LangChain Deploy Integration (Python)

Overview

An engineer ships a working LangGraph agent to Vercel. Every non-trivial request returns FUNCTION_INVOCATION_TIMEOUT. The Python runtime on Vercel defaults to a 10-second cap (P35) — a three-tool agent with one RAG round easily runs 20-40s. Local dev never exposed the wall because uvicorn on a laptop has no timeout. Two fixes apply together and each is load-bearing:

// vercel.json — the baseline cap bump (Pro plan max is 60s, Enterprise 900s)
{ "functions": { "api/chat.py": { "maxDuration": 60 } } }

# app/api/chat.py — stream the response so partial output arrives before the cap
from fastapi.responses import StreamingResponse