Vercel Functions

---
name: vercel-functions
description: Vercel Functions expert guidance — Serverless Functions, Edge Functions, Fluid Compute, streaming, Cron Jobs, and runtime configuration. Use when configuring, debugging, or optimizing server-side code running on Vercel.
metadata:
  priority: 8
  docs:
    - "https://vercel.com/docs/functions"
    - "https://vercel.com/docs/functions/runtimes"
  sitemap: "https://vercel.com/sitemap/docs.xml"
  pathPatterns:
    - 'api/**/*.*'
    - 'pages/api/**'
    - 'src/pages/api/**'
    - 'app/**/route.*'
    - 'src/app/**/route.*'
    - 'apps/*/api/**/*.*'
    - 'apps/*/app/**/route.*'
    - 'apps/*/src/app/**/route.*'
    - 'apps/*/pages/api/**'
    - 'vercel.json'
    - 'apps/*/vercel.json'
  bashPatterns:
    - '\bvercel\s+dev\b'
    - '\bvercel\s+logs\b'
validate:
  -
    pattern: export\s+default\s+function
    message: 'Use named exports (GET, POST, PUT, DELETE) instead of default export for route handlers'
    severity: error
  -
    pattern: NextApiRequest|NextApiResponse
    message: 'NextApiRequest/NextApiResponse are Pages Router types — use Web API Request/Response'
    severity: error
  -
    pattern: 'from\s+[''"](openai|@anthropic-ai/sdk|anthropic)[''"]|new\s+(OpenAI|Anthropic)\('
    message: 'Direct AI provider SDK detected in route handler. Use the Vercel AI SDK for streaming, tools, and provider abstraction.'
    severity: recommended
    upgradeToSkill: ai-sdk
    upgradeWhy: 'Replace vendor-locked provider SDKs with @ai-sdk/openai or @ai-sdk/anthropic for unified streaming and tool support.'
    skipIfFileContains: '@ai-sdk/|from\s+[''"](ai)[''"]|import.*from\s+[''"](ai)[''"]|streamText|generateText'
  -
    pattern: 'setTimeout\s*\(|setInterval\s*\(|await\s+new\s+Promise\s*\([^)]*setTimeout'
    message: 'Long-running or polling logic detected in a serverless handler. Functions have execution time limits.'
    severity: recommended
    upgradeToSkill: workflow
    upgradeWhy: 'Move delayed/polling logic to Vercel Workflow for durable execution with pause, resume, retries, and crash safety.'
    skipIfFileContains: 'use workflow|use step|@vercel/workflow'
  -
    pattern: 'writeFile(Sync)?\(|createWriteStream\(|from\s+[''"](multer|formidable)[''"]|fs\.writeFile'
    message: 'Local filesystem write detected. Serverless functions have ephemeral, read-only filesystems.'
    severity: error
    upgradeToSkill: vercel-storage
    upgradeWhy: 'Replace local filesystem writes with Vercel Blob, Neon, or Upstash for persistent, platform-native storage.'
    skipIfFileContains: '@vercel/blob|@upstash/|@neondatabase/'
  -
    pattern: 'export\s+(async\s+)?function\s+(GET|POST|PUT|PATCH|DELETE)\b'
    message: 'Route handler has no observability instrumentation. Add logging and error tracking for production debugging.'
    severity: warn
    upgradeToSkill: observability
    upgradeWhy: 'Add structured logging, error tracking, and OTel instrumentation to route handlers.'
    skipIfFileContains: 'console\.error|logger\.|captureException|Sentry|@vercel/otel|withTracing'
  -
    pattern: 'from\s+[''""](lru-cache|node-cache|memory-cache)[''""]|new\s+(LRUCache|NodeCache|Map)\(\s*\).*cache'
    message: 'In-process memory cache detected in serverless function. Process memory is not shared across invocations.'
    severity: recommended
    upgradeToSkill: runtime-cache
    upgradeWhy: 'Replace in-process caches with Vercel Runtime Cache (getCache from @vercel/functions) for region-aware caching that persists across invocations.'
    skipIfFileContains: 'getCache|from\s+[''""]\@vercel/functions[''""]'
  -
    pattern: 'maxRetries\s*[=:]|retryCount\s*[=:]|retry\s*\(\s*|for\s*\([^)]*retry|while\s*\([^)]*retry'
    message: 'Manual retry logic detected. Use Vercel Workflow DevKit for automatic retries with durable execution.'
    severity: recommended
