CORS Proxy Getting Started: Quickstart Guide (2026)

Getting started overview

CORS Proxy, specifically the cors-anywhere implementation hosted on Heroku, provides a mechanism to bypass Cross-Origin Resource Sharing (CORS) restrictions. These restrictions are a security feature enforced by web browsers, preventing web pages from making requests to a different domain than the one that served the web page, unless the target domain explicitly allows it. For developers, this often manifests as a "CORS error" when attempting to fetch data from a third-party API during local development or in client-side applications.

The cors-anywhere proxy works by prepending its URL to the target API's URL. When a request is sent to the proxy, the proxy then forwards that request to the original target API. Upon receiving the response, the proxy adds the necessary Access-Control-Allow-Origin and other CORS-related headers before returning the response to the client. This allows the browser to accept the cross-origin response without triggering a CORS error.

This getting started guide focuses on using the publicly available instance of cors-anywhere hosted on Heroku at https://cors-anywhere.herokuapp.com/. While convenient for development and testing, it is important to note that this public instance is a free, open-source project and is subject to potential rate limits and reliability issues. For production environments, hosting your own instance or using dedicated proxy services is generally recommended.

The process for getting started is straightforward, primarily involving constructing the correct URL for your requests. There are no accounts or API keys required for the public Heroku instance.

Here's a quick reference table outlining the steps:

Create an account and get keys

For the public cors-anywhere instance available at https://cors-anywhere.herokuapp.com/, there is no account creation process or API key requirement. This instance is maintained as a public utility to assist developers with CORS issues during the development phase.

Developers who require higher reliability, custom configurations, or wish to avoid potential public proxy rate limits can choose to deploy their own instance of cors-anywhere. This typically involves:

1. Cloning the cors-anywhere GitHub repository.
2. Installing Node.js dependencies.
3. Deploying the application to a platform like Heroku or a virtual private server.

When hosting your own instance, you control access and can implement custom logic, but this falls outside the scope of using the public "getting started" proxy. For the purposes of this guide, assume you are using the public Heroku-hosted version.

Your first request

Making your first request with CORS Proxy involves a simple modification to your target API URL. The core principle is to prepend https://cors-anywhere.herokuapp.com/ to the URL of the API you wish to access.

Prerequisites

• A target API endpoint that you want to call.
• A web browser with developer tools, or a command-line tool like curl, or a client-side JavaScript environment.

Example: Fetching data with JavaScript

Consider an example where you want to fetch data from https://api.example.com/data. If api.example.com does not send the appropriate CORS headers, your browser-based JavaScript application would receive a CORS error.

To bypass this using CORS Proxy, modify your request as follows:

const targetUrl = 'https://api.example.com/data';
const proxyUrl = 'https://cors-anywhere.herokuapp.com/';

fetch(proxyUrl + targetUrl)
  .then(response => {
    if (!response.ok) {
      throw new Error(`HTTP error! status: ${response.status}`);
    }
    return response.json();
  })
  .then(data => {
    console.log('Data received:', data);
    // Process your data here
  })
  .catch(error => {
    console.error('There was a problem with the fetch operation:', error);
  });

In this example, the fetch request is directed to https://cors-anywhere.herokuapp.com/https://api.example.com/data. The proxy then makes the request to https://api.example.com/data on your behalf, adds the necessary CORS headers to the response, and returns it to your browser, allowing your JavaScript code to process it.

Example: Using cURL

You can also test this from your terminal using curl. Replace https://api.example.com/data with your actual target API URL.

curl "https://cors-anywhere.herokuapp.com/https://api.example.com/data"

The response received will include the data from https://api.example.com/data, potentially with additional CORS headers added by the proxy, such as access-control-allow-origin: *.

Verifying the request

When making a request from a browser, open your browser's developer tools (usually F12 or Cmd+Option+I) and navigate to the "Network" tab. You should see the request made to cors-anywhere.herokuapp.com. Inspect the response headers to confirm that Access-Control-Allow-Origin: * (or a specific origin if the proxy is configured) is present, indicating that the proxy successfully added the necessary CORS headers.

