Free API Tester & HTTP Client

1. The five HTTP methods that matter

The dominant API design convention today is REST, which assigns meaning to the HTTP verbs:

• GET — retrieve a resource; no request body, parameters in the query string. The default method.
• POST — create a new resource; request body carries the data. Used for nearly every “submit a new…” workflow.
• PUT — replace a resource at a known URL; request body carries the new representation. Idempotent by spec — repeated PUTs produce the same end state.
• PATCH — partially modify a resource; request body carries the delta. Not idempotent in general (sequential patches may not compose; subsequent state depends on the prior).
• DELETE — remove a resource; rarely carries a body. Successful responses are typically 204 No Content.

Most API bugs that are “the request returned the wrong thing” are actually “I used the wrong verb” bugs in disguise. A tester that lets you switch verbs in a single dropdown makes this category of mistake easy to spot — you can immediately re-run the request with the other method and see whether the response changes.

2. Headers — where most API quirks live

HTTP request headers are where every Interesting API behavior lives: Authorization (your access token), Content-Type (what your body is — most servers reject JSON sent as text/plain), Accept (what response format you will accept), Cookie (session cookies that browsers add automatically and your custom client code must add explicitly), If-Match / If-None-Match (optimistic concurrency via ETags), and the entire X- family of vendor extensions. Most API bugs that are not verb bugs are header bugs — an Authorization header missing, a Content-Type uppercased differently than the server expects, a cookie scoped to the wrong domain. A tester that lets you set arbitrary headers per request is the only practical way to repro these.

3. Request body — JSON is not the only option

JSON is the dominant request body format today, but it is not the only one. Some APIs accept form-encoded (application/x-www-form-urlencoded), some multipart for file uploads (multipart/form-data), some plain text or XML or binary protocols. A tester that lets you set the body raw — whatever you paste is sent as the body — is the most flexible, but also the easiest to misuse (a JSON body sent with text/plain content-type will reject on most servers). The convention is: if you have a JSON body, set Content-Type: application/json. The tester pairs the body field with a clear content-type setting so this convention is hard to forget.

4. Response inspection — the three things you actually look at

When a request comes back, three pieces of information drive most debugging:

• Status code — the three-digit prefix that tells you the category of result: 2xx success, 3xx redirect, 4xx client error, 5xx server error. The exact code (401 Unauthorized vs 403 Forbidden is the most common distinction in security-debugging work) often points at the bug.
• Headers — the response headers carry CORS directives (access-control-allow-origin), content negotiation (content-type), caching hints (cache-control, etag), and diagnostic coconut x-request-ids.
• Body — the payload. For JSON responses, a pretty-printed tree-view is dramatically more readable than the raw one-line minified form.

5. CORS — the browser-only class of bug

Cross-Origin Resource Sharing is a browser-enforced security mechanism that requires servers to grant explicit permission for cross-origin requests; it does not exist in server-to-server calls, which is why a curl request works fine while a browser request fails with the famous “blocked by CORS” message. A browser-based API tester will encounter CORS in a way a desktop tester won’t — which is exactly the reason to use one for any front-end-to-backend workflow you intend to ship. CORS failures look like opaque error messages, but the underlying issue is always the absence of an access-control-allow-origin response header on the failing preflight (OPTIONS) request. The browser tester surfaces those failure modes at the debugging moment, not at the production-deploy moment.

6. Authentication — the four common patterns

The four common API authentication patterns and where to put each in the request:

• Bearer token — set the Authorization header to Bearer <token>. The OAuth 2 default.
• API key in a header — common headers include X-API-Key, X-Api-Key (case matters), or a vendor-specific custom header.
• Basic auth — Authorization: Basic <base64(user:pass)>. Legacy but still found.
• Cookie session — set the Cookie header directly; the tester must support this because browsers sending real credentials usually scope them to HTTP-only where your JavaScript cannot read them.

7. Why this belongs in your browser

A browser-based API tester has one specific advantage over desktop alternatives — it runs in the exact environment that your front-end code will eventually run in, surfacing CORS, cookie scope, certificate handling, and redirect policy exactly as your production code will see them. There is no upload of your request body or your auth tokens to a third-party server; everything stays in the page. The combination of in-browser execution and zero-server-side processing is unusually well-suited to a category of tool that deals with sensitive credentials on every request. The tester does not retain your auth token or your request body anywhere outside the tab.

Conclusion

An API tester is a daily-driver tool for any developer working against HTTP APIs, and a browser-based one has the specific advantage of surfacing the browser-enforced security policies (CORS, cookie scope, certificate handling) that desktop alternatives abstract away. With support for every HTTP method, arbitrary headers, raw body input, full response inspection, and zero-upload execution in the browser, this tool covers the practical surface comprehensively. For complex, persistable, multi-environment workflows, a desktop tool like Postman remains a useful companion — but for a one-off “does this endpoint work” request you are debugging right now, a one-tab-away browser tester is the right tool.