What does this Integration Langchain example do?

It demonstrates how to run a Python LangChain Expression Language (LCEL) chain with promptfoo, comparing GPT-5 against a math-focused LangChain prompt-and-output-parser pipeline.

What are the requirements to use this example?

You need Python 3.10 or higher, an OPENAI_API_KEY environment variable set, and to install dependencies via `pip install -r requirements.txt`.

When should I use this example?

Use it when you need to compare a direct LLM call against a structured LangChain pipeline on the same task, or as a template for integrating a Python LangChain chain into promptfoo as a provider.

How do I get started with this example?

Run `npx promptfoo@latest init --example integration-langchain`, then navigate to the created directory and execute `npx promptfoo eval` with your `promptfooconfig.yaml` file.

Prompt Chain

Test LangChain pipelines against multiple LLM providers

Name: Integration Langchain
Availability: OnlineOnly
Author: Promptfoo

Promptfoo example that runs a Python LangChain LCEL chain and compares it against GPT-5.4 on a math task.

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Works with langchainopenaipython

Promptfoo

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Spark score

out of 100

Updated 10 days ago

Source checked Sep 10, 2026

Version 0.123.0

Models

gpt 4o

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Why it matters

Evaluate and compare LangChain Expression Language (LCEL) chains against different language models to ensure consistent performance and validate prompt-and-output-parser pipelines before deployment.

Outcomes

What it gets done

Run Python LangChain LCEL chains with Promptfoo evaluation framework

Compare GPT model performance against custom LangChain pipelines

Execute automated tests on math-focused prompt chains with output parsers

Validate LangChain integration behavior across different provider configurations

Install

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Run in your project directory:

curl -fsSL https://spark.entire.vc/get/pfoo-integration-langchain | bash

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Steps

Steps in the chain

Initialize Promptfoo with Langchain example

Set up Python virtual environment

Install Python dependencies

Configure OpenAI API key

Run the evaluation

Overview

Integration Langchain

This promptfoo example runs a Python LangChain LCEL chain, built on ChatOpenAI with gpt-4.1-mini, and compares its output on a math-focused task against GPT-5.4 called directly. Use it as a template for evaluating a Python LangChain LCEL chain with promptfoo; it requires Python 3.10 or newer and its own virtual environment.

What it does

This is a promptfoo example (integration-langchain) that runs a Python LangChain Expression Language (LCEL) chain as a promptfoo provider and compares it against GPT-5.4 directly. The LangChain side is a math-focused prompt-and-output-parser pipeline built on ChatOpenAI(model="gpt-4.1-mini") calling the Chat Completions API, so the eval effectively compares a raw model call to the same task run through a LangChain chain.

When to use - and when NOT to

Use it as a starting template when you need to evaluate a Python LangChain LCEL chain with promptfoo, or specifically compare a LangChain pipeline's output against a direct model call on the same math-oriented task. It requires Python 3.10 or newer and its own virtual environment with requirements.txt installed, so it's not a fit if your provider isn't a Python LangChain chain.

Inputs and outputs

Input: an OPENAI_API_KEY used by both the GPT-5.4 and the LangChain/gpt-4.1-mini providers, plus the example's own LCEL chain and prompt/parser configuration. Output: standard promptfoo eval results comparing the two providers' answers on the same math prompts.

Integrations

promptfoo eval framework (promptfoo eval)
Python LangChain Expression Language (LCEL), via ChatOpenAI(model="gpt-4.1-mini") and the OpenAI Chat Completions API
GPT-5.4 as the comparison provider

Who it's for

Developers building or evaluating a LangChain-based pipeline in Python who want a working promptfoo template for comparing it against a direct model call.

Source README

integration-langchain (Langchain Python)

You can run this example with:

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:

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

Set the OpenAI API key used by both providers:

export OPENAI_API_KEY=your-api-key

Then run the eval:

npx promptfoo eval

FAQ

Common questions

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