling: '模型服务计费受限，请检查账号余额或订阅状态。', providerCapacity: '模型服务暂时满载。', rateLimit: '模型请求太频繁被限流了。', network: '网络连接失败，请检查网络。', provider: '模型服务返回错误。', stepCap: '达到工具调用步数上限，任务可能没做完。发消息让它继续。', tool: '工具调用失败，看一下上面的工具结果再决定要不要重试。', permission: '这一轮在等权限确认时结束了，重新发消息会再问一次。', restarted: '本地应用重启，上一轮没有完成', sandboxBoundaryClosed: '本地应用重启时，等待确认的「允许访问工作区以外的内容」请求已按拒绝关闭。重新发消息可以再决定一次。', executionState: { erroredTool: '这一轮有工具执行出错，先看它的结果，再决定要不要重发。', toolRan: '这一轮已经执行过工具，可能已经产生实际改动，重发前先看工具结果。' } },
   },
   'zh-TW': {
     actions: { stopFailedTitle: '停止失敗', stopFailedFallback: '任務操作失敗，請稍後重試。', refreshSessionsFailedTitle: '重新整理任務列表失敗', refreshSessionsFailedFallback: '重新整理任務列表失敗，請稍後重試。', conversationErrorTitle: '任務出錯', conversationErrorFallback: '任務執行失敗，請稍後重試。', regenerateStartedTitle: '已發起重新生成', regenerateStartedDescription: '正在生成新的一輪迴答', branchCreatedTitle: '已建立分支', branchCreatedDescription: (name) => `新任務 ${name}`, revisionStartedTitle: '已建立修改版草稿', revisionStartedDescription: '原任務仍會保留；修改後傳送將在新版本中繼續', revisionReadyTitle: '可以修改並重發了', revisionReadyDescription: '已回到該訊息之前；編輯後傳送即可', revisionUnavailableTitle: '暫時無法編輯這條訊息', revisionAttachmentsUnsupported: '包含附件的歷史訊息暫不支援編輯並重發，請複製文字後建立訊息。', revisionTransformedTextUnsupported: '透過顯式技能傳送的歷史訊息暫不支援編輯並重發，請複製文字後重新選擇技能。', revisionDraftAttachmentConflict: 'Composer 中已有待發送附件，請先發送或移除附件，再編輯歷史訊息。', revisionCommandUnsupported: '修改訊息時不能執行 /compact、/side 或編排命令，請取消修改後再試。', revisionAlreadyActive: '已有一條訊息正在修改，請先發送或取消目前修改。', revisionCancelLabel: '取消', revisionBannerTitle: '正在修改已傳送訊息', revisionBannerDetail: '· 傳送後建立新版本', revisionUnchanged: '內容沒有變化。如需重新回答，請使用“重新生成”。', operationFailedTitle: '操作失敗', operationFailedFallback: '任務操作失敗，請稍後重試。', attachmentFailedTitle: '新增附件失敗', imageAttachmentNotDirectTitle: '圖片已作為附件新增', imageAttachmentNotDirectDescription: '目前模型不會直接接收圖片。圖片已作為附件提供給模型。', tryAgain: '請稍後重試。', modelReboundTitle: '已切換到可用模型', modelReboundDescription: (modelId) => `原任務使用的連線已不可用${modelId ? ` · ${modelId}` : ''}`, messageReadFailedTitle: '讀取任務失敗', scrollMainToBottom: '滾動主對話到底部' },
@@ -914,7 +914,7 @@ const COPY = {
       reauth: { label: '上次連線測試鑑權失敗', tooltip: '最近一次連線測試回傳鑑權失敗（401 / 403），金鑰可能已過期或被吊銷。這不會攔截發送，但若傳送失敗請到 設定 · 模型 重新登入。' },
       testError: { label: '上次連線測試失敗', tooltip: '最近一次連線測試因網路 / 超時 / 5xx 失敗。這不會攔截發送，但若問題持續請到 設定 · 模型 檢查 Base URL / 代理。' },
     },
-    turnError: { streamTruncated: '回應中途斷開。', requestRejected: '模型服務拒絕了請求，請檢查模型與請求設定。', retryExhausted: '已達到自動重試次數上限。', retryDeclined: { side_effects: '本次已有工具活動，為避免重複操作，未自動重試。請先檢查工具結果。', observable_output: '本次已有部分輸出，未自動重試。請先檢查已保留的內容。', policy: '依目前重試規則，本次未自動重試。', budget: '本次執行預算已用盡，未自動重試。' }, unknown: '出錯了，暫時無法確定原因。', contextOverflow: '上下文超出模型視窗限制，減少附件或開啟新任務。', timeout: '模型請求逾時，重新傳送訊息重試。', auth: '模型鑑權失敗，請到設定裡重新連線或登入。', providerBilling: '模型服務計費受限，請檢查帳號餘額或訂閱狀態。', providerCapacity: '模型服務暫時滿載，請等待幾分鐘或切換模型。', rateLimit: '模型請求太頻繁而受到速率限制，請稍候再傳送訊息重試。', network: '網路連線失敗，檢查網路後重新傳送訊息。', provider: '模型服務回傳錯誤，稍後重試或切換模型。', stepCap: '達到工具呼叫步數上限，任務可能尚未完成。傳送訊息讓它繼續。', tool: '工具呼叫失敗，先看上面的工具結果再決定是否重試。', permission: '這一輪在等待權限確認時結束，重新傳送訊息會再詢問一次。', restarted: '本機應用程式重啟，上一輪沒有完成', sandboxBoundaryClosed: '本機應用程式重啟時，等待確認的「允許存取工作區以外的內容」請求已按拒絕關閉。重新傳送訊息可以再次決定。', executionState: { erroredTool: '這一輪有工具執行出錯，先看它的結果，再決定是否重發。', toolRan: '這一輪已經執行過工具，可能已經產生實際變更，重發前先看工具結果。' } },
+    turnError: { streamTruncated: '回應中途斷開。', requestRejected: '模型服務拒絕了請求，請檢查模型與請求設定。', retryExhausted: '已達到自動重試次數上限。', retryDeclined: { side_effects: '本次已有工具活動，為避免重複操作，未自動重試。請先檢查工具結果。', observable_output: '本次已有部分輸出，未自動重試。請先檢查已保留的內容。', policy: '依目前重試規則，本次未自動重試。', budget: '本次執行預算已用盡，未自動重試。' }, unknown: '出錯了，暫時無法確定原因。', contextOverflow: '上下文超出模型視窗限制，減少附件或開啟新任務。', timeout: '模型請求逾時。', auth: '模型鑑權失敗，請到設定裡重新連線或登入。', providerBilling: '模型服務計費受限，請檢查帳號餘額或訂閱狀態。', providerCapacity: '模型服務暫時滿載。', rateLimit: '模型請求太頻繁而受到速率限制。', network: '網路連線失敗，請檢查網路。', provider: '模型服務回傳錯誤。', stepCap: '達到工具呼叫步數上限，任務可能尚未完成。傳送訊息讓它繼續。', tool: '工具呼叫失敗，先看上面的工具結果再決定是否重試。', permission: '這一輪在等待權限確認時結束，重新傳送訊息會再詢問一次。', restarted: '本機應用程式重啟，上一輪沒有完成', sandboxBoundaryClosed: '本機應用程式重啟時，等待確認的「允許存取工作區以外的內容」請求已按拒絕關閉。重新傳送訊息可以再次決定。', executionState: { erroredTool: '這一輪有工具執行出錯，先看它的結果，再決定是否重發。', toolRan: '這一輪已經執行過工具，可能已經產生實際變更，重發前先看工具結果。' } },
   },
   en: {
     actions: { stopFailedTitle: 'Failed to stop', stopFailedFallback: 'The task action failed. Try again later.', refreshSessionsFailedTitle: 'Failed to refresh tasks', refreshSessionsFailedFallback: 'The task list could not be refreshed. Try again later.', conversationErrorTitle: 'Task error', conversationErrorFallback: 'The task run failed. Try again later.', regenerateStartedTitle: 'Regeneration started', regenerateStartedDescription: 'Generating a new response', branchCreatedTitle: 'Branch created', branchCreatedDescription: (name) => `New task: ${name}`, revisionStartedTitle: 'Edit draft ready', revisionStartedDescription: 'The original task is kept; sending creates a new version', revisionReadyTitle: 'Ready to edit and resend', revisionReadyDescription: 'Rewound to before that message; edit and send when ready', revisionUnavailableTitle: 'This message cannot be edited yet', revisionAttachmentsUnsupported: 'Edit & resend does not yet support historical attachments. Copy the text into a new message instead.', revisionTransformedTextUnsupported: 'Edit & resend does not yet support messages sent with an explicit skill. Copy the text and select the skill again instead.', revisionDraftAttachmentConflict: 'The composer already has pending attachments. Send or remove them before editing a sent message.', revisionCommandUnsupported: 'You cannot run /compact, /side, or orchestration commands while editing a sent message. Cancel the edit first.', revisionAlreadyActive: 'Another message is already being edited. Send or cancel that edit first.', revisionCancelLabel: 'Cancel', revisionBannerTitle: 'Editing sent message', revisionBannerDetail: '· New version on send', revisionUnchanged: 'Nothing changed. Use Regenerate if you only want a new answer.', operationFailedTitle: 'Action failed', operationFailedFallback: 'The task action failed. Try again later.', attachmentFailedTitle: 'Failed to add attachment', imageAttachmentNotDirectTitle: 'Image added as an attachment', imageAttachmentNotDirectDescription: 'The current model does not receive images directly. The image has been provided as an attachment.', tryAgain: 'Try again later.', modelReboundTitle: 'Switched to an available model', modelReboundDescription: (modelId) => `The previous connection is unavailable${modelId ? ` · ${modelId}` : ''}`, messageReadFailedTitle: 'Failed to load task', scrollMainToBottom: 'Scroll main conversation to bottom' },
@@ -1161,7 +1161,7 @@ const COPY = {
       reauth: { label: 'Last connection test failed authentication', tooltip: 'The latest test returned 401 / 403. Sending is not blocked, but sign in again under Settings · Models if it fails.' },
       testError: { label: 'Last connection test failed', tooltip: 'The latest test failed because of a network, timeout, or 5xx error. Sending is not blocked; check Base URL or proxy settings if it persists.' },
     },
-    turnError: { streamTruncated: 'The response stream ended before completion.', requestRejected: 'The model service rejected the request. Check the model and request configuration.', retryExhausted: 'The automatic retry limit was reached.', retryDeclined: { side_effects: 'Tool activity already occurred in this attempt. Automatic retry was declined to avoid repeating operations. Check the tool results first.', observable_output: 'This attempt already produced output, so it was not retried automatically. Check the retained content first.', policy: 'This attempt was not retried under the current retry policy.', budget: 'The execution budget was exhausted, so this attempt was not retried automatically.' }, unknown: 'Something went wrong; the cause is unknown.', contextOverflow: 'Context exceeded the model window. Reduce attachments or start a new task.', timeout: 'The model request timed out. Send a message to retry.', auth: 'Model authentication failed. Reconnect or sign in again from Settings.', providerBilling: 'Model billing is restricted. Check the account balance or subscription.', providerCapacity: 'The model service is temporarily at capacity. Wait a few minutes, or switch models.', rateLimit: 'Requests were rate-limited. Wait a moment, then send a message to retry.', network: 'The network connection failed. Check the network, then send a message again.', provider: 'The model service returned an error. Retry later, or switch models.', stepCap: 'The tool-step limit was reached, so the task may be incomplete. Send a message to continue.', tool: 'A tool call failed. Check the tool result above before deciding whether to retry.', permission: 'This turn ended while waiting for permission. Send a message and it will ask again.', restarted: 'The app restarted before the previous turn completed', sandboxBoundaryClosed: 'The app restarted, so the pending request to reach outside the workspace was closed as denied. Send a message to decide again.', executionState: { erroredTool: 'A tool errored during this turn. Read its result before deciding whether to send another message.', toolRan: 'Tools already ran during this turn and may have made real changes. Read their results before sending another message.' } },
+    turnError: { streamTruncated: 'The response stream ended before completion.', requestRejected: 'The model service rejected the request. Check the model and request configuration.', retryExhausted: 'The automatic retry limit was reached.', retryDeclined: { side_effects: 'Tool activity already occurred in this attempt. Automatic retry was declined to avoid repeating operations. Check the tool results first.', observable_output: 'This attempt already produced output, so it was not retried automatically. Check the retained content first.', policy: 'This attempt was not retried under the current retry policy.', budget: 'The execution budget was exhausted, so this attempt was not retried automatically.' }, unknown: 'Something went wrong; the cause is unknown.', contextOverflow: 'Context exceeded the model window. Reduce attachments or start a new task.', timeout: 'The model request timed out.', auth: 'Model authentication failed. Reconnect or sign in again from Settings.', providerBilling: 'Model billing is restricted. Check the account balance or subscription.', providerCapacity: 'The model service is temporarily at capacity.', rateLimit: 'Requests were rate-limited.', network: 'The network connection failed. Check the network.', provider: 'The model service returned an error.', stepCap: 'The tool-step limit was reached, so the task may be incomplete. Send a message to continue.', tool: 'A tool call failed. Check the tool result above before deciding whether to retry.', permission: 'This turn ended while waiting for permission. Send a message and it will ask again.', restarted: 'The app restarted before the previous turn completed', sandboxBoundaryClosed: 'The app restarted, so the pending request to reach outside the workspace was closed as denied. Send a message to decide again.', executionState: { erroredTool: 'A tool errored during this turn. Read its result before deciding whether to send another message.', toolRan: 'Tools already ran during this turn and may have made real changes. Read their results before sending another message.' } },
   },
 } satisfies UiCatalog<DesktopConversationCopy>;
 
