# 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

```python
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.