A/B testing prompt variations in LangChain

Introduction

A/B testing prompt variations helps you find which prompt works best by comparing different versions.

You want to see which prompt gets better answers from the AI.

You are unsure how to phrase a question for best results.

You want to improve user experience by testing different prompt styles.

You want to compare performance of multiple prompt ideas quickly.

Syntax

LangChain

from langchain.prompts import PromptTemplate
from langchain.chains import LLMChain

prompt_a = PromptTemplate(input_variables=["input"], template="Tell me a joke about {input}.")
prompt_b = PromptTemplate(input_variables=["input"], template="Make a funny story about {input}.")

chain_a = LLMChain(llm=llm, prompt=prompt_a)
chain_b = LLMChain(llm=llm, prompt=prompt_b)

response_a = chain_a.run(input="cats")
response_b = chain_b.run(input="cats")

Use different PromptTemplate objects for each variation.

Run each prompt through the LLMChain separately to compare outputs.

Examples

Two prompt variations for poetry and story writing.

LangChain

prompt1 = PromptTemplate(input_variables=["topic"], template="Write a poem about {topic}.")
prompt2 = PromptTemplate(input_variables=["topic"], template="Write a short story about {topic}.")

Run both prompts with the same input to compare results.

LangChain

chain1 = LLMChain(llm=llm, prompt=prompt1)
chain2 = LLMChain(llm=llm, prompt=prompt2)

result1 = chain1.run(topic="rain")
result2 = chain2.run(topic="rain")

Sample Program

This example creates two prompt versions about dogs. It runs both and prints their outputs to compare which is funnier or better.

LangChain

from langchain.llms import OpenAI
from langchain.prompts import PromptTemplate
from langchain.chains import LLMChain

llm = OpenAI(temperature=0.7)

prompt_a = PromptTemplate(input_variables=["input"], template="Tell me a joke about {input}.")
prompt_b = PromptTemplate(input_variables=["input"], template="Make a funny story about {input}.")

chain_a = LLMChain(llm=llm, prompt=prompt_a)
chain_b = LLMChain(llm=llm, prompt=prompt_b)

response_a = chain_a.run(input="dogs")
response_b = chain_b.run(input="dogs")

print("Prompt A response:", response_a)
print("Prompt B response:", response_b)

Important Notes

Keep prompt variations simple and focused on one change at a time.

Use the same input for fair comparison.

Check outputs carefully to decide which prompt works best.

Summary

A/B testing helps find the best prompt by comparing different versions.

Create separate PromptTemplate objects for each variation.

Run each prompt with the same input and compare results.