How to beginner · 3 min read

How to use prompt template in LangChain

Q: How to use prompt template in LangChain

Use ChatPromptTemplate from langchain_core.prompts to define reusable prompt templates with variables. Fill these templates dynamically by calling format with your input values, then pass the formatted prompt to a LangChain chat model like ChatOpenAI.

Quick answer

Use ChatPromptTemplate from langchain_core.prompts to define reusable prompt templates with variables. Fill these templates dynamically by calling format with your input values, then pass the formatted prompt to a LangChain chat model like ChatOpenAI.

PREREQUISITES

Python 3.8+
OpenAI API key (free tier works)
pip install langchain_openai>=0.2.0 langchain_core>=0.2.0

Setup

Install the required LangChain packages and set your OpenAI API key as an environment variable.

bash

pip install langchain_openai langchain_core

Step by step

Create a ChatPromptTemplate with input variables, format it with values, and send the prompt to a ChatOpenAI model to get a response.

python

import os
from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate

# Initialize the chat model
chat = ChatOpenAI(model_name="gpt-4o", temperature=0, openai_api_key=os.environ["OPENAI_API_KEY"])

# Define a prompt template with variables
prompt_template = ChatPromptTemplate.from_template(
    "Translate the following English text to French: {text}"
)

# Format the prompt with input
formatted_prompt = prompt_template.format(text="Hello, how are you?")

# Send the prompt to the model
response = chat(formatted_prompt)

print(response.content)

output

Bonjour, comment ça va ?

Common variations

Use multiple input variables in the template, e.g., "Summarize the text: {text} in {language}".
Use other chat models like claude-3-5-sonnet-20241022 with Anthropic SDK.
Use async calls with LangChain's async client methods.

python

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
import asyncio

async def async_example():
    chat = ChatOpenAI(model_name="gpt-4o", temperature=0, openai_api_key=os.environ["OPENAI_API_KEY"])
    prompt = ChatPromptTemplate.from_template(
        "Summarize the following text in {language}: {text}"
    )
    formatted = prompt.format(text="LangChain simplifies prompt management.", language="Spanish")
    response = await chat.acall(formatted)
    print(response.content)

asyncio.run(async_example())

output

LangChain simplifica la gestión de prompts.

Troubleshooting

If you get a KeyError on formatting, ensure all variables in the template are provided in format().
If the API key is missing or invalid, set OPENAI_API_KEY correctly in your environment.
For model errors, verify the model name is current and supported.

✅

Key Takeaways

Use ChatPromptTemplate to create reusable, variable-driven prompts.
Always format the prompt with all required variables before sending to the model.
LangChain supports both synchronous and asynchronous usage patterns.
Keep your API keys secure and environment variables properly configured.
Verify model names regularly as they may update over time.

Verified 2026-04 · gpt-4o, claude-3-5-sonnet-20241022

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