LangChain Expression Language crashes on pydantic v2 inputs

May 15, 2025
langchain expression-language pydantic
1 min read

LangChain Expression Language crashes on pydantic v2 inputs

Hooked a simple chain into LEL:

from langchain_experimental.expression_language import chain
from pydantic import BaseModel

class Input(BaseModel):
    q: str

result = chain("{q}").run(Input(q="hi"))

Traceback:

AttributeError: 'str' object has no attribute 'model_dump'

Python 3.12.1, langchain 0.3.0, expression-language 0.0.5, pydantic 2.8.2. Same code worked with pydantic 1.10.

Cause: LEL still calls model_dump() on BaseModel v1. In v2 the method is model_dump() but returns dict by default only if config set, otherwise raises.

Fix options:

pin pydantic: pip install 'pydantic<2'
or add model_config = {"ser_json_tuples":True} to the class and access .model_dump() explicitly

PR #42 merged but not released; install nightly:

pip install git+https://github.com/langchain-ai/langchain@master#subdirectory=libs/expression_language

Went with the nightly; crash resolved.

If import still fails ensure langchain_experimental updated; older wheels pull expression‑language 0.0.4 which lacks the patch.