# Symptom

```
ImportError: cannot import name 'ChatOpenAI' from 'langchain'
```

## Confirm

Run `python -c "import langchain; print(langchain.__version__)"`. If it is 1.x, the old import path is gone by design, not by accident.

## Cause

LangChain v1 shrank the `langchain` namespace to agents, messages, tools, chat models (init functions), and embeddings. Model classes moved to provider packages. Training data and old tutorials still teach the pre-v1 paths.

## Fix

```python
from langchain_openai import ChatOpenAI  # needs pip install langchain-openai
```

or provider-agnostic:

```python
from langchain.chat_models import init_chat_model
model = init_chat_model("openai:gpt-5.5")
```

## Verify

Import the class and construct it. Then grep the codebase for other stale imports from `langchain` (`langchain.chat_models` classes, `langchain.llms`, `langchain.embeddings` classes); they fail the same way and it is cheaper to fix them all now.