Vercel Local Dev Loop

Overview

Run Vercel serverless functions and API routes locally using vercel dev. Covers environment variable management, hot reload, local testing patterns, and framework-specific dev servers.

Prerequisites

Completed vercel-install-auth setup
Project linked via vercel link
Node.js 18+ with npm/pnpm

Instructions

Step 1: Pull Environment Variables

# Pull env vars from Vercel to local .env files
vercel env pull .env.development.local

# This creates .env.development.local with all Development-scoped vars:
# VERCEL="1"
# VERCEL_ENV="development"
# DATABASE_URL="postgres://..."
# API_SECRET="sk-..."

Step 2: Start Local Dev Server

# vercel dev starts a local server that emulates the Vercel platform
vercel dev

# Output:
# Vercel CLI 39.x.x — dev command
# > Ready on http://localhost:3000

# With a specific port
vercel dev --listen 8080

# With debug logging
vercel dev --debug

vercel dev provides:

Serverless function emulation at /api/* routes
Automatic TypeScript compilation
vercel.json rewrites, redirects, and headers applied locally
Environment variables loaded from .env*.local files
Framework detection (Next.js, Nuxt, SvelteKit, etc.)

Step 3: Test Serverless Functions Locally

# Test your API route
curl http://localhost:3000/api/hello
# {"message":"Hello from Vercel Serverless Function!"}

# Test with POST body
curl -X POST http://localhost:3000/api/users \
  -H "Content-Type: application/json" \
  -d '{"name":"test","email":"test@example.com"}'

# Test with query parameters
curl "http://localhost:3000/api/search?q=vercel&limit=10"

Step 4: Framework-Specific Dev Servers

For frameworks with their own dev server, use those instead of vercel dev:

# Next.js — built-in Vercel compatibility
npx next dev
# API routes at pages/api/* or app/api/* work identically

# Nuxt
npx nuxi dev

# SvelteKit
npm run dev

# Astro
npx astro dev

The framework dev servers handle API routes natively. Use vercel dev only for plain serverless function projects without a framework.

Step 5: Local Environment Variable Management

# Add a new env var for development only
vercel env add MY_VAR development
# Prompts for value, stores encrypted on Vercel

# List all env vars
vercel env ls

# Remove an env var
vercel env rm MY_VAR development

# Pull updated vars after changes
vercel env pull .env.development.local

Step 6: Testing with Vitest

// api/__tests__/hello.test.ts
import { describe, it, expect, vi } from 'vitest';

// Mock the Vercel request/response
function createMockReq(overrides = {}) {
  return { method: 'GET', query: {}, body: null, ...overrides };
}

function createMockRes() {
  const res: any = {};
  res.status = vi.fn().mockReturnValue(res);
  res.json = vi.fn().mockReturnValue(res);
  res.send = vi.fn().mockReturnValue(res);
  return res;
}

describe('GET /api/hello', () => {
  it('returns 200 with message', async () => {
    const handler = (await import('../hello')).default;
    const req = createMockReq();
    const res = createMockRes();

    handler(req, res);

    expect(res.status).toHaveBeenCalledWith(200);
    expect(res.json).toHaveBeenCalledWith(
      expect.objectContaining({ message: expect.any(String) })
    );
  });
});

`.env` File Hierarchy

Vercel loads environment files in this order (later files override earlier):

File	Environment	Git
`.env`	All	Commit
`.env.local`	All (local only)	Ignore
`.env.development`	Development	Commit
`.env.development.local`	Development (local only)	Ignore

Output

Local dev server running with serverless function emulation
Environment variables synced from Vercel to local .env files
Hot reload on file changes
Test suite for serverless function handlers

Error Handling

Error	Cause	Solution
`vercel dev` hangs on start	Port already in use	Kill the process on port 3000 or use `--listen 8080`
`Error: No framework detected`	Missing package.json or framework	Add a build framework or use plain functions in `api/`
Env var undefined locally	Not pulled from Vercel	Run `vercel env pull .env.development.local`
`FUNCTION_INVOCATION_TIMEOUT`	Function exceeds 10s locally	Check for unresolved promises or infinite loops
TypeScript errors in `api/`	Missing `@vercel/node` types	`npm install --save-dev @vercel/node`

Resources

Next Steps

Proceed to vercel-sdk-patterns for production-ready REST API integration patterns.