diff --git a/apps/desktop/stories/app-shell.stories.tsx b/apps/desktop/stories/app-shell.stories.tsx
index 629a81c71c..428e72dd4d 100644
--- a/apps/desktop/stories/app-shell.stories.tsx
+++ b/apps/desktop/stories/app-shell.stories.tsx
@@ -799,7 +799,7 @@ export const ProviderStreamTruncated: Story = {
       chat={{ messages: [
         user('msg-st-1', 'turn-st', 4, '检查项目的构建结果。'),
         { type: 'assistant', id: 'msg-st-answer', turnId: 'turn-st', ts: NOW - 199_000, text: '构建已完成，我继续检查输出。', modelId: 'claude-sonnet-4-5' },
-        { type: 'turn_state', id: 'state-st-failed', turnId: 'turn-st', ts: NOW - 198_000, status: 'failed', errorClass: 'stream_truncated', retry: { decision: 'declined', because: 'side_effects' } },
+        { type: 'turn_state', id: 'state-st-failed', turnId: 'turn-st', ts: NOW - 198_000, status: 'failed', errorClass: 'stream_truncated', failureMessage: 'Response stream ended without a finish reason. (status=502, requestId=req-stream-4502)', retry: { decision: 'declined', because: 'side_effects' } },
       ] }}
     />
   ),
@@ -820,7 +820,7 @@ export const ProviderRateLimited: Story = {
         messages: [
           user('msg-r-1', 'turn-r', 4, '再生成三个对照方案，越详细越好。'),
           { type: 'turn_state', id: 'state-r-running', turnId: 'turn-r', ts: NOW - 200_000, status: 'running' },
-          { type: 'turn_state', id: 'state-r-failed', turnId: 'turn-r', ts: NOW - 198_000, status: 'failed', errorClass: 'rate_limit' },
+          { type: 'turn_state', id: 'state-r-failed', turnId: 'turn-r', ts: NOW - 198_000, status: 'failed', errorClass: 'rate_limit', failureMessage: 'Quota exceeded for this account. Check the provider quota before submitting another request. (code=insufficient_quota, status=429, requestId=req-4502)' },
         ],
       }}
     />
@@ -834,6 +834,43 @@ export const ProviderRateLimited: Story = {
   },
 };
 
