Prompt to Build a Cloudflare Worker API Proxy

Copy-paste AI prompt to build a Cloudflare Worker that proxies and rate-limits external API calls, adds auth headers, and caches responses.

Use this prompt to build a Cloudflare Worker that sits in front of an external API, injects auth headers, rate-limits by IP using Workers KV, and caches responses — so clients never see your upstream API key.

Main Prompt

You are building a Cloudflare Worker using TypeScript and the Workers runtime (not Node.js).
The Worker will proxy requests to an upstream API (e.g., OpenAI at https://api.openai.com).

Task: create a production-ready API proxy Worker.

Requirements:
- Use `wrangler` v3 for local dev. Scaffold with `bun create cloudflare@latest` and choose
  "Hello World" Worker with TypeScript.
- Upstream URL: read from a Wrangler secret `UPSTREAM_URL` (string).
- Auth: inject `Authorization: Bearer ${env.UPSTREAM_API_KEY}` on every proxied request,
  where `UPSTREAM_API_KEY` is a Wrangler secret. Never expose this header to the client.
- CORS: allow only origins in `env.ALLOWED_ORIGINS` (comma-separated string secret).
  Return a `403` for disallowed origins. Handle preflight OPTIONS requests.
- Rate limiting: use Workers KV binding `RATE_LIMIT_KV`.
  - Key: `rl:${ip}` where ip is `request.headers.get('CF-Connecting-IP')`.
  - Value: request count for the current UTC minute (TTL = 60 s).
  - Limit: 60 requests/minute per IP. Return `429` with `Retry-After: 60` if exceeded.
- Caching: for GET requests, check `caches.default` before proxying upstream. Cache
  successful responses with `Cache-Control: public, max-age=300`.
- Strip the following headers from the upstream response before returning to the client:
  `x-powered-by`, `server`, `cf-ray`.
- Wrangler config: declare the KV namespace binding and all secrets in `wrangler.toml`.
- Do NOT use Node.js APIs (`fs`, `path`, `Buffer`) — Workers runtime only.

Stop and list all planned files before writing code.

Implementation Notes

Cloudflare Workers receive a Request and return a Response — avoid express-style patterns.
caches.default is the Cloudflare edge cache; it only works in production. Use MINIFLARE_CACHE for local dev testing or mock the cache.
CF-Connecting-IP is injected by Cloudflare — it is not spoofable from the public internet, but test locally with a hardcoded fallback IP.
Wrangler secrets are set with wrangler secret put UPSTREAM_API_KEY — never store them in wrangler.toml or committed .env files.

Expected File Changes

wrangler.toml                  (new)
src/index.ts                   (new — Worker entrypoint)
src/cors.ts                    (new — CORS helper)
src/rate-limit.ts              (new — KV rate limiter)
package.json                   (new)
tsconfig.json                  (new)
.dev.vars                      (new — local dev secrets, gitignored)
.gitignore                     (edited — add .dev.vars)

Acceptance Criteria

wrangler dev starts without errors and proxies a GET / to the upstream URL.
An IP sending 61 requests in one minute receives a 429 on the 61st request.
A request from a non-allowed origin receives a 403.
The Authorization header does not appear in the response or in any client-visible header.
wrangler deploy succeeds and the Worker is live on workers.dev.

Test Commands

wrangler dev &
# test normal proxy
curl http://localhost:8787/ -H "Origin: https://myapp.com"
# test CORS rejection
curl http://localhost:8787/ -H "Origin: https://evil.com"
# test rate limit (requires 61 rapid requests)
for i in $(seq 1 62); do curl -s -o /dev/null -w "%{http_code}\n" http://localhost:8787/; done

Common AI Mistakes

Using process.env instead of the env parameter passed to the Worker fetch handler.
Forgetting to handle OPTIONS preflight requests, breaking CORS for POST/PUT calls.
Storing UPSTREAM_API_KEY in wrangler.toml as a plain variable instead of a secret.
Using node:buffer or other Node.js built-ins, which are not available in the Workers runtime.

Fix Prompt

The Worker fails with a runtime error or leaks the API key. Fix in order:
1. Replace `process.env.X` with `env.X` everywhere — Workers use the `env` handler parameter.
2. Add an OPTIONS handler before the proxy logic that returns the CORS headers with a 204 status.
3. Move `UPSTREAM_API_KEY` from `wrangler.toml` [vars] to a secret: `wrangler secret put UPSTREAM_API_KEY`.
Show only the corrected diff.