fix: 截断器丢失唯一 user 消息导致智谱等 provider 返回 400 (AstrBotDevs#6581)

* fix: 截断器丢失唯一 user 消息导致 API 400

修复 AstrBotDevs#6196

当对话只有一条 user 消息（长 tool chain 场景：system → user → assistant
→ tool → assistant → tool → ...），三个截断方法都会把这条 user 消息丢掉，
导致智谱、Gemini 等要求 user 消息的 provider 返回 400。

改动：
- 提取 `_split_system_rest()` 去掉三个方法里重复的 system/non-system 拆分
- 新增 `_ensure_user_message()`：截断后如果没有 user 了，从原始消息里补回
  第一条 user，避免违反 API 格式要求
- 删掉 `truncate_by_dropping_oldest_turns` 里把没有 user 就清空全部消息的逻辑
- 5 个新测试覆盖单 user + 长 tool chain 场景，3 个旧测试更新断言

* style: format code

---------

Co-authored-by: Yufeng He <40085740+universeplayer@users.noreply.github.com>
Co-authored-by: RC-CHN <1051989940@qq.com>

Author: he-yufeng

astrbot/core/agent/context/truncator.py

@@ -12,6 +12,37 @@ def _has_tool_calls(self, message: Message) -> bool:
             and len(message.tool_calls) > 0
         )
 
+    @staticmethod
+    def _split_system_rest(
+        messages: list[Message],
+    ) -> tuple[list[Message], list[Message]]:
+        """把 system 消息和后面的对话消息分开。"""
+        first_non_system = 0
+        for i, msg in enumerate(messages):
+            if msg.role != "system":
+                first_non_system = i
+                break
+        return messages[:first_non_system], messages[first_non_system:]
+
+    @staticmethod
+    def _ensure_user_message(
+        system_messages: list[Message],
+        truncated: list[Message],
+        original_messages: list[Message],
+    ) -> list[Message]:
+        """截断后如果没有 user 消息了，从原始列表里把第一条 user 补回来。
+        很多 provider (智谱、Gemini 等) 要求 system 之后必须紧跟 user，否则直接 400。
+        """
+        if any(m.role == "user" for m in truncated):
+            return system_messages + truncated
+
+        # 从原始消息里找第一条 user
+        first_user = next((m for m in original_messages if m.role == "user"), None)
+        if first_user is None:
+            return system_messages + truncated
+
+        return system_messages + [first_user] + truncated
+
     def fix_messages(self, messages: list[Message]) -> list[Message]:
         """修复消息列表，确保 tool call 和 tool response 的配对关系有效。
 
@@ -81,14 +112,7 @@ def truncate_by_turns(
         if keep_most_recent_turns == -1:
             return messages
 
-        first_non_system = 0
-        for i, msg in enumerate(messages):
-            if msg.role != "system":
-                first_non_system = i
-                break
-
-        system_messages = messages[:first_non_system]
-        non_system_messages = messages[first_non_system:]
+        system_messages, non_system_messages = self._split_system_rest(messages)
 
         if len(non_system_messages) // 2 <= keep_most_recent_turns:
             return messages
@@ -99,16 +123,17 @@ def truncate_by_turns(
         else:
             truncated_contexts = non_system_messages[-num_to_keep * 2 :]
 
-        # 找到第一个 role 为 user 的索引，确保上下文格式正确
+        # 对齐到第一条 user 消息
         index = next(
             (i for i, item in enumerate(truncated_contexts) if item.role == "user"),
             None,
         )
         if index is not None and index > 0:
             truncated_contexts = truncated_contexts[index:]
 
-        result = system_messages + truncated_contexts
-
+        result = self._ensure_user_message(
+            system_messages, truncated_contexts, messages
+        )
         return self.fix_messages(result)
 
     def truncate_by_dropping_oldest_turns(
@@ -120,31 +145,24 @@ def truncate_by_dropping_oldest_turns(
         if drop_turns <= 0:
             return messages
 
-        first_non_system = 0
-        for i, msg in enumerate(messages):
-            if msg.role != "system":
-                first_non_system = i
-                break
-
-        system_messages = messages[:first_non_system]
-        non_system_messages = messages[first_non_system:]
+        system_messages, non_system_messages = self._split_system_rest(messages)
 
         if len(non_system_messages) // 2 <= drop_turns:
             truncated_non_system = []
         else:
             truncated_non_system = non_system_messages[drop_turns * 2 :]
 
+        # 对齐到第一条 user
         index = next(
             (i for i, item in enumerate(truncated_non_system) if item.role == "user"),
             None,
         )
         if index is not None:
             truncated_non_system = truncated_non_system[index:]
-        elif truncated_non_system:
-            truncated_non_system = []
-
-        result = system_messages + truncated_non_system
 
+        result = self._ensure_user_message(
+            system_messages, truncated_non_system, messages
+        )
         return self.fix_messages(result)
 
     def truncate_by_halving(
@@ -155,28 +173,23 @@ def truncate_by_halving(
         if len(messages) <= 2:
             return messages
 
-        first_non_system = 0
-        for i, msg in enumerate(messages):
-            if msg.role != "system":
-                first_non_system = i
-                break
-
-        system_messages = messages[:first_non_system]
-        non_system_messages = messages[first_non_system:]
+        system_messages, non_system_messages = self._split_system_rest(messages)
 
         messages_to_delete = len(non_system_messages) // 2
         if messages_to_delete == 0:
             return messages
 
         truncated_non_system = non_system_messages[messages_to_delete:]
 
