What is LangServe?

LangServe is a library that turns any LCEL chain into a production-ready REST API in a few lines of code. It wraps FastAPI and exposes standard endpoints — /invoke, /batch, /stream, and /stream_log — so clients can call your chain over HTTP without any custom FastAPI code.

pip install langserve[all] fastapi uvicorn

from fastapi import FastAPI
from langserve import add_routes
from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

app = FastAPI(title="My LLM API")

chain = (
    ChatPromptTemplate.from_template("Answer: {question}")
    | ChatOpenAI()
    | StrOutputParser()
)

add_routes(app, chain, path="/qa")

# uvicorn server:app --host 0.0.0.0 --port 8000

Once running, the /qa/invoke endpoint accepts POST with {"input": {"question": "..."}}, /qa/stream returns an SSE stream, and /qa/playground serves an interactive browser UI. LangServe also generates an OpenAPI schema at /docs automatically.

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