# integration-langchain (Langchain Python)

You can run this example with:

```bash
npx promptfoo@latest init --example integration-langchain
cd integration-langchain
```

## Usage

This example shows how to run a Python LangChain Expression Language (LCEL) chain with Promptfoo. It compares GPT-5.4 with a math-focused LangChain prompt-and-output-parser pipeline using `ChatOpenAI(model="gpt-4.1-mini")` and the Chat Completions API.

This example requires Python 3.10 or newer. Create and activate a virtual environment, then
install the requirements:

```sh
python3 -m venv venv
source venv/bin/activate
python -m pip install -r requirements.txt
```

Set the OpenAI API key used by both providers:

```sh
export OPENAI_API_KEY=your-api-key
```

Then run the eval:

```bash
npx promptfoo@latest eval --no-cache
```

Keep the virtual environment active when running Promptfoo: the `exec:python` provider uses `python` from your `PATH`. The requirements list only the two LangChain packages imported by this example; the OpenAI SDK is installed by `langchain-openai`.