MessagesPlaceholder in "Create New Prompt" panel

[NIKsun]

Describe the feature or potential improvement

Hi, I've bumped into a little lack of functionality when trying to use Langfuse UI for prompt management. 

Issue

I want to store a prompt that could be described in Langchain terms like:

prompt = ChatPromptTemplate.from_messages([
    ("system", "You are a helpful assistant."),
    ("placeholder", "{chat_history}"),
    ("user", "{input}"),
    ("placeholder", "{agent_scratchpad}"),
])

But I can't choose a placeholder "role" in the creation prompt panel:

Potential solution

Extend the role selector with the "placeholder" option.

Additional information

No response

[marcklingen]

thanks for raising this. This is a mostly langchain specific change, we could generalize it to {chat_history} across our prompt management integrations. Currently you can achieve this by manually adding the history within the returned chat message array at runtime.

[mrdrprofuroboros]

Actually there's no validation, so creating such placeholder prompt from langfuse API works
but once you try to edit it in the UI it just shows unusable and blank

btw placeholder is a great feature, it is not only for "chat_history", you can have multiple placeholders in langchain and it is really convenient and way cleaner than adding in runtime

LangSmith supports placeholders :)

[marcklingen]

thanks for sharing!

[ericfeunekes]

I would suggest something like a named placeholder in the prompt template, which you could put anywhere. Then in the prompt compilation you would pass <some_name>=[<list of messages] and the list of messages would be expanded in place. You could assume it is always a list of messages since a single message is served by existing functionality.

The named placeholder would allow for custom functionality like message history and likely other things that I can't think of right now.

[marcklingen]

thanks for sharing! agree on your take of how to implement this

[debkanchan]

+1 I need this. Currently the workaround I use if using prompt template for system message only then inalying inside ChatPromptTemplate.fromMessages like this:

ChatPromptTemplate.fromMessages([
  SystemMessagePromptTemplate.fromTemplate(prompt.getLangchainPrompt()),
  new MessagesPlaceholder("messages"),
])

One catch here is that (that's also implicitly mentioned in the original post) is that the placeholder is able to take any arbitrary key and not just messages key as in langgraph it is common to inlay multiple chat histories using different keys like this:

ChatPromptTemplate.fromMessages([
  SystemMessagePromptTemplate.fromTemplate(prompt.getLangchainPrompt()),
  new MessagesPlaceholder("messages"),
  new MessagesPlaceholder("agent_scratchpad")
])