vercel.json unfortunately doesn't support Environment variable. You can use Edge middleware to achieve this. Read more about it in our documentation here or more generally on the Edge Middleware page,

Another potential alternative if you're using Next.js would be to define your rewrites in next.config.js which supports environment variable references.

Thank you very much for your help. Let me try serverless fucntion

I am facing some problems and I hope to get some help，I tried to follow the document

try create a file middleware.ts in root dir

1import { rewrite } from '@vercel/edge';23export const config = {4  matcher: ['/api/:path*'],5};6export default async function middleware(req: Request) {7  const url = new URL(req.url);89  const path = url.pathname.replace(/^\/api/, '');10  url.pathname = path;11  url.host = process.env.API_HOST || "localhost";1213  return rewrite(url);14}

What I want to achieve is to access all paths under the /api endpoint, proxying to the API host specified in the environment variable API_HOST (like: api.example.com:8080).

But it appears to be 502 when I try to access it