Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Package (Required)

• langchain

Related Issues / PRs

#33035

Reproduction Steps / Example Code (Python)

from langchain.chains import LLMChain
from langchain_core.prompts import PromptTemplate
from langchain_community.llms.fake import FakeListLLM

llm = FakeListLLM(responses=["hello"])
prompt = PromptTemplate(input_variables=["input"], template="{input}")
chain = LLMChain(llm=llm, prompt=prompt)
chain.save("/tmp/test_chain.yaml")  # Works on 0.3.27, fails on 0.3.28

Error Message and Stack Trace

Traceback (most recent call last):
  File "<string>", line 9, in <module>
  File "langchain/chains/base.py", line 785, in save
    raise NotImplementedError(msg)
NotImplementedError: Chain verbose=False prompt=PromptTemplate(input_variables=['input'], input_types={}, partial_variables={}, template='{input}') llm=FakeListLLM(responses=['hello']) output_parser=StrOutputParser() llm_kwargs={} does not support saving.

Description

• fix: update method calls from dict to model_dump in Chain #33035 changed Chain.save() from self.dict() to self.model_dump().
• dict() injects _type via _chain_type; model_dump() inherits from Pydantic BaseModel and never includes it.
• save() now always hits "_type" not in chain_dict and raises NotImplementedError.
• This is a regression from 0.3.27 -> 0.3.28.

System Info

> langchain_core: 0.3.83
> langchain: 0.3.28
> langchain_community: 0.3.31
> Python Version: 3.10.17
> OS: Darwin