Design Edge Functions with Claude Code: Cloudflare Workers, JWT at Edge, Rate Limiting

#claudecode #cloudflare #typescript #performance

Introduction

For use cases requiring low latency even with serverless — distribute processing to the edge with Cloudflare Workers and Vercel Edge Runtime for sub-10ms responses globally. Let Claude Code design this.

CLAUDE.md Rules

## Edge Function Rules
- Use cases: auth check, rate limiting, A/B test, geo-routing
- Max CPU time: 50ms
- Memory: < 128MB
- No filesystem (use KV/D1/R2)
- No Node.js modules (Web API only)

Generated Implementation

// JWT verification at edge (Web Crypto API)
async function verifyJWT(token: string, secret: string) {
  const [headerB64, payloadB64, signatureB64] = token.split('.');
  const key = await crypto.subtle.importKey(
    'raw',
    new TextEncoder().encode(secret),
    { name: 'HMAC', hash: 'SHA-256' },
    false,
    ['verify']
  );
  const valid = await crypto.subtle.verify(
    'HMAC', key, decodeBase64(signatureB64),
    new TextEncoder().encode(`${headerB64}.${payloadB64}`)
  );
  if (!valid) return null;
  return JSON.parse(atob(payloadB64));
}

// Rate limiting with KV
async function checkRateLimit(request: Request, env: Env) {
  const ip = request.headers.get('CF-Connecting-IP') ?? 'unknown';
  const key = `ratelimit:${ip}:${Math.floor(Date.now() / 60_000)}`;
  const count = parseInt(await env.RATE_LIMIT_KV.get(key) ?? '0');

  if (count >= 100) return { allowed: false, retryAfterSeconds: 60 };
  await env.RATE_LIMIT_KV.put(key, String(count + 1), { expirationTtl: 90 });
  return { allowed: true };
}

// A/B test with sticky cookies
async function abTest(request: Request) {
  const cookies = parseCookies(request.headers.get('Cookie') ?? '');
  let variant = cookies['ab_checkout'];

  if (!variant) {
    const hash = await hashString(userId + 'checkout_v2');
    variant = hash[0] < 128 ? 'control' : 'treatment';
  }

  const response = await fetch(variant === 'treatment' ? '/checkout-v2' : '/checkout', request);
  if (!cookies['ab_checkout']) {
    response.headers.append('Set-Cookie', `ab_checkout=${variant}; Max-Age=604800`);
  }
  return response;
}