Deno Deploy CORS Proxy

Serve any HTTPS endpoint with permissive CORS headers so that browsers can call the original API from client-side code.

How It Works

Prefix the URL you want to reach with the proxy origin. The proxy forwards the request, strips the browser's Origin header (a common reason for 403 responses), and mirrors the downstream response while adding:

• Access-Control-Allow-* headers that echo requested methods and headers
• Access-Control-Expose-Headers so metadata like Content-Length and Location stays readable
• A rewritten Location when the upstream issues redirects so clients continue through the proxy

Usage Example

// Instead of doing this and hitting a CORS error …
const res = await fetch("https://api.example.com/data");

// … call the proxy with the destination URL appended
const proxied = await fetch("https://cors.deno.dev/https://api.example.com/data");
const data = await proxied.json();

Local Development

1. Install Deno 2.4.5 or newer.
2. From this directory run deno task dev to start a hot-reloading server on port 8000.
3. Visit http://localhost:8000/https://api.example.com/health to verify it proxies correctly.

The server only requires network access and read access to README.md, which is reflected in the deno.json task definitions.

Deploying to Deno Deploy

You can deploy directly from the command line or through the Deno Deploy dashboard.

CLI deployment (recommended)

1. Authenticate once with deno deploy login.
2. Deploy the current directory: deno deploy --project=cors-proxy --prod main.ts.
   • Replace cors-proxy with your preferred project name. The command will create the project on first run.
3. After a successful deploy, Deno Deploy prints the production URL (for example https://cors-proxy.deno.dev). Use that origin in your client code.

Dashboard deployment

1. Go to https://dash.deno.com/new and choose GitHub or Manual deployment.
2. Select this repository and point the entry point at main.ts.
3. Choose the United States or your preferred region and click Deploy. Updates push automatically whenever you commit to the selected branch.

Testing Redirect Behaviour

Some APIs respond with 30x redirects. The proxy rewrites Location headers so that the browser follows the redirect through the proxy instead of going direct to the upstream domain. You can test this locally by requesting https://cors.deno.dev/https://httpbin.org/redirect/1 and watching the request chain in your devtools network tab.

Troubleshooting

• 403 or 5xx from upstream – Check the proxy logs in Deno Deploy. The upstream API might require additional headers (e.g. API key). Add them to the original fetch call; the proxy forwards custom headers unchanged.
• Slow responses – The proxy streams the downstream body, so latency comes from the target server. Consider deploying the proxy in the same region as the upstream API.
• Large uploads – Deno Deploy currently limits request bodies to 10 MiB (as of September 27, 2025). Larger uploads will fail before reaching the upstream.

License

MIT