bug(sdk-python): get_langchain_prompt drops the closing brace of nested JSON #18131

Describe the bug

get_langchain_prompt() drops a closing brace when a prompt contains compact nested JSON, so LangChain renders malformed JSON. This is the Python counterpart of #18027 (reported for the JS SDK).

Prompt text: Reply with JSON like {"user": {"name": "{{name}}"}}

get_langchain_prompt() returns Reply with JSON like {{"user": {{"name": "{name}"}} (two escaped opening braces, one escaped closing pair). PromptTemplate.from_template(...).format(name="Ann") renders Reply with JSON like {"user": {"name": "Ann"}, which is missing the final }. No error is raised. Each additional }} run in deeper nesting loses one more brace. Chat prompts are affected the same way, since ChatPromptClient.get_langchain_prompt() uses the same helper. Multi-line JSON, where closing braces are not adjacent, is not affected.

Cause: BasePromptClient._escape_json_for_langchain in langfuse/model.py treats every }} as an already-escaped pair and passes it through unchanged, even when the first } closes a JSON brace that the function doubled earlier.

To reproduce (no server needed)

from langchain_core.prompts import PromptTemplate
from langfuse.api import Prompt_Text
from langfuse.model import TextPromptClient

p = TextPromptClient(Prompt_Text(type="text", name="t", version=1, config={}, tags=[], labels=[],
                                 prompt='Example: {"a": {"b": 1}}'))
print(p.get_langchain_prompt())                                         # Example: {{"a": {{"b": 1}}
print(PromptTemplate.from_template(p.get_langchain_prompt()).format())  # Example: {"a": {"b": 1}

SDK and integration versions

langfuse-python main (4.16.0, commit 618e4702), langchain-core 1.2.22, Python 3.12. Client-side only, reproduced offline.

Are you interested in contributing a fix?

Yes, I have a small fix with regression tests ready for langfuse-python.

Disclosure: found, reproduced and the draft fix tested locally with the help of an AI coding agent (Claude Code); reviewed before filing.