# 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.

**Type:** Prompt  
**Tools:** Cursor, Claude Code, Codex, Windsurf  
**Stack:** Cloudflare, TypeScript  
**Difficulty:** medium  
**Updated:** 2026-06-08

---

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

```txt title="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

```txt
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

```bash
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

```txt title="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.
```