trim_messages causes an IndexError when invoked with strategy="last", include_system=True, and empty messages

[sudosays]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangChain documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

Example Code

from langchain_core.messages import trim_messages

trim_messages(max_tokens=10, token_counter=lambda x: 0, include_system=True).invoke([])

Error Message and Stack Trace (if applicable)

Error type: IndexError

Stack trace:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/runnables/base.py", line 4706, in invoke
    return self._call_with_config(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/runnables/base.py", line 1923, in _call_with_config
    context.run(
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/runnables/config.py", line 396, in call_func_with_variable_args
    return func(input, **kwargs)  # type: ignore[call-arg]
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/runnables/base.py", line 4562, in _invoke
    output = call_func_with_variable_args(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/runnables/config.py", line 396, in call_func_with_variable_args
    return func(input, **kwargs)  # type: ignore[call-arg]
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/messages/utils.py", line 853, in trim_messages
    return _last_max_tokens(
           ^^^^^^^^^^^^^^^^^
  File "/home/julius/harbour/documents/clinigma/casa/.venv/lib/python3.12/site-packages/langchain_core/messages/utils.py", line 962, in _last_max_tokens
    swapped_system = include_system and isinstance(messages[0], SystemMessage)
                                                   ~~~~~~~~^^^
IndexError: list index out of range

Description

What is the issue?

• Trying to use trim_messages with include_system=True in a RunnableWithMessageHistory without a message already in a MessageHistory

Expected behaviour:

• an empty list is returned

Current behaviour:

• Raises an IndexError and crashes

This was found when setting up a RunnableWithMessageHistory where there were no initial messages in the store to pull from.

The documentation currently provides examples where prior messages are available, which could be a pitfall to new users.

Testing

I have tested all other combinations of argument values for:

• strategy
• allow_partials
• include_system
• starts_on (partially with only "human" and "system")
• ends_on (partially with only "human" and "system")

The error is only triggered when strategy="last" and include_system=True

System Info

System Information

OS: Linux
OS Version: #1 SMP PREEMPT_DYNAMIC Thu, 12 Sep 2024 17:21:02 +0000
Python Version: 3.12.6 (main, Sep 8 2024, 13:18:56) [GCC 14.2.1 20240805]

Package Information

langchain_core: 0.3.6
langchain: 0.3.1
langchain_community: 0.3.1
langsmith: 0.1.128
langchain_openai: 0.2.0
langchain_text_splitters: 0.3.0

Optional packages not installed

langgraph
langserve

Other Dependencies

aiohttp: 3.10.6
async-timeout: Installed. No version info available.
dataclasses-json: 0.6.7
httpx: 0.27.2
jsonpatch: 1.33
numpy: 1.26.4
openai: 1.48.0
orjson: 3.10.7
packaging: 24.1
pydantic: 2.9.2
pydantic-settings: 2.5.2
PyYAML: 6.0.2
requests: 2.32.3
SQLAlchemy: 2.0.35
tenacity: 8.5.0
tiktoken: 0.7.0
typing-extensions: 4.12.2