fix(agent): preserve context when summarization fails (#6286)

app/agent/middleware/summarization.py

@@ -0,0 +1,59 @@
+"""Agent 会话上下文压缩中间件。"""
+
+from langchain.agents.middleware import SummarizationMiddleware
+from langchain_core.messages import AnyMessage
+from langchain_core.messages.utils import get_buffer_string
+
+
+class ContextSummarizationError(RuntimeError):
+    """摘要不可用且原有会话上下文未被替换。"""
+
+
+class ContextPreservingSummarizationMiddleware(SummarizationMiddleware):
+    """摘要失败时中止状态更新，避免永久丢失既有会话上下文。"""
+
+    _ERROR_MESSAGE = "会话上下文压缩失败，原有上下文已保留，请稍后重试"
+    _UNSUMMARIZABLE_MESSAGE = (
+        "会话历史中存在无法压缩的超长内容，原有上下文已保留，"
+        "请新建或清空会话后继续"
+    )
+
+    @classmethod
+    def _require_valid_summary(cls, summary: str) -> str:
+        """拒绝无法继续承载会话上下文的空摘要。"""
+        if not summary:
+            raise ContextSummarizationError(cls._ERROR_MESSAGE)
+        return summary
+
+    def _prepare_summary_input(
+        self, messages_to_summarize: list[AnyMessage]
+    ) -> str:
+        """复用 LangChain 裁剪策略生成摘要模型输入。"""
+        trimmed_messages = self._trim_messages_for_summary(messages_to_summarize)
+        if not trimmed_messages:
+            raise ContextSummarizationError(self._UNSUMMARIZABLE_MESSAGE)
+        return get_buffer_string(trimmed_messages, format="xml")
+
+    def _create_summary(self, messages_to_summarize: list[AnyMessage]) -> str:
+        """同步摘要失败时保持原图状态。"""
+        formatted_messages = self._prepare_summary_input(messages_to_summarize)
+        try:
+            response = self.model.invoke(
+                self.summary_prompt.format(messages=formatted_messages).rstrip(),
+                config={"metadata": {"lc_source": "summarization"}},
+            )
+        except Exception as err:
+            raise ContextSummarizationError(self._ERROR_MESSAGE) from err
+        return self._require_valid_summary(response.text.strip())
+
+    async def _acreate_summary(self, messages_to_summarize: list[AnyMessage]) -> str:
+        """异步摘要失败时保持原图状态。"""
+        formatted_messages = self._prepare_summary_input(messages_to_summarize)
+        try:
+            response = await self.model.ainvoke(
+                self.summary_prompt.format(messages=formatted_messages).rstrip(),
+                config={"metadata": {"lc_source": "summarization"}},
+            )
+        except Exception as err:
+            raise ContextSummarizationError(self._ERROR_MESSAGE) from err
+        return self._require_valid_summary(response.text.strip())