from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser
from langchain_core.runnables import RunnableParallel, RunnablePassthrough

prompt = ChatPromptTemplate.from_template(
    "Answer using ONLY this context.\n\n{context}\n\nQuestion: {question}"
)

chain = (
    RunnableParallel(
        context=retriever | (lambda docs: "\n\n".join(d.page_content for d in docs)),
        question=RunnablePassthrough(),      # keep the original question
    )
    | prompt
    | model
    | StrOutputParser()
)

for chunk in chain.stream("What changed in the refund policy?"):
    print(chunk, end="", flush=True)