fix: ensure tool call/response pairing in context truncation (#5417)

* fix: ensure tool call/response pairing in context truncation

* refactor: simplify fix_messages to single-pass state machine

Author: Luna-channel

astrbot/core/agent/context/truncator.py

@@ -4,19 +4,60 @@
 class ContextTruncator:
     """Context truncator."""
 
+    def _has_tool_calls(self, message: Message) -> bool:
+        """Check if a message contains tool calls."""
+        return (
+            message.role == "assistant"
+            and message.tool_calls is not None
+            and len(message.tool_calls) > 0
+        )
+
     def fix_messages(self, messages: list[Message]) -> list[Message]:
-        fixed_messages = []
-        for message in messages:
-            if message.role == "tool":
-                # tool block 前面必须要有 user 和 assistant block
-                if len(fixed_messages) < 2:
-                    # 这种情况可能是上下文被截断导致的
-                    # 我们直接将之前的上下文都清空
-                    fixed_messages = []
-                else:
-                    fixed_messages.append(message)
-            else:
-                fixed_messages.append(message)
+        """修复消息列表，确保 tool call 和 tool response 的配对关系有效。
+
+        此方法确保：
+        1. 每个 `tool` 消息前面都有一个包含 tool_calls 的 `assistant` 消息
+        2. 每个包含 tool_calls 的 `assistant` 消息后面都有对应的 `tool` 响应
+
+        这是 OpenAI Chat Completions API 规范的要求（Gemini 对此执行严格检查）。
+        """
+        if not messages:
+            return messages
+
+        fixed_messages: list[Message] = []
+        pending_assistant: Message | None = None
+        pending_tools: list[Message] = []
+
+        def flush_pending_if_valid() -> None:
+            nonlocal pending_assistant, pending_tools
+            if pending_assistant is not None and pending_tools:
+                fixed_messages.append(pending_assistant)
+                fixed_messages.extend(pending_tools)
+            pending_assistant = None
+            pending_tools = []
+
+        for msg in messages:
+            if msg.role == "tool":
+                # 只有在有挂起的 assistant(tool_calls) 时才记录 tool 响应
+                if pending_assistant is not None:
+                    pending_tools.append(msg)
+                # else: 孤立的 tool 消息，直接忽略
+                continue
+
+            if self._has_tool_calls(msg):
+                # 遇到新的 assistant(tool_calls) 前，先处理旧的 pending 链
+                flush_pending_if_valid()
+                pending_assistant = msg
+                continue
+
+            # 非 tool，且不含 tool_calls 的消息
+            # 先结束任何 pending 链，再正常追加
+            flush_pending_if_valid()
+            fixed_messages.append(msg)
+
+        # 结束时处理最后一个 pending 链
+        flush_pending_if_valid()
+
         return fixed_messages
 
     def truncate_by_turns(