+// The same turn moves from running to its durable terminal contribution.
+function FailureArrival() {
+  const [failed, fail] = useReducer(() => true, false);
+  useEffect(() => {
+    window.addEventListener('storybook:turn-failed', fail);
+    return () => window.removeEventListener('storybook:turn-failed', fail);
+  }, []);
+  return <ComposedShell chat={{ messages: [
+    user('msg-arrival', 'turn-arrival', 4, '检查结果。'),
+    { type: 'assistant', id: 'answer-arrival', turnId: 'turn-arrival', ts: NOW - 200_000, text: '已经得到部分结果。', modelId: 'claude-sonnet-4-5' },
+    { type: 'turn_state', id: 'state-arrival', turnId: 'turn-arrival', ts: NOW - 198_000, status: failed ? 'failed' : 'running', ...(failed ? { errorClass: 'stream_truncated', failureMessage: 'Response stream ended without a finish reason.', retry: { decision: 'declined' as const, because: 'observable_output' as const } } : {}) },
+  ] }} />;
+}
+
+export const FailureArrivesLive: Story = {
+  render: () => <FailureArrival />,
+  play: async ({ canvasElement }) => {
+    expect(canvasElement.querySelector('.maka-turn-failed-banner')).toBeNull();
+    window.dispatchEvent(new Event('storybook:turn-failed'));
+    await waitFor(() => expect(canvasElement.querySelector('.maka-turn-failed-banner')?.textContent).toContain('本次已有部分输出'));
+    const toggle = canvasElement.querySelector<HTMLButtonElement>('.maka-turn-failed-banner button[aria-expanded]')!;
+    expect(toggle.getAttribute('aria-expanded')).toBe('false');
+    toggle.focus();
+    toggle.dispatchEvent(new MouseEvent('click', { bubbles: true }));
+    await waitFor(() => expect(toggle.getAttribute('aria-expanded')).toBe('true'));
+    expect(document.activeElement).toBe(toggle);
+    expect(canvasElement.querySelector('.maka-turn-failure-detail')?.textContent).toBe('Response stream ended without a finish reason.');
+  },
+};
+
+export const LongFailureDiagnostic: Story = {
+  render: () => <ComposedShell chat={{ messages: [
+    user('msg-long-error', 'turn-long-error', 4, '检查模型配置。'),
+    { type: 'turn_state', id: 'state-long-error', turnId: 'turn-long-error', ts: NOW - 198_000, status: 'failed', errorClass: 'request_rejected', failureMessage: 'The provider rejected this request.\n' + 'configuration-'.repeat(145) + '\n(status=400, requestId=req-long-4502)' },
+  ] }} />,
+};
+
 // Real path: the provider throttles a live request and Runtime schedules a
 // retry. The running turn swaps its working phrase for the retry Banner
 // (`ModelProviderRetryIndicator`) — the "retrying" state no story reached.
diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index c4a5285eac..b495caed0d 100644
--- a/packages/core/src/session.ts
+++ b/packages/core/src/session.ts
@@ -17,7 +17,11 @@
  * under the License.
  */
 
-import { isModelRetryDecision, type ModelRetryDecision } from './model-failure.js';
+import {
+  MODEL_FAILURE_MESSAGE_MAX_BYTES,
+  isModelRetryDecision,
+  type ModelRetryDecision,
+} from './model-failure.js';
 
 import {
   decodeMessageContent,
@@ -912,6 +916,7 @@ export interface TurnStateMessage {
   /** Diagnostic source for user/renderer-triggered aborts, e.g. renderer.stop_button. */
   abortSource?: string;
   errorClass?: string;
+  failureMessage?: string;
   retry?: ModelRetryDecision;
 }
 
@@ -1121,6 +1126,7 @@ export interface TurnRecord {
   abortedAt?: number;
   abortSource?: string;
   errorClass?: string;
+  failureMessage?: string;
   retry?: ModelRetryDecision;
 }
 
@@ -1261,6 +1267,7 @@ const TURN_STATE_MESSAGE_SHAPE = defineObjectShape<TurnStateMessage>()(
     'abortedAt',
     'abortSource',
     'errorClass',
+    'failureMessage',
     'retry',
   ],
   ['partialOutputRetained'],
@@ -1542,6 +1549,10 @@ function decodeMessage(
         (message.abortedAt === undefined || isFiniteNumber(message.abortedAt)) &&
         isOptionalString(message.abortSource) &&
         isOptionalString(message.errorClass) &&
+        (message.failureMessage === undefined ||
+          (typeof message.failureMessage === 'string' &&
+            new TextEncoder().encode(message.failureMessage).byteLength <=
+              MODEL_FAILURE_MESSAGE_MAX_BYTES)) &&
         (message.retry === undefined || isModelRetryDecision(message.retry))
       )
         return pickShape(message as unknown as TurnStateMessage, TURN_STATE_MESSAGE_SHAPE);
@@ -1831,6 +1842,7 @@ export function deriveTurnRecords(messages: readonly StoredMessage[]): TurnRecor
         ...(latestState.abortedAt !== undefined ? { abortedAt: latestState.abortedAt } : {}),
         ...(latestState.abortSource ? { abortSource: latestState.abortSource } : {}),
         ...(latestState.errorClass ? { errorClass: latestState.errorClass } : {}),
+        ...(latestState.failureMessage ? { failureMessage: latestState.failureMessage } : {}),
         ...(latestState.retry ? { retry: latestState.retry } : {}),
       };
     }
diff --git a/packages/runtime-host/src/__tests__/session-transcript-pager.test.ts b/packages/runtime-host/src/__tests__/session-transcript-pager.test.ts
index 23410e23f6..00fc141529 100644
--- a/packages/runtime-host/src/__tests__/session-transcript-pager.test.ts
+++ b/packages/runtime-host/src/__tests__/session-transcript-pager.test.ts
@@ -88,6 +88,7 @@ test('preserves the canonical retry decision in shared bootstrap and later pages
       status: 'failed',
       errorClass: 'stream_truncated',
       retry: { decision: 'declined', because: 'side_effects' },
+      failureMessage: 'Private provider diagnostic',
     },
   ];
   const reader = transcriptReader(durable);
@@ -101,6 +102,7 @@ test('preserves the canonical retry decision in shared bootstrap and later pages
     maxBytes: 1024,
     projection: 'shared',
   });
+  assert.equal(decodeBootstrap(bootstrap.durable)[0]?.failureMessage, undefined);
   assert.deepEqual(decodeBootstrap(bootstrap.durable)[0]?.retry, {
     decision: 'declined',
     because: 'side_effects',
@@ -113,6 +115,7 @@ test('preserves the canonical retry decision in shared bootstrap and later pages
     status: 'failed',
     errorClass: 'stream_truncated',
     retry: { decision: 'exhausted', attempts: 2 },
+    failureMessage: 'Another private diagnostic',
   });
   updateSubscriberTranscriptHighWater(state, 1);
   const page = await readSessionTranscriptPage({
@@ -128,6 +131,7 @@ test('preserves the canonical retry decision in shared bootstrap and later pages
       maxBytes: 1024,
     },
   });
+  assert.equal(decodeBootstrap(page)[0]?.failureMessage, undefined);
   assert.deepEqual(decodeBootstrap(page)[0]?.retry, { decision: 'exhausted', attempts: 2 });
 });
 
diff --git a/packages/runtime-host/src/__tests__/session-turns.test.ts b/packages/runtime-host/src/__tests__/session-turns.test.ts
index 5e89f7a914..d311a68ed9 100644
--- a/packages/runtime-host/src/__tests__/session-turns.test.ts
+++ b/packages/runtime-host/src/__tests__/session-turns.test.ts
@@ -18,6 +18,7 @@
  */
 
 import assert from 'node:assert/strict';
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import test from 'node:test';
 import {
   decodeSessionTurnsQueryResult,
@@ -100,6 +101,7 @@ test('bounds turn diagnostics before publishing a contribution', () => {
         ts: 1,
         status: 'failed',
         errorClass: '失败'.repeat(100_000),
+        failureMessage: '失败'.repeat(100_000),
         retry: { decision: 'declined', because: 'side_effects' },
       },
     },
@@ -119,7 +121,11 @@ test('bounds turn diagnostics before publishing a contribution', () => {
     }),
   );
   const turn = projectSessionTurnContribution(contribution);
-  assert.deepEqual(turn?.retry, { decision: 'declined', because: 'side_effects' });
+  assert.ok(turn);
+  assert.ok(turn.failureMessage);
+  assert.ok(Buffer.byteLength(turn.failureMessage) <= MODEL_FAILURE_MESSAGE_MAX_BYTES);
+  assert.equal(turn.failureMessage, contribution.latestState!.message.failureMessage);
+  assert.deepEqual(turn.retry, { decision: 'declined', because: 'side_effects' });
 });
 
 test('rejects invalid turn-state references before publishing a contribution', () => {
diff --git a/packages/runtime-host/src/protocol/index.ts b/packages/runtime-host/src/protocol/index.ts
index 4d3e75ccca..20fe84108d 100644
--- a/packages/runtime-host/src/protocol/index.ts
+++ b/packages/runtime-host/src/protocol/index.ts
@@ -101,7 +101,9 @@ export const RUNTIME_HOST_REGISTRATION_SCHEMA_VERSION = 1 as const;
 export const RUNTIME_HOST_PROTOCOL_VERSION = 0 as const;
 // Increment when the same protocol version no longer guarantees safe Client-Host
 // interoperability. Mismatches are rejected before domain commands are admitted.
-export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 129 as const;
+export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 130 as const;
+// 130: Turn contributions carry the optional bounded `failureMessage` diagnostic.
+// Epoch-129 peers reject this added field on the strict contribution shape.
 // 129: Turn states and Turn records drop `partialOutputRetained`. The fact was
 // derived twice — once from the Turn's output rows, once off the state message
 // — and read by nothing; older peers require the field on both.
diff --git a/packages/runtime-host/src/protocol/session-turns.ts b/packages/runtime-host/src/protocol/session-turns.ts
index 0a3f682381..136f7883f4 100644
--- a/packages/runtime-host/src/protocol/session-turns.ts
+++ b/packages/runtime-host/src/protocol/session-turns.ts
@@ -17,6 +17,7 @@
  * under the License.
  */
 
