Vercel Functions

Vercel Functions

You are an expert in Vercel Functions — the compute layer of the Vercel platform.

Function Types

Serverless Functions (Node.js)

• Full Node.js runtime, all npm packages available
• Default for Next.js API routes, Server Actions, Server Components
• Cold starts: 800ms–2.5s (with DB connections)
• Max duration: 10s (Hobby), 300s (Pro default), 800s (Fluid Compute Pro/Enterprise)

// app/api/hello/route.ts
export async function GET() {
  return Response.json({ message: 'Hello from Node.js' })
}

Edge Functions (V8 Isolates)

• Lightweight V8 runtime, Web Standard APIs only
• Ultra-low cold starts ( {

await processAnalytics(data) await sendNotification(data) })

return response }

### Next.js `after` (equivalent)

```ts
import { after } from 'next/server'

export async function POST(req: Request) {
  const data = await req.json()

  after(async () => {
    await logToAnalytics(data)
  })

  return Response.json({ ok: true })
}

Streaming

Zero-config streaming for both runtimes. Essential for AI applications.

export async function POST(req: Request) {
  const encoder = new TextEncoder()
  const stream = new ReadableStream({
    async start(controller) {
      for (const chunk of data) {
        controller.enqueue(encoder.encode(chunk))
        await new Promise(r => setTimeout(r, 100))
      }
      controller.close()
    },
  })

  return new Response(stream, {
    headers: { 'Content-Type': 'text/event-stream' },
  })
}

For AI streaming, use the AI SDK's toUIMessageStreamResponse() (for chat UIs with useChat) which handles SSE formatting automatically.

Cron Jobs

Schedule function invocations via vercel.json:

{
  "crons": [
    {
      "path": "/api/daily-report",
      "schedule": "0 8 * * *"
    },
    {
      "path": "/api/cleanup",
      "schedule": "0 */6 * * *"
    }
  ]
}

The cron endpoint receives a normal HTTP request. Verify it's from Vercel:

export async function GET(req: Request) {
  const authHeader = req.headers.get('authorization')
  if (authHeader !== `Bearer ${process.env.CRON_SECRET}`) {
    return new Response('Unauthorized', { status: 401 })
  }
  // Do scheduled work
  return Response.json({ ok: true })
}

Configuration via vercel.json

Deprecation notice: Support for the legacy now.json config file will be removed on March 31, 2026. Rename now.json to vercel.json (no content changes required).

{
  "functions": {
    "app/api/heavy/**": {
      "maxDuration": 300,
      "memory": 1024
    },
    "app/api/edge/**": {
      "runtime": "edge"
    }
  }
}

Timeout Limits

All plans now default to 300s execution time with Fluid Compute.

| Plan | Default | Max | |------|---------|-----| | Hobby | 300s | 300s | | Pro | 300s | 800s | | Enterprise | 300s | 800s |

Common Pitfalls

1. Cold starts with DB connections: Use connection pooling (e.g., Neon's @neondatabase/serverless)
2. Edge limitations: No fs, no native modules, limited crypto — use Node.js runtime if needed
3. Timeout exceeded: Use Fluid Compute for long-running tasks, or Workflow DevKit for very long processes
4. Bundle size: Python runtime supports up to 500MB; Node.js has smaller limits
5. Environment variables: Available in all functions automatically; use vercel env pull for local dev

Function Runtime Diagnostics

Timeout Diagnostics

504 Gateway Timeout?
├─ All plans default to 300s with Fluid Compute
├─ Pro/Enterprise: configurable up to 800s
├─ Long-running task?
│  ├─ Under 5 min → Use Fluid Compute with streaming
│  ├─ Up to 15 min → Use Vercel Functions with `maxDuration` in vercel.json
│  └─ Hours/days → Use Workflow DevKit (DurableAgent or workflow steps)
└─ DB query slow? → Add connection pooling, check cold start, use Edge Config

500 Error Diagnostics

500 Internal Server Error?
├─ Check Vercel Runtime Logs (Dashboard → Deployments → Functions tab)
├─ Missing env vars? → Compare `.env.local` against Vercel dashboard settings
├─ Import error? → Verify package is in `dependencies`, not `devDependencies`
└─ Uncaught exception? → Wrap handler in try/catch, use `after()` for error reporting

Invocation Failure Diagnostics

"FUNCTION_INVOCATION_FAILED"?
├─ Memory exceeded? → Increase `memory` in vercel.json (up to 3008 MB on Pro)
├─ Crashed during init? → Check top-level await or heavy imports at module scope
└─ Edge Function crash? → Check for Node.js APIs not available in Edge runtime

Cold Start Diagnostics

Cold start latency > 1s?
├─ Using Node.js runtime? → Consider Edge Functions for latency-sensitive routes
├─ Large function bundle? → Audit imports, use dynamic imports, tree-shake
├─ DB connection in cold start? → Use connection pooling (Neon serverless driver)
└─ Enable Fluid Compute to reuse warm instances across requests

Edge Function Timeout Diagnostics

"EDGE_FUNCTION_INVOCATION_TIMEOUT"?
├─ Edge Functions have 25s hard limit (not configurable)
├─ Move heavy computation to Node.js Serverless Functions
└─ Use streaming to start response early, process in background with `waitUntil`

Official Documentation

• Vercel Functions
• Serverless Functions
• Edge Functions
• Fluid Compute
• Streaming
• Cron Jobs
• GitHub: Vercel

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: build-with-dhiraj
• Source: build-with-dhiraj/ai-workflow-framework-portability-kit
• License: MIT

Install and usage instructions live in the source repository linked above.