From a59b2f04a66df2f027787c35135e945665a0b608 Mon Sep 17 00:00:00 2001
From: Yufeng He <40085740+he-yufeng@users.noreply.github.com>
Date: Thu, 18 Jun 2026 15:29:24 +0800
Subject: [PATCH] fix(truncator): keep an all-system context in the system half

_split_system_rest initialized first_non_system to 0, so when every message is
a system message the loop never breaks and the split is reversed: the system
messages land in the non-system half. truncate_by_halving then treats them as
droppable turns and discards half of them (e.g. 4 system messages -> 2). System
messages must never be dropped. Default first_non_system to len(messages) so an
all-system list stays entirely in the system half; mixed and empty inputs are
unchanged.
---
 astrbot/core/agent/context/truncator.py |  4 +++-
 tests/agent/test_truncator.py           | 23 +++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/astrbot/core/agent/context/truncator.py b/astrbot/core/agent/context/truncator.py
index 9abf574336..e742fb7d18 100644
--- a/astrbot/core/agent/context/truncator.py
+++ b/astrbot/core/agent/context/truncator.py
@@ -21,7 +21,9 @@ def _split_system_rest(
         Returns:
             tuple: (system_messages, non_system_messages)
         """
-        first_non_system = 0
+        # Default to the end so an all-system list lands entirely in the system
+        # half; otherwise the loop never breaks and the split would be reversed.
+        first_non_system = len(messages)
         for i, msg in enumerate(messages):
             if msg.role != "system":
                 first_non_system = i
diff --git a/tests/agent/test_truncator.py b/tests/agent/test_truncator.py
index 7dac80f9ce..bbcc9db706 100644
--- a/tests/agent/test_truncator.py
+++ b/tests/agent/test_truncator.py
@@ -376,6 +376,29 @@ def test_all_system_messages(self):
         if len(result) > 0:
             assert all(msg.role == "system" for msg in result)
 
+    def test_split_system_rest_with_only_system_messages(self):
+        """All-system input must land entirely in the system half, not the rest."""
+        truncator = ContextTruncator()
+        messages = [
+            self.create_message("system", "System 1"),
+            self.create_message("system", "System 2"),
+        ]
+
+        system_messages, non_system_messages = truncator._split_system_rest(messages)
+
+        assert len(system_messages) == 2
+        assert non_system_messages == []
+
+    def test_halving_keeps_all_system_messages(self):
+        """Halving must never drop system messages, even with no turns present."""
+        truncator = ContextTruncator()
+        messages = [self.create_message("system", f"System {i}") for i in range(4)]
+
+        result = truncator.truncate_by_halving(messages)
+
+        assert len(result) == 4
+        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]: