Vercel Edge Functions vs Serverless Functions: Which to Use and When

Vercel offers two function runtimes and the choice matters for performance, cost, and capability. Most developers default to Serverless Functions because that's what Next.js API routes use by default — but Edge Functions are often the better choice for AI applications and many API patterns.

The Core Difference

When to Use Edge Functions

1. AI Streaming Chat

Edge Functions were built for streaming. No cold start means the first token reaches the user in milliseconds. Use the Edge runtime for any route that streams an LLM response.

// app/api/chat/route.ts — Edge runtime
export const runtime = 'edge';
 
export async function POST(req: Request) {
  // Native streaming — works perfectly on Edge
  const stream = await openai.chat.completions.create({
    model: 'gpt-4o-mini',
    messages: await req.json(),
    stream: true,
  });
  return new Response(stream.toReadableStream());
}

2. Auth Middleware

Validating a JWT or session cookie on every request is cheap and fast on the Edge. No cold start means no added latency to your auth check.

// middleware.ts — runs on Edge automatically
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';
 
export function middleware(request: NextRequest) {
  const token = request.cookies.get('session')?.value;
  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
  // Validate JWT using Web Crypto API (available on Edge)
  return NextResponse.next();
}
 
export const config = {
  matcher: ['/dashboard/:path*', '/api/protected/:path*'],
};

3. Geolocation and A/B Testing

Edge Functions have access to the request's geolocation headers. Use them for region-specific content, currency localisation, or A/B test routing — all at zero-cold-start speed.

When to Use Serverless Functions

1. Database Queries with Connection Pooling

Most database drivers use TCP connections, which aren't available in V8 isolates. Use Serverless Functions (Node.js runtime) for direct database access, or use an HTTP-based database client (Neon, PlanetScale, Supabase) on the Edge.

// app/api/documents/route.ts — Node.js runtime (default)
// DO NOT add export const runtime = 'edge' here
 
import { db } from '@/lib/db'; // Drizzle + standard pg driver
 
export async function GET() {
  const docs = await db.select().from(documents);
  return Response.json(docs);
}

2. Heavy npm Packages

PDF processing, image manipulation, or any package using native Node.js modules requires the Serverless runtime. Edge Functions only support pure JavaScript/TypeScript.

3. Long-Running Operations

If a task might exceed 30 seconds, you need Serverless (up to 60s on Pro) or — better — a background job service like Trigger.dev or Inngest.

The Hybrid Pattern for AI Apps

Most production AI applications use both runtimes:

• Edge: streaming chat, auth middleware, A/B routing, geolocation
• Serverless: database reads/writes, file uploads, heavy processing, webhook handlers

// Edge: streaming chat endpoint
// app/api/chat/route.ts
export const runtime = 'edge';
 
// Serverless: save conversation to database
// app/api/conversations/route.ts
// (no runtime export = Node.js serverless)

You can mix runtimes across routes in the same Next.js project. Set export const runtime = 'edge' only on the specific routes that benefit from it.