Suppress "Placeholders have not been resolved" warning when MessagesPlaceholder use is intentional (LangChain integration)

[s0ftnote]

Summary

ChatPromptClient.compile() (and get_langchain_prompt() which calls it
under the hood) unconditionally logs:

Placeholders ['...'] have not been resolved. Pass them as keyword arguments to compile().

even when leaving placeholders unresolved is intentional — i.e. the
caller follows the officially documented LangChain integration pattern
where unresolved placeholders are deliberately returned as
MessagesPlaceholder for LangChain's ChatPromptTemplate to resolve at
.invoke() time.

Result: production logs are flooded with one WARNING per request, masking
real signals.

Reproduction (langfuse 4.6.1, official docs example)

from langfuse import get_client
from langchain_core.prompts import ChatPromptTemplate

langfuse = get_client()

# Create a chat prompt with a placeholder (per docs example)
langfuse.create_prompt(
    name="movie-critic-chat",
    type="chat",
    prompt=[
        {"role": "system", "content": "You are an expert movie critic"},
        {"type": "placeholder", "name": "chat_history"},
        {"role": "user", "content": "What should I watch next?"},
    ],
    labels=["production"],
)

# Documented LangChain integration:
# https://langfuse.com/docs/prompt-management/features/message-placeholders
prompt = langfuse.get_prompt("movie-critic-chat")
langchain_prompt = ChatPromptTemplate.from_template(prompt.get_langchain_prompt())

# → logs WARNING: Placeholders ['chat_history'] have not been resolved.
#   even though leaving it unresolved is EXACTLY what the docs tell you
#   to do for LangChain integration.

Why this is a problem

1. The docs at /docs/prompt-management/features/message-placeholders
   (LangChain Python tab) endorse this exact pattern — MessagesPlaceholder
   objects in the output are the documented goal.
2. SDK can't distinguish "intentional unresolved (LangChain hand-off)" from
   "misuse (forgot kwargs)" and warns in both cases.
3. No public API to silence: no silent= / suppress_warnings= kwarg on
   compile() or get_langchain_prompt().
4. logging.getLogger("langfuse").setLevel(...) is not a reliable
   workaround — see bug: changing langfuse log level in python doesn't affect the logger langfuse#4976 reporting setLevel doesn't
   affect certain SDK-emitted warnings.
5. Forces every LangChain integrator to install a logging.Filter that
   string-matches the warning message — brittle (breaks on copy edits)
   and easy to over-silence.

Suggested fix

Add a silent=True (or suppress_unresolved_warning=True) kwarg to:

• ChatPromptClient.compile(**kwargs, silent: bool = False)
• ChatPromptClient.get_langchain_prompt(**kwargs, silent: bool = False)

When silent=True, the
"Placeholders ['...'] have not been resolved" warning is skipped, but
other validation warnings (e.g. "Placeholder X must contain a list of
chat messages") still fire — those are genuine misuse signals.

Alternatively, get_langchain_prompt() could pass silent=True to its
internal compile() call by default, since by definition any unresolved
placeholder returned from this method is intended for LangChain.

Version info

• langfuse-python: 4.6.1 (also reproduces on 4.5.1 + main HEAD)
• langchain-core: 1.3.2
• Python: 3.11

Current workaround we use

contextlib.contextmanager that scopes a logging.Filter matching the
substring "have not been resolved" over the
chat_skeleton.get_langchain_prompt() call site. Fragile (depends on
warning string), but scoped narrowly so other langfuse warnings still
propagate.

Happy to send a PR if the maintainers agree with the kwarg shape.

[hassiebp]

Thanks for raising this - happy to review a PR 👍