API Client — Free Postman Alternative
Test REST APIs directly in your browser. All HTTP methods, authentication, collections, history, and environment variables — no signup, no installation.
What is an API Client?
An API client is a tool that lets developers send HTTP requests to web APIs and inspect the responses. Whether you are building a new API, debugging an existing integration, or exploring a third-party service, an API client is essential for testing endpoints, verifying authentication, and understanding response data before writing any code.
Our API Client is a browser-based alternative to tools like Postman and Insomnia. It runs entirely in your browser using the native Fetch API, with no installation, no signup, and no data sent to any server. Your requests, collections, history, and environment variables are all stored privately in your browser's localStorage.
Built for developers who want a fast, clean, privacy-first way to test APIs, our tool covers all the essentials: every HTTP method (GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS), query parameters, headers, multiple body types (JSON, form data, URL-encoded, raw), authentication (Bearer, Basic, API Key), collections for organizing requests, history for replaying past calls, and environment variables for managing different deployment targets.
How to Use the API Client
- Choose an HTTP method from the dropdown (GET, POST, etc.).
- Enter the URL of the API endpoint you want to test. You can use environment variables like
{{baseUrl}}. - Add query parameters, headers, body, or auth using the tabs below the URL bar. Each tab remembers your input as you switch between them.
- Click Send and see the response instantly — status code, response time, size, headers, and body.
- Save successful requests to collections for later, or click an entry in history to replay it.
- Use “Open in JSON Formatter” to send the response body to our JSON Formatter tool for pretty-printing and tree view inspection.
Features
- All HTTP Methods — GET, POST, PUT, PATCH, DELETE, HEAD, and OPTIONS for testing any REST API.
- Query Parameters — Add, edit, enable, and disable query params without rewriting the URL.
- Custom Headers — Send any headers your API requires, including content type, accept, cookies, and custom auth schemes.
- Multiple Body Types — JSON, form-data, URL-encoded, and raw text. Automatically sets the right Content-Type header.
- Authentication — Built-in support for Bearer tokens, Basic auth, and API keys (header or query).
- Collections — Organize your requests into named collections. Export and import as JSON for backup or sharing.
- History — Automatically saves the last 50 requests. Click any entry to replay it with one click.
- Environment Variables — Define variables like
{{baseUrl}}and{{apiKey}}for easy switching between environments. - cURL Export — Copy any request as a cURL command with one click for sharing or debugging.
- JSON Formatter Integration — Send response bodies to our JSON Formatter tool for pretty-printing and tree view.
- 100% Client-Side — All requests go directly from your browser to the API. Your data never touches our servers.
Understanding CORS Errors
Because our API Client runs in your browser, requests are subject to the same Cross-Origin Resource Sharing (CORS) rules that your regular web applications follow. If an API does not explicitly allow cross-origin requests from stringtoolsapp.com, your browser will block the request before it is ever sent.
This is not a bug in our tool — it is a fundamental browser security feature. Most modern public APIs support CORS and will work fine. Examples include GitHub API, JSONPlaceholder, OpenWeather, most Stripe endpoints, Firebase, and many others. If you see a CORS error, it means the specific API you are calling has not enabled cross-origin access.
If you control the API, the fix is simple: add the Access-Control-Allow-Origin: * header (or a more restrictive allow-list) to your API responses. If you do not control the API, you will need to use a desktop tool like Postman, Insomnia, or curl — those tools do not run in a browser, so CORS does not apply to them.
We deliberately do not offer a third-party CORS proxy option because it would route your requests (including any auth tokens and sensitive data) through a server we do not control, which would break our privacy-first promise. We believe transparency about this limitation is better than compromising your data security.
Privacy and Security
All your API requests are made directly from your browser to the target API server. StringToolsApp never sees your request URLs, headers, request bodies, authentication tokens, or response data. There is no backend, no proxy, and no logging of your API traffic.
Your collections, history, and environment variables are stored in your browser's localStorage, which means they persist across browser sessions on the same device but are never uploaded anywhere. If you clear your browser data, you will lose your saved requests — so we recommend exporting your collections as JSON periodically for backup.
Frequently Asked Questions
Is this a free Postman alternative?
Yes. Our API Client is a completely free, browser-based alternative to Postman. It supports all the core features you need for API testing — HTTP methods, headers, authentication, request body, collections, history, and environment variables — without any signup or installation.
Why do some API requests fail with CORS errors?
CORS (Cross-Origin Resource Sharing) is a browser security feature that blocks requests to APIs that do not explicitly allow them. This is not a bug in our tool — it is enforced by your browser. If you control the API, add the Access-Control-Allow-Origin header to enable cross-origin requests. For APIs you do not control, use a desktop tool like Postman or curl instead.
Where are my requests and collections stored?
All your requests, collections, history, and environment variables are stored entirely in your browser's localStorage. Nothing is sent to our servers. Your data persists across browser sessions on the same device but is private to you.
Can I export my collections?
Yes. Click the Collections sidebar and use the Export button to download all your saved collections as a JSON file. You can re-import them later using the Import button, which is useful for backup or moving between devices.