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import { decodeCanonicalMessage, type TurnRecord, type TurnStateMessage } from '@maka/core/session';
 import { truncateUtf8 } from '@maka/core/diagnostic-log';
 import {
@@ -164,6 +165,15 @@ function projectTurnStateMessageForWire(message: TurnStateMessage): TurnStateMes
     ...(message.errorClass
       ? { errorClass: truncateUtf8(message.errorClass, SESSION_TURN_DIAGNOSTIC_MAX_BYTES) }
       : {}),
+    ...(message.failureMessage
+      ? {
+          failureMessage: truncateUtf8(
+            message.failureMessage,
+            MODEL_FAILURE_MESSAGE_MAX_BYTES,
+            '…',
+          ),
+        }
+      : {}),
     ...(message.retry ? { retry: message.retry } : {}),
   };
 }
@@ -197,6 +207,7 @@ export function projectSessionTurnContribution(
     ...(state.abortedAt !== undefined ? { abortedAt: state.abortedAt } : {}),
     ...(state.abortSource ? { abortSource: state.abortSource } : {}),
     ...(state.errorClass ? { errorClass: state.errorClass } : {}),
+    ...(state.failureMessage ? { failureMessage: state.failureMessage } : {}),
     ...(state.retry ? { retry: state.retry } : {}),
   };
 }
diff --git a/packages/runtime/src/__tests__/ai-sdk-backend.test.ts b/packages/runtime/src/__tests__/ai-sdk-backend.test.ts
index 06a04e4b93..83a86c1c6e 100644
--- a/packages/runtime/src/__tests__/ai-sdk-backend.test.ts
+++ b/packages/runtime/src/__tests__/ai-sdk-backend.test.ts
@@ -7280,7 +7280,7 @@ describe('AiSdkBackend model history', () => {
 });
 
 describe('AiSdkBackend error surfaces', () => {
-  test('generalizes model setup errors before emitting renderer events', async () => {
+  test('preserves model setup diagnostics in renderer events', async () => {
     const backend = createBackend({
       connection: connection(),
       apiKey: 'sk-live-secret-token-value',
@@ -7300,8 +7300,7 @@ describe('AiSdkBackend error surfaces', () => {
     const error = events.find(
       (event): event is Extract<SessionEvent, { type: 'error' }> => event.type === 'error',
     );
-    assert.equal(error?.message, '401 Authorization: Bearer [redacted]');
-    assert.equal(JSON.stringify(events).includes('sk-live-secret-token-value'), false);
+    assert.equal(error?.message, '401 Authorization: Bearer sk-live-secret-token-value');
   });
 
   test('stops after a T1 rejection only after sibling tool calls settle', async () => {
diff --git a/packages/runtime/src/__tests__/model-adapter.test.ts b/packages/runtime/src/__tests__/model-adapter.test.ts
index fe1814a860..a56a940153 100644
--- a/packages/runtime/src/__tests__/model-adapter.test.ts
+++ b/packages/runtime/src/__tests__/model-adapter.test.ts
@@ -917,13 +917,13 @@ describe('ModelAdapter stream and error normalization', () => {
     assert.equal(event.message, 'fetch failed');
   });
 
-  test('retains a safe bounded summary from an unknown structured provider error', () => {
+  test('retains an unredacted bounded summary from an unknown structured provider error', () => {
     const adapter = newAdapter();
     const failure = adapter.normalizeFailure({
       type: 'error',
       error: {
         code: 'provider_error',
-        message: `provider exploded api_key=sk-live-secret-token-value ${'x'.repeat(4_000)}`,
+        message: `provider exploded api_key=sk-test-diagnostic-value ${'x'.repeat(4_000)}`,
       },
       request_id: 'req-123',
     });
@@ -931,10 +931,9 @@ describe('ModelAdapter stream and error normalization', () => {
 
     assert.equal(event.reason, 'unknown');
     assert.equal(event.code, 'provider_error');
-    assert.match(event.message, /^provider exploded api_key=\[redacted\]/);
+    assert.ok(event.message.startsWith('provider exploded api_key=sk-test-diagnostic-value '));
     assert.match(event.message, /… \(code=provider_error, requestId=req-123\)$/);
     assert.equal(Buffer.byteLength(event.message, 'utf8') <= 2 * 1024, true);
-    assert.equal(event.message.includes('sk-live-secret-token-value'), false);
   });
 
   test('normalizes cache and reasoning usage variants in the adapter module', () => {
diff --git a/packages/runtime/src/__tests__/provider-error-classification.test.ts b/packages/runtime/src/__tests__/provider-error-classification.test.ts
index b3f8dd6ad6..40a0f44cc8 100644
--- a/packages/runtime/src/__tests__/provider-error-classification.test.ts
+++ b/packages/runtime/src/__tests__/provider-error-classification.test.ts
@@ -210,7 +210,7 @@ describe('Provider error classification', () => {
   test('extracts allowlisted fields from JSON string failures without copying the payload', () => {
     const summary = providerModelFailure(
       JSON.stringify({
-        error: { message: 'provider rejected request', code: 'bad_request' },
+        error: { message: 'Invalid api_key=sk-test-diagnostic-value', code: 'bad_request' },
         request_id: 'req-123',
         prompt: 'private customer text',
         headers: { 'x-debug': 'internal' },
@@ -218,7 +218,7 @@ describe('Provider error classification', () => {
     );
 
     assert.partialDeepStrictEqual(summary, {
-      message: 'provider rejected request (code=bad_request, requestId=req-123)',
+      message: 'Invalid api_key=sk-test-diagnostic-value (code=bad_request, requestId=req-123)',
       code: 'bad_request',
     });
     assert.equal(JSON.stringify(summary).includes('private customer text'), false);
diff --git a/packages/runtime/src/__tests__/runtime-event-read-model.test.ts b/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
index 880ffdefd3..aea8ca2ef5 100644
--- a/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
+++ b/packages/runtime/src/__tests__/runtime-event-read-model.test.ts
@@ -18,13 +18,14 @@
  */
 
 import assert from 'node:assert/strict';
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import { describe, test } from 'node:test';
 import type { RuntimeInvocationRecord } from '@maka/core/runtime-invocation';
 import type { CreateSessionInput, SessionListFilter } from '@maka/core/runtime-inputs';
 import type { RuntimeEvent, RuntimeEventActions } from '@maka/core/runtime-event';
 import { runtimeEventHasModelVisibleContent } from '@maka/core/runtime-event';
 import type { SessionHeader, SessionSummary, StoredMessage, TurnRecord } from '@maka/core/session';
-import { deriveTurnRecords } from '@maka/core/session';
+import { deriveTurnRecords, decodeCanonicalMessage } from '@maka/core/session';
 import {
   isHardRuntimeEventReadModelDiagnostic,
   isUnclaimedRuntimeEventDiagnostic,
@@ -1579,6 +1580,53 @@ describe('projectRuntimeEventsToStoredMessages', () => {
     assert.deepStrictEqual(out.diagnostics, []);
   });
 
+  test('failure diagnostics survive terminal projection and serialization round-trip', () => {
+    const message = 'Quota exceeded for api_key=sk-test-diagnostic-value (status=429)';
+    const out = projectRuntimeEventsToStoredMessages(
+      [
+        ev({
+          id: 'provider-failed',
+          status: 'failed',
+          content: {
+            kind: 'error',
+            message,
+            retry: { decision: 'declined', because: 'side_effects' },
+          },
+          actions: { endInvocation: true, stateDelta: { failureClass: 'rate_limit' } },
+        }),
+      ],
+      { invocations: [endedAs('failed', 'rate_limit')] },
+    );
+    const live = deriveTurnRecords(out.messages)[0];
+    const roundTripped = deriveTurnRecords(
+      JSON.parse(JSON.stringify(out.messages)).map(decodeCanonicalMessage),
+    )[0];
+    assert.equal(live.failureMessage, message);
+    assert.deepEqual(roundTripped, live);
+    assert.equal(live.errorClass, 'rate_limit');
+    assert.throws(() =>
+      decodeCanonicalMessage({ ...out.messages[0], failureMessage: '界'.repeat(2048) }),
+    );
+    assert.deepEqual(live.retry, { decision: 'declined', because: 'side_effects' });
+  });
+
+  test('bounds terminal diagnostics before publishing decodable turn states', () => {
+    const out = projectRuntimeEventsToStoredMessages(
+      [
+        ev({
+          status: 'failed',
+          content: { kind: 'error', message: '界'.repeat(2048) },
+          actions: { endInvocation: true, stateDelta: { failureClass: 'unknown' } },
+        }),
+      ],
+      { invocations: [endedAs('failed', 'unknown')] },
+    );
+    const turn = deriveTurnRecords(out.messages)[0];
+    assert.ok(turn.failureMessage?.startsWith('界'));
+    assert.ok(Buffer.byteLength(turn.failureMessage!) <= MODEL_FAILURE_MESSAGE_MAX_BYTES);
+    assert.doesNotThrow(() => out.messages.map(decodeCanonicalMessage));
+  });
+
   test('a session written with the retired context_budget_exhausted reads back as context_overflow', () => {
     // The runtime no longer decides locally that a request cannot be made to
     // fit, so that outcome is gone from the live contract. Sessions persisted
diff --git a/packages/runtime/src/__tests__/session-manager-terminal-ledger.test.ts b/packages/runtime/src/__tests__/session-manager-terminal-ledger.test.ts
index 9fc4afd975..46729664d0 100644
--- a/packages/runtime/src/__tests__/session-manager-terminal-ledger.test.ts
+++ b/packages/runtime/src/__tests__/session-manager-terminal-ledger.test.ts
@@ -20,6 +20,7 @@
 import { deferred, nextId } from '@maka/core/test-only/async-primitives';
 import { describe, test } from 'node:test';
 import assert from 'node:assert/strict';
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import { setTimeout as timerDelay } from 'node:timers/promises';
 import { deriveTurnRecords } from '@maka/core/session';
 import { DurableStoreWriteError, RunSealedError } from '@maka/core/runtime-event-store';
@@ -221,6 +222,7 @@ describe('SessionManager terminal ledger invariants', () => {
   });
 