Common next steps

Once you've successfully made your first proxied request, consider these common next steps for effective development:

1. Explore Different HTTP Methods: Test your proxy with various HTTP methods (POST, PUT, DELETE, etc.) if your target API supports them. Ensure that the proxy correctly forwards all method types and request bodies. The HTTP methods documentation provides details on common request types.

2. Handle Request Headers: Many APIs require custom headers for authentication (e.g., Authorization) or content negotiation (e.g., Content-Type). Verify that your client-side code correctly sends these headers and that the proxy forwards them to the target API. For instance, when using fetch, include an headers object in your request options.

3. Test with Query Parameters and Body Data: Ensure that complex URLs with query parameters (e.g., ?param1=value1&param2=value2) are correctly handled. For POST/PUT requests, confirm that the request body (JSON, form data) is properly transmitted through the proxy to the target API.

4. Understand Rate Limits: The public cors-anywhere.herokuapp.com instance may enforce rate limits, especially under heavy usage. If you encounter 429 Too Many Requests errors, this is likely the cause. For sustained development or testing, consider deploying your own instance of cors-anywhere or using a dedicated proxy service.

5. Configure Your Own Instance (for Production/Heavy Use): If the public instance proves unreliable or insufficient for your needs, deploying your own version of cors-anywhere offers greater control. You can configure specific origins, rate limits, and other parameters. Instructions for self-hosting are available in the cors-anywhere GitHub repository.

6. Implement Error Handling: Beyond basic success checks, implement robust error handling for network issues, API-specific errors, and proxy-related failures (e.g., 5xx errors from the proxy itself). This improves the resilience of your application.

Troubleshooting the first call

When your first call through the CORS Proxy doesn't work as expected, consider these common troubleshooting steps:

• Check the Proxied URL Format: Double-check that you have correctly prepended https://cors-anywhere.herokuapp.com/ to your target API URL. A common mistake is missing the trailing slash on the proxy URL or incorrectly concatenating the strings.

  Incorrect: cors-anywhere.herokuapp.comapi.example.com/data
  Correct: cors-anywhere.herokuapp.com/https://api.example.com/data

• Verify Target API Accessibility: Ensure that the target API URL itself is correct and accessible. Try making a direct request to the target API (without the proxy) using curl or a tool like Postman to confirm it's online and responding as expected. If the target API is down or returns an error directly, the proxy will simply forward that error.

• Inspect Browser Network Tab: Use your browser's developer tools (Network tab) to inspect the request and response. Look for:

  • The actual URL being requested (it should start with cors-anywhere.herokuapp.com).
  • The HTTP status code of the response from the proxy.
  • The response headers, specifically looking for Access-Control-Allow-Origin. If this header is missing or incorrect, the browser might still block the response.
  • Any error messages in the console.

• CORS Proxy Rate Limiting (429 Error): If you receive an HTTP 429 Too Many Requests status code from cors-anywhere.herokuapp.com, it indicates that the public instance is experiencing high traffic or you've exceeded its temporary rate limits. This is a common issue with free, public proxies. You may need to wait, try again later, or consider deploying your own instance of the proxy. The IETF RFC 6585 on Additional HTTP Status Codes details the 429 status code.

• Preflight Request Issues (OPTIONS Method): For complex requests (those with custom headers, non-GET methods, or specific Content-Types), browsers often send a "preflight" OPTIONS request first. The proxy needs to handle this correctly. The cors-anywhere proxy is designed to do so, but if you're using a self-hosted version, ensure it's configured properly. If the preflight request fails, the actual request will not be sent.

• Browser Extensions: Occasionally, other browser extensions (especially those related to ad-blocking or privacy) can interfere with network requests. Try disabling extensions temporarily or testing in an incognito window.

• Check Console Errors: Look for any specific error messages in your browser's JavaScript console. These can often provide clues about why the request failed, such as network errors or issues with parsing the response.