How to use API Tester online, free

Build and send HTTP requests (GET, POST, PUT, DELETE, etc.) with custom headers, query parameters, and a request body, then inspect the full response - similar to lightweight Postman-style testing directly in your browser. Requests go out directly from your browser first, which is what makes testing your own localhost dev server possible - a server-hosted tool can never reach your machine, but your own browser can. If the browser can't complete a request directly (usually a public API blocking cross-origin requests), it automatically falls back to a server-side proxy so the request still goes through.

You should not have to open an IDE to format a snippet from Slack, tidy a JSON blob, or check a hash. Paste it here, copy the result, move on.

API Tester is a good fit when testing REST endpoints during feature development.

Why this exists

API Tester is built around a few practical wins, not a long feature list:

• Fast API debugging without installing external desktop clients.
• Supports common request methods, headers, params, and payload testing.
• Shows full response details for troubleshooting backend behavior.
• Great for quick endpoint checks during frontend and backend development.

Walkthrough

1. Choose HTTP method and URL. Pick GET/POST/PUT/DELETE and enter the exact endpoint you want to test.
2. Add query params and headers. Set required headers like Authorization and Content-Type plus any query string values.
3. Provide body if needed. For POST/PUT/PATCH requests, paste JSON or other payload format expected by the API.
4. Send and inspect response. Review status code, headers, and response body to validate behavior and debug quickly.

Jobs it is built for

• Testing REST endpoints during feature development.
• Validating auth headers and token-based access flows.
• Checking API error responses and edge-case behavior.
• Verifying staging APIs before release or QA handoff.

Worth knowing before you start

• Save sample payloads for repeat endpoint tests to speed up debugging.
• Start with a simple GET to verify connectivity before complex body requests.
• Match Content-Type exactly to your backend parser expectations.
• Use realistic test data to reproduce production-like failures safely.

What not to expect

• Forgetting auth headers and assuming the endpoint itself is broken.
• Sending JSON with wrong Content-Type and misreading parse errors.
• Ignoring response headers when debugging caching or CORS behavior.
• Testing only success paths without checking error handling responses.

Privacy, in one paragraph

API Tester needs the ToolBox API for work a browser cannot do. What you submit is processed, then discarded. It is not kept as a library of your files.

Related tools worth opening next

If this is one step in a longer job, these usually come after it:

• HTTP Header Viewer - Fetch a URL and inspect its response headers
• URL Parser - Parse and analyze URL components
• JSON Formatter - Format and validate JSON data

Short answers

Can this call any API, including ones requiring authentication?

You can add custom headers (including Authorization headers for tokens/API keys) to your request, so most authenticated APIs can be tested as long as you have valid credentials to include.

Will I run into CORS issues testing some APIs?

Requests are sent directly from your browser first, and some public APIs that don't allow cross-origin requests (no permissive CORS headers) will block that direct attempt - this tool then automatically retries the same request through a server-side proxy, so most public APIs still work even when the direct browser request is blocked.

Can this reach my localhost dev server?

Yes - because the request is sent directly from your own browser rather than from a remote server, it can reach http://localhost or 127.0.0.1 just like any other page open in your browser can. It still needs your local server to allow the request (CORS), which most dev frameworks permit by default in development.

What can I see in the response?

Typically the response status code, headers, and body - letting you fully inspect what an endpoint actually returned, not just whether it succeeded.

Open the API Tester when you are ready. It is free, and you do not need an account.