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.