   test('error streams persist a failed terminal fact without non-terminal error ledger rows', async () => {
+    const diagnostic = 'Provider failed with api_key=sk-test-diagnostic-value';
     const store = new TinySessionStore();
     const { manager, runStore, session } = await makeHarness(
       [
@@ -228,7 +230,7 @@ describe('SessionManager terminal ledger invariants', () => {
           type: 'error',
           recoverable: false,
           reason: 'stream_truncated',
-          message: 'Response stream ended without a finish reason.',
+          message: diagnostic,
           retry: { decision: 'declined', because: 'side_effects' },
         },
         { type: 'complete', stopReason: 'end_turn' },
@@ -281,13 +283,14 @@ describe('SessionManager terminal ledger invariants', () => {
     const terminal = restored.find(isTerminalRuntimeEvent)!;
     assert.equal(
       terminal.content?.kind === 'error' ? terminal.content.message : undefined,
-      'Response stream ended without a finish reason.',
+      diagnostic,
     );
     assert.equal(runtimeEventHasModelVisibleContent(terminal), false);
     assert.ok(Buffer.byteLength(JSON.stringify(terminal)) < 4096);
     const cold = projectRuntimeEventsToStoredMessages(restored, { invocations: [run] });
     assert.deepEqual(cold.diagnostics, []);
     const coldTurn = deriveTurnRecords(cold.messages).find((turn) => turn.turnId === 'turn-1')!;
+    assert.equal(coldTurn.failureMessage, diagnostic);
     assert.equal(coldTurn.errorClass, turnState.errorClass);
     assert.deepEqual(coldTurn.retry, turnState.retry);
   });
@@ -1228,6 +1231,33 @@ describe('SessionManager terminal ledger invariants', () => {
     );
   });
 
+  test('caught failures keep diagnostic text within the terminal byte budget', async () => {
+    const store = new TinySessionStore();
+    const runStore = new TinyAgentRunStore();
+    const session = await store.create(makeInput());
+    const run = new AgentRun({
+      sessionId: session.id,
+      header: session,
+      userInput: { turnId: 'turn-1', text: 'hello' },
+      runStore,
+      runtimeEventStore: runStore,
+      newId: nextId(),
+      now: nextNow(40_000),
+      hooks: inertAgentRunHooks(store),
+    });
+    const prefix = 'api_key=sk-test-diagnostic-value ';
+    await run.recordFailure(new Error(prefix + '界'.repeat(2048)));
+    await run.finalize();
+    const terminal = (await runStore.readRuntimeEvents(session.id, run.runId)).find(
+      isTerminalRuntimeEvent,
+    );
+    const content = terminal?.content;
+    assert.equal(content?.kind, 'error');
+    if (content?.kind !== 'error') throw new Error('missing failure diagnostic');
+    assert.ok(content.message.startsWith(prefix));
+    assert.ok(Buffer.byteLength(content.message) <= MODEL_FAILURE_MESSAGE_MAX_BYTES);
+  });
+
   test('direct AgentRun finalize synthesizes a failed terminal fact when no terminal event was recorded', async () => {
     const store = new TinySessionStore();
     const runStore = new TinyAgentRunStore();
diff --git a/packages/runtime/src/__tests__/session-manager.test.ts b/packages/runtime/src/__tests__/session-manager.test.ts
index 012352af12..124d79ff28 100644
--- a/packages/runtime/src/__tests__/session-manager.test.ts
+++ b/packages/runtime/src/__tests__/session-manager.test.ts
@@ -8170,6 +8170,7 @@ describe('SessionManager permission mode updates', () => {
       ts: 103,
       status: 'failed',
       errorClass: 'tool_failed',
+      failureMessage: 'tool failed',
     });
     assert.strictEqual(runtimeEvents.filter((event) => event.status === 'failed').length, 1);
   });
diff --git a/packages/runtime/src/__tests__/subscription-model-fetch.test.ts b/packages/runtime/src/__tests__/subscription-model-fetch.test.ts
index 66e6492f40..fc0152d6ca 100644
--- a/packages/runtime/src/__tests__/subscription-model-fetch.test.ts
+++ b/packages/runtime/src/__tests__/subscription-model-fetch.test.ts
@@ -260,7 +260,12 @@ describe('subscription model fetch', () => {
       fetchFn: async () => {
         attempts += 1;
         return Response.json(
-          { error: { message: 'account is not authorized', code: 'account_not_authorized' } },
+          {
+            error: {
+              message: 'Invalid api_key=sk-test-diagnostic-value',
+              code: 'account_not_authorized',
+            },
+          },
           { status: 403, headers: { 'x-request-id': 'req-codex-403' } },
         );
       },
@@ -275,6 +280,7 @@ describe('subscription model fetch', () => {
       (error) => {
         assert.ok(error instanceof Error);
         assert.match(error.message, /Codex OAuth request failed: HTTP 403/);
+        assert.ok(error.message.includes('api_key=sk-test-diagnostic-value'));
         assert.equal((error as { statusCode?: unknown }).statusCode, 403);
         assert.deepEqual((error as { data?: unknown }).data, {
           error: { code: 'account_not_authorized' },
diff --git a/packages/runtime/src/agent-run.ts b/packages/runtime/src/agent-run.ts
index 5eb3564da0..28e4c86550 100644
--- a/packages/runtime/src/agent-run.ts
+++ b/packages/runtime/src/agent-run.ts
@@ -53,6 +53,8 @@ import type { ModelCallCommit } from '@maka/core/agent-run';
 import { Buffer } from 'node:buffer';
 import { isDeepStrictEqual } from 'node:util';
 import { redactSecrets } from '@maka/core/redaction';
+import { truncateUtf8 } from '@maka/core/diagnostic-log';
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import {
   MODEL_CALL_ATTEMPT_EVENT_TYPE,
   type ModelCallAttempt,
@@ -1211,7 +1213,10 @@ export class AgentRun {
       return;
     }
     this.finalStatus = { status: 'blocked', blockedReason: 'unknown' };
-    this.markRunFailed(error instanceof Error ? error.name : 'unknown', errorMessage(error));
+    this.markRunFailed(
+      error instanceof Error ? error.name : 'unknown',
+      error instanceof Error ? error.message : String(error),
+    );
   }
 
   async finalize(): Promise<void> {
@@ -1454,12 +1459,13 @@ export class AgentRun {
   /**
    * Remember why this run is going to fail.
    *
+   * Preserve diagnostic text without trace redaction, within the byte budget.
    * Nothing is written here: the terminal RuntimeEvent carries the failure, and
    * it is committed once, at the end, by `commitTerminalRun`.
    */
   private markRunFailed(failureClass: string, message: string): void {
     this.failureClass = failureClass;
-    this.failureMessage = redactTraceString(message);
+    this.failureMessage = truncateUtf8(message, MODEL_FAILURE_MESSAGE_MAX_BYTES, '…');
   }
 
   /**
diff --git a/packages/runtime/src/provider-error-classification.ts b/packages/runtime/src/provider-error-classification.ts
index 370c5725ad..762c6b1c27 100644
--- a/packages/runtime/src/provider-error-classification.ts
+++ b/packages/runtime/src/provider-error-classification.ts
@@ -20,7 +20,7 @@
 import { RetryError } from 'ai';
 import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
 import { truncateUtf8 } from '@maka/core/diagnostic-log';
-import { isAuthenticationErrorText, redactSecrets } from '@maka/core/redaction';
+import { isAuthenticationErrorText } from '@maka/core/redaction';
 import type { ModelFailure, ModelFailureKind } from './model-protocol.js';
 
