How do I use LangChain with RESTful APIs?

To use LangChain with RESTful APIs, you typically create custom tools or integrations that allow LangChain components like chains or agents to interact with external services. LangChain provides built-in utilities and patterns to simplify connecting language models (LLMs) to APIs, enabling workflows where the LLM can trigger API calls, process responses, and incorporate the data into its output. This involves writing code to handle HTTP requests, parse responses, and integrate the logic into LangChain’s framework.

First, define a custom tool or use LangChain’s APIRequestTool to interact with APIs. For example, if you need to fetch data from a weather API, you might create a tool using Python’s requests library. Here’s a simplified example:

from langchain.tools import tool
import requests

@tool
def get_weather(city: str) -> str:
 """Fetches current weather for a city using a REST API."""
 url = f"https://api.weather.com/v1/{city}/conditions"
 response = requests.get(url)
 return response.json()["weather_description"]

This tool can be added to a LangChain agent, allowing the LLM to decide when to call it based on user input (e.g., “What’s the weather in Tokyo?”). The agent automatically parses the query, invokes the tool, and combines the API response with its own output. For more complex APIs requiring authentication or POST requests, you’d extend this pattern by adding headers, handling tokens, or using LangChain’s BaseModel to validate inputs.

Second, structure chains to sequence API calls and LLM processing. For instance, you might create a chain that first calls an API to retrieve raw data, then uses an LLM to summarize it. Using SimpleSequentialChain:

from langchain.chains import SimpleSequentialChain
from langchain.llms import OpenAI

llm = OpenAI()
chain = SimpleSequentialChain(
 chains=[get_weather_tool_chain, llm_chain_for_summary]
)

Here, get_weather_tool_chain handles the API call, and llm_chain_for_summary processes the result. This approach works well for stateless APIs. For stateful interactions (e.g., multi-step workflows), use agents with memory to track context across API calls.

Finally, handle errors and edge cases. APIs might return incomplete data, time out, or require rate limiting. Wrap API calls in try-except blocks, validate responses with Pydantic, and use LangChain’s built-in retry mechanisms. For example:

from tenacity import retry, stop_after_attempt

@retry(stop=stop_after_attempt(3))
def safe_api_call(url):
 # implementation with error handling

Always test API integrations separately before combining them with LangChain. This ensures reliability and helps debug issues like incorrect parameters or authentication errors. By combining LangChain’s orchestration with robust API client code, you can build reliable, data-aware LLM applications.