fix: improve JSON parsing robustness in chat_json and plan_outline fallback

- Add multi-strategy JSON parsing in llm_client.chat_json (control char removal, nested JSON extraction, single-quote fix, markdown code block handling)
- Add ValueError-specific handler in plan_outline to extract JSON from error messages before falling back

backend/app/services/report_agent.py

@@ -1392,6 +1392,43 @@ class ReportAgent:
             logger.info(t('report.outlinePlanDone', count=len(sections)))
             return outline
             
+        except ValueError as e:
+            # JSON 解析失败，尝试用更宽松的方式提取 JSON
+            logger.warning(t('report.outlinePlanFailed', error=str(e)))
+            try:
+                # 尝试从错误消息中提取 JSON
+                error_msg = str(e)
+                json_start = error_msg.find('{')
+                json_end = error_msg.rfind('}') + 1
+                if json_start >= 0 and json_end > json_start:
+                    json_str = error_msg[json_start:json_end]
+                    response = json.loads(json_str)
+                    sections = []
+                    for section_data in response.get("sections", []):
+                        sections.append(ReportSection(
+                            title=section_data.get("title", ""),
+                            content=""
+                        ))
+                    outline = ReportOutline(
+                        title=response.get("title", "模拟分析报告"),
+                        summary=response.get("summary", ""),
+                        sections=sections
+                    )
+                    outline = self._ensure_prediction_outline(outline)
+                    logger.info(t('report.outlinePlanDone', count=len(sections)))
+                    return outline
+            except Exception:
+                pass
+            # Fallback 默认大纲
+            return self._ensure_prediction_outline(ReportOutline(
+                title="未来预测报告",
+                summary="基于模拟预测的未来趋势与风险分析",
+                sections=[
+                    ReportSection(title="预测场景与核心发现"),
+                    ReportSection(title="人群行为预测分析"),
+                    ReportSection(title="趋势展望与风险提示")
+                ]
+            ))
         except Exception as e:
             logger.error(t('report.outlinePlanFailed', error=str(e)))
             # 返回默认大纲（3个章节，作为fallback）

backend/app/utils/llm_client.py

@@ -101,8 +101,51 @@ class LLMClient:
         cleaned_response = re.sub(r'\n?```\s*$', '', cleaned_response)
         cleaned_response = cleaned_response.strip()
 
+        # 尝试直接解析
         try:
             return json.loads(cleaned_response)
         except json.JSONDecodeError:
-            raise ValueError(f"LLM返回的JSON格式无效: {cleaned_response}")
+            pass
+
+        # 策略2: 尝试提取 JSON 对象（可能嵌套在其他文本中）
+        json_patterns = [
+            r'\{[^{}]*(?:\{[^{}]*\}[^{}]*)*\}',  # 简单嵌套
+            r'(\{"[^"]*"\s*:\s*)',  # JSON 开始位置
+        ]
+        for pattern in json_patterns:
+            match = re.search(pattern, cleaned_response, re.DOTALL)
+            if match:
+                try:
+                    # 尝试从匹配位置开始解析
+                    candidate = match.group(0) if match.group(0).startswith('{') else match.group(1)
+                    # 向后扩展直到找到完整对象
+                    for end in range(len(candidate), len(cleaned_response) + 1):
+                        try:
+                            result = json.loads(cleaned_response[match.start():end])
+                            return result
+                        except json.JSONDecodeError:
+                            continue
+                except Exception:
+                    pass
+
+        # 策略3: 尝试修复常见 JSON 问题
+        fixed = cleaned_response
+        # 移除控制字符
+        fixed = re.sub(r'[\x00-\x1f\x7f-\x9f]', '', fixed)
+        # 修复单引号
+        fixed = re.sub(r"'", '"', fixed)
+        try:
+            return json.loads(fixed)
+        except json.JSONDecodeError:
+            pass
+
+        # 策略4: 查找 ```json ... ``` 块
+        code_block_match = re.search(r'```json\s*\n?(.*?)\n?```', cleaned_response, re.DOTALL | re.IGNORECASE)
+        if code_block_match:
+            try:
+                return json.loads(code_block_match.group(1).strip())
+            except json.JSONDecodeError:
+                pass
+
+        raise ValueError(f"LLM返回的JSON格式无效: {cleaned_response[:500]}")