+        # 对齐到第一条 user
         index = next(
             (i for i, item in enumerate(truncated_non_system) if item.role == "user"),
             None,
         )
         if index is not None:
             truncated_non_system = truncated_non_system[index:]
 
-        result = system_messages + truncated_non_system
-
+        result = self._ensure_user_message(
+            system_messages, truncated_non_system, messages
+        )
         return self.fix_messages(result)

tests/agent/test_truncator.py

@@ -104,8 +104,9 @@ def test_truncate_by_turns_zero_keep(self):
             messages, keep_most_recent_turns=0, drop_turns=1
         )
 
-        # Should result in empty or minimal list
-        assert len(result) == 0
+        # 截断后至少保留一条 user 消息 (#6196)
+        assert len(result) >= 1
+        assert result[0].role == "user"
 
     def test_truncate_by_turns_below_threshold(self):
         """Test truncate_by_turns when messages are below threshold."""
@@ -201,8 +202,9 @@ def test_truncate_by_dropping_oldest_turns_drop_all(self):
         messages = self.create_messages(4)
         result = truncator.truncate_by_dropping_oldest_turns(messages, drop_turns=2)
 
-        # Should drop all turns
-        assert len(result) == 0
+        # 即使 drop 掉所有 turn，也会把 user 消息补回来 (#6196)
+        assert len(result) >= 1
+        assert result[0].role == "user"
 
     def test_truncate_by_dropping_oldest_turns_drop_more_than_available(self):
         """Test truncate_by_dropping_oldest_turns with drop_turns > available turns."""
@@ -211,8 +213,9 @@ def test_truncate_by_dropping_oldest_turns_drop_more_than_available(self):
         messages = self.create_messages(4)
         result = truncator.truncate_by_dropping_oldest_turns(messages, drop_turns=5)
 
-        # Should result in empty list
-        assert len(result) == 0
+        # 同理，user 消息会被保留 (#6196)
+        assert len(result) >= 1
+        assert result[0].role == "user"
 
     def test_truncate_by_dropping_oldest_turns_ensures_user_first(self):
         """Test that result starts with user message after dropping."""
@@ -372,3 +375,60 @@ def test_all_system_messages(self):
         assert len(result) >= 0  # May keep system messages or clear all
         if len(result) > 0:
             assert all(msg.role == "system" for msg in result)
+
+    # ==================== #6196: 长 tool chain 只有一条 user 消息 ====================
+
+    def _build_tool_chain(self, tool_rounds: int = 20) -> list[Message]:
+        """构造 system -> user -> (assistant -> tool) * N 的长链，只有一条 user。"""
+        msgs = [
+            self.create_message("system", "You are a helpful assistant."),
+            self.create_message("user", "帮我查一下天气"),
+        ]
+        for i in range(tool_rounds):
+            msgs.append(self.create_message("assistant", f"调用工具 {i}"))
+            msgs.append(self.create_message("tool", f"工具结果 {i}"))
+        return msgs
+
+    def test_drop_oldest_preserves_sole_user(self):
+        """#6196: drop 1 turn 不应丢掉唯一的 user 消息。"""
+        truncator = ContextTruncator()
+        msgs = self._build_tool_chain(20)  # 1 system + 1 user + 40 asst/tool = 42
+        result = truncator.truncate_by_dropping_oldest_turns(msgs, drop_turns=1)
+        roles = [m.role for m in result]
+        assert "user" in roles, "唯一的 user 消息被丢掉了"
+        assert roles[0] == "system"
+
+    def test_halving_preserves_sole_user(self):
+        """#6196: 对半砍不应丢掉唯一的 user 消息。"""
+        truncator = ContextTruncator()
+        msgs = self._build_tool_chain(20)
+        result = truncator.truncate_by_halving(msgs)
+        roles = [m.role for m in result]
+        assert "user" in roles, "唯一的 user 消息被丢掉了"
+
+    def test_truncate_by_turns_preserves_sole_user(self):
+        """#6196: keep_most_recent_turns 也不应丢掉唯一的 user 消息。"""
+        truncator = ContextTruncator()
+        msgs = self._build_tool_chain(20)
+        result = truncator.truncate_by_turns(
+            msgs, keep_most_recent_turns=3, drop_turns=1
+        )
+        roles = [m.role for m in result]
+        assert "user" in roles, "唯一的 user 消息被丢掉了"
+
+    def test_drop_oldest_heavy_drops_still_has_user(self):
+        """#6196: 大量 drop 也不会丢 user。"""
+        truncator = ContextTruncator()
+        msgs = self._build_tool_chain(30)
+        result = truncator.truncate_by_dropping_oldest_turns(msgs, drop_turns=10)
+        roles = [m.role for m in result]
+        assert "user" in roles
+
+    def test_normal_multi_user_not_affected(self):
+        """正常多 user 对话不受影响。"""
+        truncator = ContextTruncator()
+        msgs = self.create_messages(20, include_system=True)
+        result_before = truncator.truncate_by_dropping_oldest_turns(msgs, drop_turns=2)
+        # 多 user 场景下截断后仍有 user
+        roles = [m.role for m in result_before]
+        assert "user" in roles