feat(ai-writing): 各阶段模型调用自动容错，失败时换下一个模型继续

参考多智能体会商的容错思路：意图解析/大纲/正文/审核四个 LLM
环节失败时，按 config.yaml models 顺序换下一个模型重跑该环节，
并通过 model_switch 事件在前端时间线提示「『X』模型<原因>，
已自动切换到『Y』继续」。正文逐章节、审核逐维度独立切换，
已生成的章节/维度保留；意图解析成功的模型还会写回 state
供下游阶段沿用，避免每阶段重复经历同一次失败。

- nodes/_model_fallback.py: 候选模型链 + 异常分类 + model_switch
  事件构造（沿用 ROUNDTABLE_MODEL_FALLBACK[_MAX] env 开关）
- intent_parser/writer_outline/writer_draft/editor: 失败循环换模型
- 前端 types/reducer/timeline: 新增 model_switch 事件类型，按
  stage 复位对应流式缓冲并以 amber 系统提示渲染

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/open-canvas/components/ai-writing/AIWritingTimeline.tsx

@@ -19,6 +19,7 @@ const DISPLAYED_TYPES = new Set([
   'await_user',
   'resumed',
   'notice',
+  'model_switch',
   'done',
   'error',
 ]);

frontend-web/src/open-canvas/components/ai-writing/TimelineStep.tsx

@@ -13,6 +13,7 @@ interface TimelineStepProps {
 function statusOf(event: ProgressEvent, isLast: boolean): StepStatus {
   if (event.type === 'error') return 'error';
   if (event.type === 'notice') return 'warning';
+  if (event.type === 'model_switch') return 'warning';
   if (event.type === 'await_user') return 'paused';
   // step_start 只有在列表末尾（后续事件还未到达）时才显示转圈，否则已有后续事件说明该步骤已完成
   if (event.type === 'step_start') return isLast ? 'running' : 'done';
@@ -30,8 +31,11 @@ const AGENT_COLOR: Record<string, string> = {
 
 export function TimelineStep({ event, isLast, children }: TimelineStepProps) {
   const status = statusOf(event, isLast ?? true);
-  // notice=非致命告警（如 ES 波动、模型兜底），error=致命错误，两者都展示「标题 + 真实原因」
-  const isAlert = event.type === 'error' || event.type === 'notice';
+  // notice=非致命告警（如 ES 波动、模型兜底），error=致命错误，model_switch=模型自动容错切换（amber 同 notice）
+  const isAlert =
+    event.type === 'error' ||
+    event.type === 'notice' ||
+    event.type === 'model_switch';
   const isError = event.type === 'error';
 
   return (

frontend-web/src/open-canvas/hooks/useAIWritingStream.ts

@@ -388,6 +388,37 @@ function handleEvent(prev: AIWritingState, event: ProgressEvent): AIWritingState
     case 'notice':
       return base;
 
+    case 'model_switch': {
+      // 模型自动容错：丢弃当前阶段已累积的流式缓冲，避免上一个失败模型的半截输出
+      // 与下一个模型的输出粘在一起；时间线展示侧由 TimelineStep 按 amber 渲染（同 notice）。
+      const stage = event.modelSwitch?.stage;
+      const sectionIndex = event.modelSwitch?.sectionIndex;
+      const resetByStage: Partial<AIWritingState> = (() => {
+        switch (stage) {
+          case 'intent':
+            return { intentStreamingText: '', intentThinkingText: '' };
+          case 'outline':
+            return { outlineStreamingText: '', outlineThinkingText: '' };
+          case 'draft': {
+            // 只清掉失败那一节的流式片段，已写好的章节保持不动
+            if (typeof sectionIndex === 'number') {
+              return {
+                streamingDraftSections: prev.streamingDraftSections.filter(
+                  (s) => s.sectionIndex !== sectionIndex,
+                ),
+              };
+            }
+            return { streamingDraftSections: [], streamingDraftTitle: '' };
+          }
+          case 'review':
+            return { streamingReviewItems: [] };
+          default:
+            return {};
+        }
+      })();
+      return { ...base, ...resetByStage };
+    }
+
     default:
       return base;
   }
@@ -506,6 +537,22 @@ export function normalizeProgressDict(parsed: Record<string, unknown>, sessionId
     out.reviewItemIssues = (out.issues as Record<string, unknown>[]).map(normalizeReviewIssue);
   }
 
