diff --git a/pytests/test_maisaka_timing_gate.py b/pytests/test_maisaka_timing_gate.py
index a2f79b8a..ec7eaf98 100644
--- a/pytests/test_maisaka_timing_gate.py
+++ b/pytests/test_maisaka_timing_gate.py
@@ -238,3 +238,47 @@ def test_finish_tool_removes_empty_assistant_history_message() -> None:
     )
 
     assert runtime._chat_history == []
+
+
+def test_timing_gate_head_trim_keeps_short_history() -> None:
+    messages = [
+        AssistantMessage(content="第一条消息", timestamp=datetime.now()),
+        AssistantMessage(content="第二条消息", timestamp=datetime.now()),
+    ]
+
+    trimmed_messages = MaisakaHeartFlowChatting._drop_head_context_messages(
+        messages,
+        drop_context_count=3,
+    )
+
+    assert trimmed_messages == messages
+
+
+def test_timing_gate_head_trim_keeps_history_within_config_limit() -> None:
+    messages = [
+        AssistantMessage(content=f"消息 {index}", timestamp=datetime.now())
+        for index in range(10)
+    ]
+
+    trimmed_messages = MaisakaHeartFlowChatting._drop_head_context_messages(
+        messages,
+        drop_context_count=7,
+        trim_threshold_context_count=10,
+    )
+
+    assert trimmed_messages == messages
+
+
+def test_timing_gate_head_trim_applies_after_config_limit_exceeded() -> None:
+    messages = [
+        AssistantMessage(content=f"消息 {index}", timestamp=datetime.now())
+        for index in range(11)
+    ]
+
+    trimmed_messages = MaisakaHeartFlowChatting._drop_head_context_messages(
+        messages,
+        drop_context_count=7,
+        trim_threshold_context_count=10,
+    )
+
+    assert trimmed_messages == messages[7:]
diff --git a/src/maisaka/runtime.py b/src/maisaka/runtime.py
index 77eed427..e3ccf739 100644
--- a/src/maisaka/runtime.py
+++ b/src/maisaka/runtime.py
@@ -614,6 +614,7 @@ class MaisakaHeartFlowChatting:
         sub_agent_history = self._drop_head_context_messages(
             selected_history,
             drop_head_context_count,
+            trim_threshold_context_count=context_message_limit,
         )
         if extra_messages:
             sub_agent_history.extend(list(extra_messages))
@@ -637,12 +638,21 @@ class MaisakaHeartFlowChatting:
     def _drop_head_context_messages(
         chat_history: Sequence[LLMContextMessage],
         drop_context_count: int,
+        *,
+        trim_threshold_context_count: int | None = None,
     ) -> list[LLMContextMessage]:
         """从已选上下文头部丢弃指定数量的普通上下文消息。"""
 
         if drop_context_count <= 0:
             return list(chat_history)
 
+        context_message_count = sum(1 for message in chat_history if message.count_in_context)
+        if trim_threshold_context_count is not None and context_message_count <= trim_threshold_context_count:
+            return list(chat_history)
+
+        if context_message_count <= drop_context_count:
+            return list(chat_history)
+
         first_kept_index = 0
         dropped_context_count = 0
         while (