    upgradeToSkill: workflow
    upgradeWhy: 'Replace manual retry loops with Workflow DevKit steps that provide automatic retries, crash safety, and observability.'
    skipIfFileContains: 'use workflow|use step|@vercel/workflow|from\s+[''""](workflow)[''""]'
  -
    pattern: 'from\s+[''"](express)[''""]|require\s*\(\s*[''"](express)[''""\)]'
    message: 'Express.js detected in a Vercel project. Vercel Functions use the Web Request/Response API — Express middleware, req/res, and app.listen() do not work in serverless.'
    severity: recommended
    upgradeToSkill: vercel-functions
    upgradeWhy: 'Replace Express with Next.js route handlers (export async function GET/POST) or Vercel Functions using the Web Request/Response API.'
    skipIfFileContains: 'export\s+(async\s+)?function\s+(GET|POST|PUT|PATCH|DELETE)|from\s+[''""](next/server|@vercel/functions)[''""]'
retrieval:
  aliases:
    - serverless functions
    - api routes
    - edge functions
    - lambda
  intents:
    - create serverless function
    - configure function runtime
    - optimize cold starts
    - add api route
  entities:
    - Serverless Functions
    - Edge Functions
    - Fluid Compute
    - streaming
    - Cron Jobs
chainTo:
  -
    pattern: 'from\s+[''\"](openai|@anthropic-ai/sdk|anthropic)[''"]|new\s+(OpenAI|Anthropic)\('
    targetSkill: ai-sdk
    message: 'Direct AI provider SDK in route handler — loading AI SDK guidance for unified streaming and tool support.'
  -
    pattern: 'setTimeout\s*\(|setInterval\s*\(|await\s+new\s+Promise\s*\([^)]*setTimeout'
    targetSkill: workflow
    message: 'Long-running or polling logic in serverless handler — loading Workflow DevKit for durable execution.'
  -
    pattern: 'writeFile(Sync)?\(|createWriteStream\(|from\s+[''\"](multer|formidable)[''"]|fs\.writeFile'
    targetSkill: vercel-storage
    message: 'Local filesystem write in serverless function — loading Vercel Storage guidance for platform-native persistence.'
  -
    pattern: 'from\s+[''""]@vercel/(postgres|kv)[''""]'
    targetSkill: vercel-storage
    message: '@vercel/postgres and @vercel/kv are sunset — loading Vercel Storage guidance for Neon and Upstash migration.'
  -
    pattern: 'generateObject\s*\(|streamObject\s*\(|toDataStreamResponse|maxSteps\b|CoreMessage\b'
    targetSkill: ai-sdk
    message: 'Deprecated AI SDK v5 API detected — loading AI SDK v6 guidance for migration.'
  -
    pattern: 'while\s*\(\s*true\s*\)\s*\{|for\s*\(\s*;\s*;\s*\)\s*\{|setInterval\s*\(\s*async'
    targetSkill: workflow
    message: 'Polling loop in serverless function detected — loading Workflow DevKit for durable, crash-safe execution with pause/resume.'
    skipIfFileContains: "use workflow|use step|from\\s+['\"]workflow['\"]"
  -
    pattern: "from\\s+['\"]express['\"]|require\\s*\\(\\s*['\"]express['\"]"
    targetSkill: vercel-functions
    message: 'Express.js detected — loading Vercel Functions guidance for Web Request/Response API route handlers that replace Express middleware and routing.'
    skipIfFileContains: "export\\s+(async\\s+)?function\\s+(GET|POST|PUT|PATCH|DELETE)"
  -
    pattern: 'from\s+[''""](lru-cache|node-cache|memory-cache)[''""]|new\s+(LRUCache|NodeCache|Map)\(\s*\).*cache'
    targetSkill: runtime-cache
    message: 'In-process memory cache in serverless function — loading Runtime Cache guidance for region-aware caching that persists across invocations.'
    skipIfFileContains: 'getCache|from\s+[''""]\@vercel/functions[''""]'
  -
    pattern: 'maxRetries\s*[=:]|retryCount\s*[=:]|retry\s*\(\s*|for\s*\([^)]*retry|while\s*\([^)]*retry'
    targetSkill: workflow
    message: 'Manual retry logic in serverless handler — loading Workflow DevKit guidance for automatic retries with durable execution.'
    skipIfFileContains: 'use workflow|use step|@vercel/workflow|from\s+[''""](workflow)[''""]'