+  // model_switch（updates 通道里的 progress_events 也可能带）：折成 modelSwitch 子对象
+  if (out.type === 'model_switch' && !out.modelSwitch) {
+    out.modelSwitch = {
+      stage: (out.stage as 'intent' | 'outline' | 'draft' | 'review') ?? 'intent',
+      failedModel: String(out.failed_model ?? out.failedModel ?? ''),
+      nextModel: String(out.next_model ?? out.nextModel ?? ''),
+      reason: String(out.reason ?? ''),
+      sectionIndex: typeof out.section_index === 'number'
+        ? (out.section_index as number)
+        : (typeof out.sectionIndex === 'number' ? (out.sectionIndex as number) : undefined),
+      sectionTitle: typeof out.section_title === 'string'
+        ? (out.section_title as string)
+        : (typeof out.sectionTitle === 'string' ? (out.sectionTitle as string) : undefined),
+    };
+  }
+
   if (!out.message) out.message = '';
   return out as unknown as ProgressEvent;
 }
@@ -522,6 +569,7 @@ export function customToProgressEvent(raw: unknown, sessionId: string): Progress
     'research_progress', 'outline_chunk', 'draft_chunk',
     'review_chunk', 'review_item_done',
     'skill_search_step', 'skill_select_chunk',
+    'model_switch',
   ];
   if (!allowed.includes(type as ProgressEventType)) return null;
 
@@ -536,10 +584,30 @@ export function customToProgressEvent(raw: unknown, sessionId: string): Progress
   if (Array.isArray(merged.issues)) {
     merged.reviewItemIssues = (merged.issues as Record<string, unknown>[]).map(normalizeReviewIssue);
   }
+  if ('agent_name' in merged && !merged.agentName) merged.agentName = merged.agent_name;
   // skill_search_step：把字段折成 skillStep 子对象，方便 reducer / UI 取用
   if (type === 'skill_search_step') {
     merged.skillStep = normalizeSkillStep(o);
   }
+  // model_switch：把 snake_case 字段折成 modelSwitch 子对象（reducer + TimelineStep 取用）
+  if (type === 'model_switch') {
+    merged.modelSwitch = {
+      stage: (o.stage as 'intent' | 'outline' | 'draft' | 'review') ?? 'intent',
+      failedModel: String(o.failed_model ?? (o as Record<string, unknown>).failedModel ?? ''),
+      nextModel: String(o.next_model ?? (o as Record<string, unknown>).nextModel ?? ''),
+      reason: String(o.reason ?? ''),
+      sectionIndex: typeof o.section_index === 'number'
+        ? (o.section_index as number)
+        : (typeof (o as Record<string, unknown>).sectionIndex === 'number'
+          ? ((o as Record<string, unknown>).sectionIndex as number)
+          : undefined),
+      sectionTitle: typeof o.section_title === 'string'
+        ? (o.section_title as string)
+        : (typeof (o as Record<string, unknown>).sectionTitle === 'string'
+          ? ((o as Record<string, unknown>).sectionTitle as string)
+          : undefined),
+    };
+  }
   return merged as unknown as ProgressEvent;
 }
 

frontend-web/src/open-canvas/types/ai-writing.ts

@@ -172,6 +172,7 @@ export type ProgressEventType =
   | "await_user"
   | "resumed"
   | "notice"
+  | "model_switch"
   | "skill_search_step"
   | "skill_select_chunk"
   | "done"
@@ -215,6 +216,16 @@ export interface ProgressEvent {
   blockedSections?: BlockedSection[];
   // skill_search_step：技能检索步骤事件（list_skills / select_skills / invoke_skill）
   skillStep?: SkillSearchStep;
+  // model_switch：模型自动容错切换（某模型挂掉，换下一个模型继续）
+  // stage ∈ intent / outline / draft / review，前端 reducer 据此清掉对应阶段的流式缓冲。
+  modelSwitch?: {
+    stage: "intent" | "outline" | "draft" | "review";
+    failedModel: string;
+    nextModel: string;
+    reason: string;
+    sectionIndex?: number;
+    sectionTitle?: string;
+  };
 }
 
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