bug: ChatPromptClient.compile appends str(whole list) once per non-dict placeholder item #1971

Describe the bug

In ChatPromptClient.compile, when a placeholder value is a list and an element is not a dict, the code appends ChatMessageDict(role="NOT_GIVEN", content=str(placeholder_value)). This is the inner else of the per-item loop, at langfuse/model.py line 365 (lines 362-368).

placeholder_value is the whole list, not the current item. N non-dict items therefore produce N identical messages, each containing the full list, and no message contains its own element. The input is already invalid (the warning says "Appended as string"), but the output is duplicated, garbled content.

I would expect one message per item, for example with content=str(msg), though maintainers may prefer a different handling of invalid items (skipping them, raising, etc.). The point is that the current output repeats the whole list per item.

Steps to reproduce

from langfuse.model import ChatPromptClient
from langfuse.api import Prompt_Chat
c = ChatPromptClient(Prompt_Chat(name="p", version=1, type="chat", config={}, labels=[], tags=[], commit_message=None,
    prompt=[{"type": "chatmessage", "role": "system", "content": "hi {{x}}"}, {"type": "placeholder", "name": "history"}]))
print(c.compile(x="1", history=["a", "b"]))

Observed:

[{'role': 'system', 'content': 'hi 1'}, {'role': 'NOT_GIVEN', 'content': "['a', 'b']"}, {'role': 'NOT_GIVEN', 'content': "['a', 'b']"}]

The last two messages each contain the whole list ['a', 'b'] instead of 'a' and 'b' respectively.

Langfuse Cloud or self-hosted?

Langfuse Cloud (not server dependent; the behavior is purely client-side in the SDK)

If self-hosted, what version are you running?

N/A

SDK and integration versions

langfuse 4.17.0 (main at bf11ec1), Python 3.14.0

Additional information

Related open PR #1840 (LangChain message objects in placeholders) touches the same loop; as far as its diff shows it keeps the str(placeholder_value) fallback for other non-dict items.

Are you interested in contributing a fix for this bug?

No