 /**
@@ -383,7 +383,7 @@ function failureSummaryFromFacts(facts: ProviderErrorFacts): ProviderFailureSumm
     MODEL_FAILURE_MESSAGE_MAX_BYTES - Buffer.byteLength(suffix, 'utf8'),
   );
   const summary = `${truncateUtf8(
-    redactSecrets(message ?? 'Provider request failed'),
+    message ?? 'Provider request failed',
     messageBudget,
     '…',
   )}${suffix}`;
@@ -396,7 +396,7 @@ function failureSummaryFromFacts(facts: ProviderErrorFacts): ProviderFailureSumm
 /**
  * Projects provider errors into a small durable fingerprint. Unlike the
  * presentation summary, this intentionally excludes provider messages and
- * response bodies: even redacted free text can echo prompts or credentials.
+ * response bodies: free text can echo prompts or credentials.
  */
 export function providerFailureDiagnostic(error: unknown): ProviderFailureDiagnostic {
   const facts = extractProviderErrorFacts(error);
@@ -528,7 +528,7 @@ function boundedProviderField(value: unknown): string | undefined {
   if (typeof value !== 'string' && typeof value !== 'number') return undefined;
   const normalized = String(value).trim();
   if (!normalized) return undefined;
-  return truncateUtf8(redactSecrets(normalized), PROVIDER_FAILURE_FIELD_MAX_BYTES, '…');
+  return truncateUtf8(normalized, PROVIDER_FAILURE_FIELD_MAX_BYTES, '…');
 }
 
 function boundedProviderMessage(value: unknown, parseJson = true): string | undefined {
@@ -553,7 +553,7 @@ function boundedProviderMessage(value: unknown, parseJson = true): string | unde
     }
   }
   if (!normalized) return undefined;
-  return truncateUtf8(redactSecrets(normalized), MODEL_FAILURE_MESSAGE_MAX_BYTES, '…');
+  return truncateUtf8(normalized, MODEL_FAILURE_MESSAGE_MAX_BYTES, '…');
 }
 
 /**
diff --git a/packages/runtime/src/runtime-event-read-model.ts b/packages/runtime/src/runtime-event-read-model.ts
index fd01aecfec..61e79973dd 100644
--- a/packages/runtime/src/runtime-event-read-model.ts
+++ b/packages/runtime/src/runtime-event-read-model.ts
@@ -17,6 +17,8 @@
  * under the License.
  */
 
+import { MODEL_FAILURE_MESSAGE_MAX_BYTES } from '@maka/core/model-failure';
+import { truncateUtf8 } from '@maka/core/diagnostic-log';
 import type { RuntimeInvocationRecord } from '@maka/core/runtime-invocation';
 import type { AssistantStepContentKind, StoredMessage, TurnStatus } from '@maka/core/session';
 import type { RuntimeEvent, RuntimeEventStatus } from '@maka/core/runtime-event';
@@ -1239,6 +1241,11 @@ function projectTerminalTurnState(
     ...(status === 'aborted' ? { abortedAt: event.ts } : {}),
     ...(abortSource ? { abortSource } : {}),
     ...(status === 'failed' ? { errorClass: failureClass ?? 'unknown' } : {}),
+    ...(status === 'failed' && event.content?.kind === 'error' && event.content.message
+      ? {
+          failureMessage: truncateUtf8(event.content.message, MODEL_FAILURE_MESSAGE_MAX_BYTES, '…'),
+        }
+      : {}),
     ...(status === 'failed' && event.content?.kind === 'error' && event.content.retry
       ? { retry: event.content.retry }
       : {}),
diff --git a/packages/runtime/src/subscription-model-fetch.ts b/packages/runtime/src/subscription-model-fetch.ts
index 26b2a8cfce..348ba19876 100644
--- a/packages/runtime/src/subscription-model-fetch.ts
+++ b/packages/runtime/src/subscription-model-fetch.ts
@@ -17,7 +17,6 @@
  * under the License.
  */
 
-import { redactSecrets } from '@maka/core/redaction';
 import type { RuntimeExecutionConnection } from '@maka/core/llm-connections';
 import {
   GITHUB_COPILOT_API_VERSION,
@@ -381,7 +380,7 @@ function codexInstructionsFromBody(body: Record<string, unknown>): string {
 }
 
