ModuleNotFoundError: No module named 'langchain_openai' (or any provider package)

No module named 'langchain_openai'

You installed langchain but not the provider package. Each provider ships separately:

pip install -U langchain-openai
pip install -U langchain-anthropic

Why it happens

• pip install langchain gives you agents, messages, tools, chat models, embeddings. It deliberately does not pull in any provider SDK.
• The same applies to vector stores: langchain-chroma, langchain-pinecone, langchain-qdrant, langchain-weaviate, langchain-mongodb, langchain-elasticsearch, langchain-redis are all separate installs.
• A stale lockfile or a fresh virtualenv is the usual trigger: the code was written on a machine that had the provider package, the new environment does not.

Fix

1. Confirm: pip show langchain-openai (or the provider you need).
2. Install it: pip install -U langchain-openai.
3. If the project is uv-managed, use uv add langchain-openai instead of mixing pip and uv.

Do not reinstall langchain itself; that changes nothing. The missing piece is always the provider package.

Error catalog entry grounded in the official install and providers docs, verified 2026-09-26. Install: https://docs.langchain.com/oss/python/langchain/install ; providers: https://docs.langchain.com/oss/python/integrations/providers/overview