ChatPromptTemplate.format_messages does not handle f-strings inherited from BaseMessage, such as SystemMessage, etc

[TsinlingLi]

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 HumanMessage, SystemMessage, AIMessage
from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder

ChatPromptTemplate.from_messages([
SystemMessage("You are a helpful assistant. Answer all questions to the best of your ability in {language}"),
MessagesPlaceholder(variable_name="messages")
]).invoke({"language": "English", "messages": [HumanMessage("hello")]})

Error Message and Stack Trace (if applicable)

Unexpected results

Description

result，language Not replaced:
ChatPromptValue(messages=[SystemMessage(content='You are a helpful assistant. Answer all questions to the best of your ability in {language}', additional_kwargs={}, response_metadata={}), HumanMessage(content='hello', additional_kwargs={}, response_metadata={})])

because，No processing of objects integrated from BaseMessage

class ChatPromptTemplate(ChatPromptTemplate):
def format_messages(self, **kwargs: Any) -> list[BaseMessage]:
kwargs = self._merge_partial_and_user_variables(**kwargs)
result = []
for message_template in self.messages:
if isinstance(message_template, BaseMessage):
# No processing of objects integrated from BaseMessage
result.extend([message_template])
elif isinstance(
message_template, (BaseMessagePromptTemplate, BaseChatPromptTemplate)
):
message = message_template.format_messages(**kwargs)
result.extend(message)
else:
msg = f"Unexpected input: {message_template}"
raise ValueError(msg)
return result

System Info

python==3.12.7
langchain==0.3.14

[sirine707]

Hi there ,To fix this, you need to modify the format_messages method to process f-strings here's the updated code let me know if iam not correct !

class ChatPromptTemplate(ChatPromptTemplate):
    def format_messages(self, **kwargs: Any) -> list[BaseMessage]:
        kwargs = self._merge_partial_and_user_variables(**kwargs)
        result = []
        for message_template in self.messages:
            if isinstance(message_template, BaseMessage):
                # Process f-strings in BaseMessage objects
                formatted_content = message_template.content.format(**kwargs)
                message_template.content = formatted_content
                result.extend([message_template])
            elif isinstance(
                    message_template, (BaseMessagePromptTemplate, BaseChatPromptTemplate)
            ):
                message = message_template.format_messages(**kwargs)
                result.extend(message)
            else:
                msg = f"Unexpected input: {message_template}"
                raise ValueError(msg)
        return result