 function formatOpenAiCodexHttpError(statusCode: number, detail: string): string {
-  const compact = redactSecrets(detail).replace(/\s+/g, ' ').trim().slice(0, 240);
+  const compact = detail.replace(/\s+/g, ' ').trim().slice(0, 240);
   return compact
     ? `Codex OAuth request failed: HTTP ${statusCode} ${compact}`
     : `Codex OAuth request failed: HTTP ${statusCode}`;
@@ -396,7 +395,7 @@ function openAiCodexHttpError(
     ? 'openai_codex_edge_rejection'
     : openAiCodexProviderCode(detail);
   const rawRequestId = response.headers.get('x-request-id')?.trim();
-  const requestId = rawRequestId ? redactSecrets(rawRequestId).slice(0, 256) : undefined;
+  const requestId = rawRequestId ? rawRequestId.slice(0, 256) : undefined;
   return Object.assign(new Error(formatOpenAiCodexHttpError(response.status, detail)), {
     name: exhaustedEdgeRejection ? 'OpenAiCodexEdgeRejectionError' : 'OpenAiCodexHttpError',
     ...(exhaustedEdgeRejection ? { code: 'openai_codex_edge_rejection' } : {}),
@@ -417,7 +416,7 @@ function openAiCodexProviderCode(detail: string): string | undefined {
         : root;
     const value = error.code ?? error.type;
     if (typeof value !== 'string' && typeof value !== 'number') return undefined;
-    const normalized = redactSecrets(String(value).trim()).slice(0, 256);
+    const normalized = String(value).trim().slice(0, 256);
     return normalized || undefined;
   } catch {
     return undefined;
diff --git a/packages/ui/src/__tests__/transcript-projection.test.ts b/packages/ui/src/__tests__/transcript-projection.test.ts
index 1f154ed1c3..af975aeb8b 100644
--- a/packages/ui/src/__tests__/transcript-projection.test.ts
+++ b/packages/ui/src/__tests__/transcript-projection.test.ts
@@ -456,6 +456,11 @@ describe('turn identity moves across structural change classes', () => {
         { type: 'turn_state', id: 's1', turnId: 'turn-1', ts: 5, status: 'failed' },
       ],
     },
+    {
+      field: 'failureMessage',
+      from: [...base.slice(0, 2), { type: 'turn_state', id: 's1', turnId: 'turn-1', ts: 5, status: 'failed', errorClass: 'rate_limit' }],
+      refresh: [...base.slice(0, 2), { type: 'turn_state', id: 's1', turnId: 'turn-1', ts: 5, status: 'failed', errorClass: 'rate_limit', failureMessage: 'Quota exceeded (status=429, requestId=req-4502)' }],
+    },
     {
       field: 'assistant',
       refresh: [
diff --git a/packages/ui/src/chat-turn.tsx b/packages/ui/src/chat-turn.tsx
index 153503f4f5..58d9523d6a 100644
--- a/packages/ui/src/chat-turn.tsx
+++ b/packages/ui/src/chat-turn.tsx
@@ -695,12 +695,12 @@ export const TurnView = memo(function TurnView(props: {
                   container="section"
                   className="maka-turn-failed-banner"
                   title={props.failedReasonLabel}
-                  {...(props.safeResumeAction?.detail ?? props.failedExecutionStateLabel
-                    ? {
-                        description:
-                          props.safeResumeAction?.detail ?? props.failedExecutionStateLabel,
-                      }
-                    : {})}
+                  description={
+                    <>
+                      {props.safeResumeAction?.detail ?? props.failedExecutionStateLabel}
+                      {!turn.failureMessage && <span className="maka-turn-failure-unavailable">{copy.failureDetailsUnavailable}</span>}
+                    </>
+                  }
                   {...(props.safeResumeAction
                     ? {
                         endContent: (
@@ -718,7 +718,11 @@ export const TurnView = memo(function TurnView(props: {
                         ),
                       }
                     : {})}
-                />
+                >
+                  {turn.failureMessage && (
+                    <pre className="maka-turn-failure-detail">{turn.failureMessage}</pre>
+                  )}
+                </Banner>
               )}
               {ownsTurnChrome && props.liveStreaming && (
                 <>
diff --git a/packages/ui/src/conversation-copy.ts b/packages/ui/src/conversation-copy.ts
index 9fe705f1ff..78579d4a3d 100644
--- a/packages/ui/src/conversation-copy.ts
+++ b/packages/ui/src/conversation-copy.ts
@@ -281,6 +281,7 @@ export interface ConversationCopy {
     providerRetryStarted: (attempt: number, maxAttempts: number) => string;
     providerRetryWaiting: (attempt: number, maxAttempts: number) => string;
     providerRetryReason: Record<ProviderRetryReason, string>;
+    failureDetailsUnavailable: string;
     safeResumePending: string;
     safeResume: string;
     thinking: string;
@@ -547,7 +548,7 @@ const CONVERSATION_COPY = {
       chooseAriaLabel: (label, branch) => branch ? `选择项目：${label}，当前分支 ${branch}` : `选择项目：${label}`,
     },
     messages: {
-      you: '你', assistant: 'Maka', processing: '正在处理…', continuing: '继续中…', awaitingModelOutput: '等待模型输出…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `${formatRetryDelay(seconds, { day: '天', hour: '小时', minute: '分', second: '秒' })}后重试（${attempt}/${maxAttempts}）`, providerRetryStarted: (attempt, maxAttempts) => `正在重试（${attempt}/${maxAttempts}）`, providerRetryWaiting: (attempt, maxAttempts) => `等待重试（${attempt}/${maxAttempts}）`, providerRetryReason: { stream_truncated: '响应中途断开', network: '网络中断', provider_capacity: '模型服务暂时满载', provider_unavailable: '模型服务暂时不可用', rate_limit: '触发模型速率限制', timeout: '请求超时', unknown: '模型请求失败' }, safeResumePending: '正在检查…', safeResume: '继续这一轮', thinking: '深度思考', truncated: '已截断', copied: '已复制', copying: '复制中', copyFailed: '复制失败', copy: '复制', editMessage: '编辑并重发', editMessageDisabledRunning: '当前回答仍在进行中，结束后再编辑', editMessageDisabledAttachments: '包含附件的历史消息暂不支持编辑并重发', editMessageDisabledQuotes: '包含引用的历史消息暂不支持编辑并重发', editMessageDisabledTransformedText: '包含已展开上下文的历史消息暂不支持编辑并重发',
+      you: '你', assistant: 'Maka', processing: '正在处理…', continuing: '继续中…', awaitingModelOutput: '等待模型输出…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `${formatRetryDelay(seconds, { day: '天', hour: '小时', minute: '分', second: '秒' })}后重试（${attempt}/${maxAttempts}）`, providerRetryStarted: (attempt, maxAttempts) => `正在重试（${attempt}/${maxAttempts}）`, providerRetryWaiting: (attempt, maxAttempts) => `等待重试（${attempt}/${maxAttempts}）`, providerRetryReason: { stream_truncated: '响应中途断开', network: '网络中断', provider_capacity: '模型服务暂时满载', provider_unavailable: '模型服务暂时不可用', rate_limit: '触发模型速率限制', timeout: '请求超时', unknown: '模型请求失败' }, failureDetailsUnavailable: '无可用诊断详情。', safeResumePending: '正在检查…', safeResume: '继续这一轮', thinking: '深度思考', truncated: '已截断', copied: '已复制', copying: '复制中', copyFailed: '复制失败', copy: '复制', editMessage: '编辑并重发', editMessageDisabledRunning: '当前回答仍在进行中，结束后再编辑', editMessageDisabledAttachments: '包含附件的历史消息暂不支持编辑并重发', editMessageDisabledQuotes: '包含引用的历史消息暂不支持编辑并重发', editMessageDisabledTransformedText: '包含已展开上下文的历史消息暂不支持编辑并重发',
       editMessageDisabledDirectoryReferences: '包含文件夹引用的历史消息暂不支持编辑并重发',
       userAriaLabel: '你发送的消息', systemAriaLabel: '系统消息', assistantAriaLabel: 'Maka 的回答', answerActionsAriaLabel: (context) => `回答操作${context ? `：${context}` : ''}`, answerActionAriaLabel: (action, context) => `${action}回答${context ? `：${context}` : ''}`, messageActionAriaLabel: (action, context) => `${action}消息${context ? `：${context}` : ''}`, sourceAriaLabel: '本轮回答的来源', derivativesAriaLabel: '本轮回答的衍生', scheduledTaskTriggered: '定时任务触发', scheduledTaskTitle: (id) => `由定时任务触发 · ${id}`, legacyAutomationTriggered: '旧版自动化（仅历史）', legacyAutomationTitle: (id) => `由旧版自动化触发 · ${id} · 仅保留历史，不会再次执行`, goalContinued: 'Goal 自动继续', goalTitle: (id) => `由 Goal 继续执行 · ${id}`, agentGraphTriggered: 'Agent Graph 自动继续', agentGraphTitle: (graphId) => `由 Agent Graph 调度器触发 · ${graphId}`,
       thinkingTruncatedTitle: '部分 reasoning 已截断；显示的是最近的内容', outputTruncatedTitle: '助手输出已超过单次回合上限，超出部分未渲染。如需完整内容请重新生成或查看持久化的任务日志。', removeAttachmentAriaLabel: (name) => `移除 ${name}`, quoteLabel: '引用', quoteExpandAriaLabel: '展开引用全文', quoteCollapseAriaLabel: '收起引用', removeQuoteAriaLabel: '移除引用', aborted: '已中断', abortedByStop: '已中断 · 由停止按钮触发',
@@ -706,7 +707,7 @@ const CONVERSATION_COPY = {
       chooseAriaLabel: (label, branch) => branch ? `選擇專案：${label}，目前分支 ${branch}` : `選擇專案：${label}`,
     },
     messages: {
-      you: '你', assistant: 'Maka', processing: '正在處理…', continuing: '繼續中…', awaitingModelOutput: '等待模型輸出…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `${formatRetryDelay(seconds, { day: '天', hour: '小時', minute: '分', second: '秒' })}後重試（${attempt}/${maxAttempts}）`, providerRetryStarted: (attempt, maxAttempts) => `正在重試（${attempt}/${maxAttempts}）`, providerRetryWaiting: (attempt, maxAttempts) => `等待重試（${attempt}/${maxAttempts}）`, providerRetryReason: { stream_truncated: '回應中途斷開', network: '網路中斷', provider_capacity: '模型服務暫時滿載', provider_unavailable: '模型服務暫時不可用', rate_limit: '觸發模型速率限制', timeout: '請求超時', unknown: '模型請求失敗' }, safeResumePending: '正在檢查…', safeResume: '繼續這一輪', thinking: '深度思考', truncated: '已截斷', copied: '已複製', copying: '複製中', copyFailed: '複製失敗', copy: '複製', editMessage: '編輯並重發', editMessageDisabledRunning: '目前回答仍在進行中，結束後再編輯', editMessageDisabledAttachments: '包含附件的歷史訊息暫不支援編輯並重發', editMessageDisabledQuotes: '包含引用的歷史訊息暫不支援編輯並重發', editMessageDisabledTransformedText: '包含已展開上下文的歷史訊息暫不支援編輯並重發',
