Vercel local dev loop

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":"[email protected]"}'

# 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

• Vercel Dev Command
• Environment Variables
• Vercel Functions
• Vitest Documentation

Next Steps

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