API Tester - Free Online HTTP Request Builder

What Is an API Tester?

An API tester is a tool that lets you construct HTTP requests, send them to a server, and inspect the responses. Developers use API testers to verify that endpoints work correctly, debug unexpected behavior, test authentication flows, and prototype integrations before writing production code. This tool runs entirely in your browser using the Fetch API, so your requests go directly from your machine to the target server without passing through any intermediary.

Unlike desktop applications that require installation and updates, this browser-based API tester is available immediately from any device. It supports all standard HTTP methods, multiple authentication schemes, various body formats, and generates equivalent code in three languages. Your request history and saved collections persist in localStorage so they survive page refreshes and browser restarts.

How to Use This API Testing Tool

Start by selecting an HTTP method from the dropdown and entering the target URL. Click Send to execute the request. The response appears below with the status code, response time, body content with syntax highlighting, and response headers. You can switch between tabs to view different aspects of the response or generate equivalent code.

Building Query Parameters

Use the Params tab to add query parameters as key-value pairs. The tool automatically appends them to the URL in the correct format. This is cleaner than manually editing the URL string, especially when dealing with multiple parameters or values that need encoding. Each parameter row has a remove button, and you can add as many parameters as needed.

Setting Request Headers

The Headers tab lets you add custom HTTP headers. Common headers like Content-Type and Accept are frequently needed. Each header is a key-value pair. The tool starts with a default Content-Type of application/json, which you can modify or remove. Authentication headers are handled separately in the Auth tab to keep your credentials organized.

Request Body Options

The Body tab supports four formats. Raw JSON is the most common for REST APIs. You can type or paste JSON directly into the editor. Form Data sends the request as multipart/form-data, which is standard for file uploads and form submissions. The x-www-form-urlencoded format encodes the body as URL parameters, commonly used by older APIs and OAuth endpoints. Select None for GET, HEAD, and OPTIONS requests that do not include a body.

Authentication

Four authentication methods are supported. Bearer Token adds an Authorization header with the format "Bearer [token]". Basic Auth encodes your username and password in base64 and sends them in the Authorization header. API Key adds a custom header (default X-API-Key) with your key value. Select None when the endpoint does not require authentication. Your credentials are never stored on any server.

Understanding HTTP Methods

HTTP methods define the action you want the server to perform. Each method has specific semantics that APIs rely on for correct behavior.

GET Requests

GET retrieves data from the server without modifying anything. It is the most common HTTP method. When you visit a webpage, your browser sends a GET request. In API testing, GET requests fetch resources like user profiles, product listings, or configuration data. GET requests should not include a request body according to the HTTP specification, though some servers accept it.

POST Requests

POST sends data to the server to create a new resource. When you submit a registration form, your browser sends a POST request. In APIs, POST typically creates new records in a database. The server usually responds with the newly created resource and a 201 status code. POST requests include a body containing the data for the new resource.

PUT and PATCH Requests

PUT replaces an entire resource with the data you send. If you PUT a user object, all fields must be included because the server replaces the existing record completely. PATCH updates only the specified fields, leaving the rest unchanged. PATCH is more bandwidth-efficient when you only need to change one or two fields in a large object.

DELETE Requests

DELETE removes a resource from the server. The URL typically includes an identifier for the specific resource to delete. Successful DELETE requests usually return a 200 or 204 status code. Some APIs require authentication and authorization checks before allowing deletions.

HEAD and OPTIONS

HEAD is identical to GET except the server does not return a response body. It is useful for checking if a resource exists or reading response headers without downloading the content. OPTIONS returns the HTTP methods that the server supports for a given URL. Browsers send OPTIONS requests automatically as CORS preflight checks before making cross-origin requests.

Reading API Responses

The response section displays several pieces of information that help you understand what the server returned and diagnose issues.

Status Codes

HTTP status codes fall into five categories. 2xx codes (like 200 OK, 201 Created, 204 No Content) indicate success. 3xx codes (like 301, 302, 304) indicate redirects or cached responses. 4xx codes (like 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found) indicate client errors, meaning something was wrong with your request. 5xx codes (like 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable) indicate server errors. This tool color-codes each category for quick identification.

Response Headers

Response headers contain metadata about the response. Important headers include Content-Type (the format of the response body), Content-Length (the size of the response), Cache-Control (caching instructions), and various CORS headers that control cross-origin access. The Headers tab displays all response headers in a readable table format.

Response Body and JSON Formatting

Most modern APIs return JSON. This tool automatically detects JSON responses, formats them with proper indentation, and applies syntax highlighting. Strings appear in green, numbers in orange, booleans in purple, null values in gray, and property keys in blue. This color coding makes it significantly easier to scan large response objects and find the data you need.

Code Generation

After building a request, you can switch to the Code Generation tab to see the equivalent code in cURL, JavaScript fetch, and Python requests. This is useful when you have tested an endpoint and want to integrate it into your application. Each code snippet is fully formed and ready to copy into your project, including headers, authentication, and the request body.

cURL Commands

cURL is the universal command-line HTTP client available on Linux, macOS, and Windows. The generated cURL command includes all your headers, authentication, body content, and the appropriate flags. You can paste it directly into a terminal to execute the same request outside the browser.

JavaScript Fetch

The fetch API is the modern standard for making HTTP requests in JavaScript. The generated code uses async/await syntax with proper error handling. It includes the method, headers object, and body. You can paste it into a browser console, Node.js script, or your application code.

Python Requests

The requests library is the standard HTTP client for Python. The generated code includes the import statement, all necessary parameters, and basic error handling. It is ready to run in a Python script or Jupyter notebook.

Collections and History

The sidebar provides two ways to manage your requests. Collections let you save requests with custom names for reuse. Click "Save Current Request" to add the current request configuration to your collection. Click any saved request to load it into the builder. You can export your entire collection as JSON to share with teammates or import a collection from a JSON file.

Request history automatically logs every request you send, including the method, URL, status code, and timestamp. Click any history entry to replay that exact request. History is stored in localStorage and persists across sessions. Clear it at any time using the Clear History button.

Common API Testing Scenarios

Testing a REST API

Start with a GET request to the base resource endpoint (like /api/users) to verify you can connect and retrieve data. Then test creating a resource with POST, updating it with PUT or PATCH, and deleting it with DELETE. Check that the status codes match expectations at each step. Verify that the response body contains the correct data after each operation.

Debugging Authentication Issues

If you receive a 401 Unauthorized response, check that your token or credentials are correct and properly formatted. Try the request without authentication first to confirm the endpoint exists (you should get 401, not 404). Verify the token has not expired. Check that the authentication header name matches what the API expects (Authorization for Bearer and Basic, or a custom header for API keys).

Testing Error Handling

Send intentionally malformed requests to verify that the API returns proper error messages. Try missing required fields, invalid data types, extremely long strings, and special characters. A well-designed API should return descriptive error messages with appropriate 4xx status codes, not crash with a 500 error.

Frequently Asked Questions

Performance Comparison

Benchmark: processing speed relative to alternatives. Higher is better.