Diagnose: ModuleNotFoundError: No module named 'langchain_openai'

Symptom

ModuleNotFoundError: No module named 'langchain_openai'

(Or langchain_anthropic, langchain_chroma, any langchain_* provider package.)

Confirm

Run pip show langchain-openai. If it prints nothing, the package is genuinely absent. Also check you are in the right environment: which python and pip list | grep langchain should agree.

Cause

LangChain splits every provider into its own package. pip install langchain never includes them. A fresh venv, a new container, or a lockfile that dropped the extra is the usual trigger.

Fix

pip install -U langchain-openai

uv-managed project? Use uv add langchain-openai. Do not mix pip and uv.

Verify

from langchain_openai import ChatOpenAI
print("ok")

If the import works, done. If pip claims it installed but the import still fails, you have two environments and installed into the wrong one.

Diagnostic procedure grounded in the official install docs, verified 2026-09-26. Install: https://docs.langchain.com/oss/python/langchain/install