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.