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feat(core): support custom message separator in get_buffer_string() #34569

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Merged
ccurme merged 1 commit into from
Jan 7, 2026
Merged

feat(core): support custom message separator in get_buffer_string() #34569

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8 changes: 6 additions & 2 deletions libs/core/langchain_core/messages/utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -99,14 +99,18 @@ def _get_type(v: Any) -> str:


def get_buffer_string(
messages: Sequence[BaseMessage], human_prefix: str = "Human", ai_prefix: str = "AI"
messages: Sequence[BaseMessage],
human_prefix: str = "Human",
ai_prefix: str = "AI",
message_separator: str = "\n",
) -> str:
r"""Convert a sequence of messages to strings and concatenate them into one string.

Args:
messages: Messages to be converted to strings.
human_prefix: The prefix to prepend to contents of `HumanMessage`s.
ai_prefix: The prefix to prepend to contents of `AIMessage`.
message_separator: The separator to use between messages.

Returns:
A single string concatenation of all input messages.
Expand Down Expand Up @@ -157,7 +161,7 @@ def get_buffer_string(
message += f"{m.additional_kwargs['function_call']}"
string_messages.append(message)

return "\n".join(string_messages)
return message_separator.join(string_messages)


def _message_from_dict(message: dict) -> BaseMessage:
Expand Down
8 changes: 8 additions & 0 deletions libs/core/tests/unit_tests/test_messages.py
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Original file line number Diff line number Diff line change
Expand Up @@ -370,6 +370,14 @@ def test_multiple_msg(self) -> None:

assert get_buffer_string(msgs) == expected_output

def test_custom_message_separator(self) -> None:
msgs = [
self._HUMAN_MSG,
self._AI_MSG,
]
expected_output = "Human: human\n\nAI: ai"
assert get_buffer_string(msgs, message_separator="\n\n") == expected_output


def test_multiple_msg() -> None:
human_msg = HumanMessage(content="human", additional_kwargs={"key": "value"})
Expand Down