Vercel seems to have abandoned or deprioritized their serverless functions offering (outside Next.js) because I couldn’t find any official documentation for setting up function streaming with CORS enabled.
Eventually this ended up being quite a hassle to figure out, so I decided to make this snippet for anyone who needs it.
import { openai } from "@ai-sdk/openai"; import { streamText } from "ai"; export function OPTIONS() { return new Response("OK", { status: 200, headers: { "Access-Control-Allow-Credentials": "true", "Access-Control-Allow-Origin": "*", "Access-Control-Allow-Methods": "GET,OPTIONS,PATCH,DELETE,POST,PUT", "Access-Control-Allow-Headers": "X-CSRF-Token, X-Requested-With, Accept, Accept-Version, Content-Length, Content-MD5, Content-Type, Date, X-Api-Version", }, }); } export async function GET() { const response = streamText({ model: openai("gpt-4o-mini"), messages: [{ role: "user", content: "Write a short poem" }], }); return response.toTextStreamResponse({ headers: { "Content-Type": "text/event-stream", "Access-Control-Allow-Credentials": "true", "Access-Control-Allow-Origin": "*", "Access-Control-Allow-Methods": "GET,OPTIONS,PATCH,DELETE,POST,PUT", "Access-Control-Allow-Headers": "X-CSRF-Token, X-Requested-With, Accept, Accept-Version, Content-Length, Content-MD5, Content-Type, Date, X-Api-Version", }, }); } 
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