core: Fix IndexError when trim_messages invoked with empty list (#26896)

This prevents `trim_messages` from raising an `IndexError` when invoked
with `include_system=True`, `strategy="last"`, and an empty message
list.

Fixes #26895

Dependencies: none

libs/core/langchain_core/messages/utils.py

@@ -819,6 +819,7 @@ def trim_messages(
 
             def list_token_counter(messages: Sequence[BaseMessage]) -> int:
                 return sum(token_counter(msg) for msg in messages)  # type: ignore[arg-type, misc]
+
         else:
             list_token_counter = token_counter  # type: ignore[assignment]
     else:
@@ -956,6 +957,8 @@ def _last_max_tokens(
     ] = None,
 ) -> list[BaseMessage]:
     messages = list(messages)
+    if len(messages) == 0:
+        return []
     if end_on:
         while messages and not _is_message_type(messages[-1], end_on):
             messages.pop()

libs/core/tests/unit_tests/messages/test_utils.py

@@ -332,6 +332,19 @@ def test_trim_messages_allow_partial_text_splitter() -> None:
     assert _MESSAGES_TO_TRIM == _MESSAGES_TO_TRIM_COPY
 
 
+def test_trim_messages_include_system_strategy_last_empty_messages() -> None:
+    expected: list[BaseMessage] = []
+
+    actual = trim_messages(
+        max_tokens=10,
+        token_counter=dummy_token_counter,
+        strategy="last",
+        include_system=True,
+    ).invoke([])
+
+    assert actual == expected
+
+
 def test_trim_messages_invoke() -> None:
     actual = trim_messages(max_tokens=10, token_counter=dummy_token_counter).invoke(
         _MESSAGES_TO_TRIM