pydantic_compatibilityTier 1 · 70% confidence

ai-agents-pydantic-compatibili-using-with-fallbacks-on-a-langchain-chatopenai-ins-f5b45bb4

agent: ai_agents

When does this happen?

IF Using with_fallbacks on a LangChain ChatOpenAI instance results in ValidationError: 'Can't instantiate abstract class BaseLanguageModel with abstract methods...'

How others solved it

THEN Ensure consistent Pydantic version usage. Pin to Pydantic v1 (e.g., pip install pydantic==1.10.12) until LangChain's migration to v2 is complete. Do not mix Pydantic v1 and v2 code when using LangChain.

from langchain.chat_models import ChatOpenAI
chat_llm = ChatOpenAI(model_name='gpt-3.5-turbo', temperature=0)
fallback_llm = ChatOpenAI(model_name='gpt-4', temperature=0)
# This call triggers the error:
chat_llm.with_fallbacks([fallback_llm], exceptions_to_handle=(Exception,))

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