+      you: '你', assistant: 'Maka', processing: '正在處理…', continuing: '繼續中…', awaitingModelOutput: '等待模型輸出…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `${formatRetryDelay(seconds, { day: '天', hour: '小時', minute: '分', second: '秒' })}後重試（${attempt}/${maxAttempts}）`, providerRetryStarted: (attempt, maxAttempts) => `正在重試（${attempt}/${maxAttempts}）`, providerRetryWaiting: (attempt, maxAttempts) => `等待重試（${attempt}/${maxAttempts}）`, providerRetryReason: { stream_truncated: '回應中途斷開', network: '網路中斷', provider_capacity: '模型服務暫時滿載', provider_unavailable: '模型服務暫時不可用', rate_limit: '觸發模型速率限制', timeout: '請求超時', unknown: '模型請求失敗' }, failureDetailsUnavailable: '無可用診斷詳情。', safeResumePending: '正在檢查…', safeResume: '繼續這一輪', thinking: '深度思考', truncated: '已截斷', copied: '已複製', copying: '複製中', copyFailed: '複製失敗', copy: '複製', editMessage: '編輯並重發', editMessageDisabledRunning: '目前回答仍在進行中，結束後再編輯', editMessageDisabledAttachments: '包含附件的歷史訊息暫不支援編輯並重發', editMessageDisabledQuotes: '包含引用的歷史訊息暫不支援編輯並重發', editMessageDisabledTransformedText: '包含已展開上下文的歷史訊息暫不支援編輯並重發',
       editMessageDisabledDirectoryReferences: '包含資料夾引用的歷史訊息暫不支援編輯並重發',
       userAriaLabel: '你傳送的訊息', systemAriaLabel: '系統訊息', assistantAriaLabel: 'Maka 的回答', answerActionsAriaLabel: (context) => `回答操作${context ? `：${context}` : ''}`, answerActionAriaLabel: (action, context) => `${action}回答${context ? `：${context}` : ''}`, messageActionAriaLabel: (action, context) => `${action}訊息${context ? `：${context}` : ''}`, sourceAriaLabel: '本輪迴答的來源', derivativesAriaLabel: '本輪迴答的衍生', scheduledTaskTriggered: '定時任務觸發', scheduledTaskTitle: (id) => `由定時任務觸發 · ${id}`, legacyAutomationTriggered: '舊版自動化（僅歷史）', legacyAutomationTitle: (id) => `由舊版自動化觸發 · ${id} · 僅保留歷史，不會再次執行`, goalContinued: 'Goal 自動繼續', goalTitle: (id) => `由 Goal 繼續執行 · ${id}`, agentGraphTriggered: 'Agent Graph 自動繼續', agentGraphTitle: (graphId) => `由 Agent Graph 排程器觸發 · ${graphId}`,
       thinkingTruncatedTitle: '部分 reasoning 已截斷；顯示的是最近的內容', outputTruncatedTitle: '助手輸出已超過單次回合上限，超出部分未渲染。如需完整內容請重新生成或檢視持久化的任務記錄。', removeAttachmentAriaLabel: (name) => `移除 ${name}`, quoteLabel: '引用', quoteExpandAriaLabel: '展開引用全文', quoteCollapseAriaLabel: '收起引用', removeQuoteAriaLabel: '移除引用', aborted: '(已中斷)', abortedByStop: '(已中斷 · 由停止按鈕觸發)',
@@ -891,7 +892,7 @@ const CONVERSATION_COPY = {
       chooseAriaLabel: (label, branch) => branch ? `Choose project: ${label}, current branch ${branch}` : `Choose project: ${label}`,
     },
     messages: {
-      you: 'You', assistant: 'Maka', processing: 'Working…', continuing: 'Continuing…', awaitingModelOutput: 'Waiting for model output…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `Retrying in ${formatRetryDelay(seconds, { day: 'd', hour: 'h', minute: 'm', second: 's' })} (${attempt}/${maxAttempts})`, providerRetryStarted: (attempt, maxAttempts) => `Retrying (${attempt}/${maxAttempts})`, providerRetryWaiting: (attempt, maxAttempts) => `Waiting to retry (${attempt}/${maxAttempts})`, providerRetryReason: { stream_truncated: 'Response stream ended before completion', network: 'Network interrupted', provider_capacity: 'The model service is temporarily at capacity', provider_unavailable: 'Model service temporarily unavailable', rate_limit: 'Model rate limit reached', timeout: 'Request timed out', unknown: 'Model request failed' }, safeResumePending: 'Checking…', safeResume: 'Continue this turn', thinking: 'Thinking', truncated: 'Truncated', copied: 'Copied', copying: 'Copying', copyFailed: 'Copy failed', copy: 'Copy', editMessage: 'Edit & resend', editMessageDisabledRunning: 'Wait for this answer to finish before editing', editMessageDisabledAttachments: 'Edit & resend does not yet support messages with attachments', editMessageDisabledQuotes: 'Edit & resend does not yet support messages with quotes', editMessageDisabledTransformedText: 'Edit & resend does not yet support messages with expanded context',
+      you: 'You', assistant: 'Maka', processing: 'Working…', continuing: 'Continuing…', awaitingModelOutput: 'Waiting for model output…', providerRetryScheduled: (seconds, attempt, maxAttempts) => `Retrying in ${formatRetryDelay(seconds, { day: 'd', hour: 'h', minute: 'm', second: 's' })} (${attempt}/${maxAttempts})`, providerRetryStarted: (attempt, maxAttempts) => `Retrying (${attempt}/${maxAttempts})`, providerRetryWaiting: (attempt, maxAttempts) => `Waiting to retry (${attempt}/${maxAttempts})`, providerRetryReason: { stream_truncated: 'Response stream ended before completion', network: 'Network interrupted', provider_capacity: 'The model service is temporarily at capacity', provider_unavailable: 'Model service temporarily unavailable', rate_limit: 'Model rate limit reached', timeout: 'Request timed out', unknown: 'Model request failed' }, failureDetailsUnavailable: 'No diagnostic details are available.', safeResumePending: 'Checking…', safeResume: 'Continue this turn', thinking: 'Thinking', truncated: 'Truncated', copied: 'Copied', copying: 'Copying', copyFailed: 'Copy failed', copy: 'Copy', editMessage: 'Edit & resend', editMessageDisabledRunning: 'Wait for this answer to finish before editing', editMessageDisabledAttachments: 'Edit & resend does not yet support messages with attachments', editMessageDisabledQuotes: 'Edit & resend does not yet support messages with quotes', editMessageDisabledTransformedText: 'Edit & resend does not yet support messages with expanded context',
       editMessageDisabledDirectoryReferences: 'Edit & resend does not yet support messages with folder references',
       userAriaLabel: 'Your message', systemAriaLabel: 'System message', assistantAriaLabel: "Maka's response", answerActionsAriaLabel: (context) => `Response actions${context ? `: ${context}` : ''}`, answerActionAriaLabel: (action, context) => `${action} response${context ? `: ${context}` : ''}`, messageActionAriaLabel: (action, context) => `${action} message${context ? `: ${context}` : ''}`, sourceAriaLabel: 'Source of this response', derivativesAriaLabel: 'Responses derived from this one', scheduledTaskTriggered: 'Triggered by scheduled task', scheduledTaskTitle: (id) => `Triggered by scheduled task · ${id}`, legacyAutomationTriggered: 'Legacy Automation (history only)', legacyAutomationTitle: (id) => `Triggered by legacy Automation · ${id} · Historical only; it will not run again`, goalContinued: 'Continued by Goal', goalTitle: (id) => `Continued by Goal · ${id}`, agentGraphTriggered: 'Continued by Agent Graph', agentGraphTitle: (graphId) => `Triggered by the Agent Graph scheduler · ${graphId}`,
       thinkingTruncatedTitle: 'Some reasoning was truncated; showing the most recent content', outputTruncatedTitle: 'The assistant output exceeded the per-turn limit. Regenerate it or inspect the persisted task log for the complete content.', removeAttachmentAriaLabel: (name) => `Remove ${name}`, quoteLabel: 'Quote', quoteExpandAriaLabel: 'Show the full quoted excerpt', quoteCollapseAriaLabel: 'Collapse the quoted excerpt', removeQuoteAriaLabel: 'Remove quote', aborted: 'Interrupted', abortedByStop: 'Interrupted · Stop button',
diff --git a/packages/ui/src/materialize.ts b/packages/ui/src/materialize.ts
index 3cacfa985c..e07e5a294f 100644
--- a/packages/ui/src/materialize.ts
+++ b/packages/ui/src/materialize.ts
@@ -415,6 +415,7 @@ export interface TurnViewModel {
   abortedAt?: number;
   abortSource?: string;
   errorClass?: string;
+  failureMessage?: string;
   retry?: import('@maka/core/model-failure').ModelRetryDecision;
   user?: ChatItem;
   tools: ToolActivityItem[];
@@ -784,6 +785,7 @@ export function materializeTurns(
           ? { abortedAt: record.abortedAt }
           : {}),
         ...(record?.abortSource ? { abortSource: record.abortSource } : {}),
+        ...(record?.failureMessage ? { failureMessage: record.failureMessage } : {}),
         ...(record?.errorClass ? { errorClass: record.errorClass } : {}),
         ...(record?.retry ? { retry: record.retry } : {}),
         tools: [],
diff --git a/packages/ui/src/styles.css b/packages/ui/src/styles.css
index ecbdf18e9b..078609bed0 100644
--- a/packages/ui/src/styles.css
+++ b/packages/ui/src/styles.css
@@ -813,6 +813,26 @@
 
 /* Astryx `Banner` paints the failed-turn surface; this only sets the block
    rhythm between it and the timeline entry above it. */
+.maka-turn-failure-unavailable { display: block; }
+/* Keep Banner's immediate height change and built-in chevron rotation.
+   Animating height repeatedly lays out the transcript and triggers its
+   ResizeObserver-driven scroll following; that cost grows with history.
+   Only fade the newly mounted detail, without delaying layout or removal. */
+.maka-turn-failure-detail {
+  margin: 0;
+  white-space: pre-wrap;
+  overflow-wrap: anywhere;
+  font: inherit;
+  color: var(--foreground);
+  animation: maka-turn-failure-enter var(--duration-fast) var(--ease-standard);
+}
+@keyframes maka-turn-failure-enter {
+  from { opacity: 0; }
+  to { opacity: 1; }
+}
+@media (prefers-reduced-motion: reduce) {
+  .maka-turn-failure-detail { animation: none; }
+}
 .maka-turn-failed-banner { margin-block: var(--space-1) var(--space-0-5); }
 
 .maka-turn-lineage-row,