---

# 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)

```ts
// 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 (<1ms globally)
- Limited API surface (no full Node.js)
- Best for: auth checks, redirects, A/B testing, simple transformations

```ts
// app/api/hello/route.ts
export const runtime = 'edge'

export async function GET() {
  return new Response('Hello from the Edge')
}
```

### Bun Runtime (Public Beta)

Add `"bunVersion": "1.x"` to `vercel.json` to run Node.js functions on Bun instead. ~28% lower latency for CPU-bound workloads. Supports Next.js, Express, Hono, Nitro.

### Rust Runtime (Public Beta)

Rust functions run on Fluid Compute with HTTP streaming and Active CPU pricing. Built on the community Rust runtime. Supports environment variables up to 64 KB.

### Node.js 24 LTS

Node.js 24 LTS is now GA on Vercel for both builds and functions. Features V8 13.6, global `URLPattern`, Undici v7 for faster `fetch()`, and npm v11.

### Choosing Runtime

| Need | Runtime | Why |
|------|---------|-----|
| Full Node.js APIs, npm packages | `nodejs` | Full compatibility |
| Lower latency, CPU-bound work | `nodejs` + Bun | ~28% latency reduction |
| Ultra-low latency, simple logic | `edge` | <1ms cold start, global |
| Database connections, heavy deps | `nodejs` | Edge lacks full Node.js |
| Auth/redirect at the edge | `edge` | Fastest response |
| AI streaming | Either | Both support streaming |
| Systems-level performance | `rust` (beta) | Native speed, Fluid Compute |

## Fluid Compute

Fluid Compute is the unified execution model for all Vercel Functions (both Node.js and Edge).

Key benefits:
- **Optimized concurrency**: Multiple invocations on a single instance — up to 85% cost reduction for high-concurrency workloads
- **Extended durations**: Default 300s for all plans; up to 800s on Pro/Enterprise
- **Active CPU pricing**: Charges only while CPU is actively working, not during idle/await time. Enabled by default for all plans. Memory-only periods billed at a significantly lower rate.
- **Background processing**: `waitUntil` / `after` for post-response tasks
- **Dynamic scaling**: Automatic during traffic spikes
- **Bytecode caching**: Reduces cold starts via Rust-based runtime with pre-compiled function code
- **Multi-region failover**: Default for Enterprise when Fluid is activated

### Instance Sizes

| Size | CPU | Memory |
|------|-----|--------|
| Standard (default) | 1 vCPU | 2 GB |
| Performance | 2 vCPU | 4 GB |

Hobby projects use Standard CPU. The Basic CPU instance has been removed.

### Background Processing with `waitUntil`

```ts
// Continue work after sending response
import { waitUntil } from '@vercel/functions'

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

  // Send response immediately
  const response = Response.json({ received: true })

  // Continue processing in background
  waitUntil(async () => {
    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.

```ts
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`:

```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:

```ts
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).

```json
{
  "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](https://vercel.com/docs/functions)
- [Serverless Functions](https://vercel.com/docs/functions)
- [Edge Functions](https://vercel.com/docs/functions)
- [Fluid Compute](https://vercel.com/docs/fluid-compute)
- [Streaming](https://vercel.com/docs/functions/streaming)
- [Cron Jobs](https://vercel.com/docs/cron-jobs)
- [GitHub: Vercel](https://github.com/vercel/vercel)