API Response Simulator
Mock any API response with full control over the HTTP status code, headers, and response body. Our API response simulator lets you choose from 19 status codes, add custom headers with CORS support, write response bodies in JSON, XML, HTML, plain text, or JavaScript, and simulate network latency up to 5 seconds - all from your browser. Perfect for frontend development without a backend.
Mock API responses with custom status codes, headers, and body content. Select a preset or configure everything manually - method, path, status code, headers, body (JSON/XML/HTML/text), and latency. Perfect for frontend development without a backend.
HTTP/1.1 200 OK Date: Wed, 29 Jul 2026 02:20:50 GMT Content-Length: 160 Content-Type: application/json Server: AbackTools/1.0 [ { "id": 1, "name": "John Doe", "email": "john@example.com" }, { "id": 2, "name": "Jane Smith", "email": "jane@example.com" } ]
Why Use Our API Response Simulator?
Custom Status Codes & Responses
Mock any HTTP response with full control over the status code (200, 201, 400, 401, 403, 404, 500, etc.) and status text. Choose from 19 common HTTP status codes or set custom values to simulate any API scenario.
Flexible Body & Content Types
Write response bodies in JSON, XML, HTML, plain text, or JavaScript. The Content-Type header is automatically set based on your selection. JSON bodies are automatically formatted for readability.
Pre-built Response Presets
Choose from 8 pre-configured presets including 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Server Error, XML Response, and HTML Response - each with realistic sample data and appropriate latency simulation.
Latency Simulation & CORS Support
Simulate realistic network latency from 0ms to 5000ms to test loading states and timeouts. One-click CORS headers (Access-Control-Allow-Origin: *) enables cross-origin testing without configuration.
Common Use Cases for API Response Simulator
Frontend Development Without a Backend
Build and test your frontend application even when the backend API isn't ready yet. Simulate API responses with realistic data structures, status codes, and network latency. Iterate on your UI without waiting for backend development.
Error Handling & Edge Case Testing
Test how your application handles different error scenarios - 400 validation errors, 401 authentication failures, 403 forbidden access, 404 not found, 429 rate limiting, and 500 server errors. Ensure your error handling UI works correctly.
Loading State & UX Testing
Use the latency simulation feature to test loading spinners, skeleton screens, and timeout handling. Simulate slow 3G connections (1000-3000ms delays) to ensure your application provides good UX under poor network conditions.
API Documentation Examples
Generate realistic API response examples for your API documentation. Create responses for different endpoints, status codes, and content types. Copy the raw HTTP response format to include in your documentation.
CORS Configuration Testing
Use the built-in CORS header toggle to simulate cross-origin responses. Test how your application behaves when CORS headers are present or missing. Verify that your frontend correctly handles CORS-related errors.
Learning HTTP Protocol
Use the simulator to learn how HTTP responses are structured. See the exact format of status lines, response headers, and body content. Experiment with different status codes and content types to understand HTTP response behaviour.
Understanding API Response Simulation
What Is API Response Simulation?
API response simulation is a technique used in frontend development where you create mock HTTP responses to test how your application behaves without requiring a real backend server. It allows developers to simulate various server scenarios - successful responses, error states, slow networks, and different data formats - all from the browser. This is especially valuable when the backend team is still developing the API, or when you need to test edge cases that are hard to reproduce.
How Our API Response Simulator Works
- Choose a Preset or Configure Manually - Start from one of 8 pre-built presets covering common API scenarios, or manually configure the method, path, status code, headers, and body from scratch.
- Customise the Response - Select the HTTP status code and text, add custom headers (with optional CORS auto-inclusion), choose a body content type (JSON, XML, HTML, text, or JavaScript), and enter your response body content.
- Generate & Copy - Set an optional latency delay to simulate network conditions, then generate the raw HTTP response. Copy the formatted response with one click for use in your development workflow.
What Can You Control?
- HTTP Method & Path - Select any HTTP method (GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS) and enter a request path to make the simulation realistic.
- Status Code & Text - Choose from 19 common HTTP status codes or enter a custom status text. The simulator will show the appropriate status line format.
- Response Headers - Add, remove, enable, or disable custom response headers. Toggle CORS headers on/off with a single click.
- Response Body - Write body content in JSON, XML, HTML, plain text, or JavaScript. JSON content is automatically validated and formatted.
- Latency Simulation - Add a delay of 0-5000ms to simulate network latency for realistic loading state testing.
Privacy, Security & Availability
The API Response Simulator runs entirely in your browser. No data you enter - including API paths, headers, response bodies, or any content - is ever sent to our servers or any external service. All response generation and formatting happens locally using standard JavaScript APIs. The tool is completely free with no signup required, no API keys, and no usage limits.
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Frequently Asked Questions About API Response Simulator
An API response simulator is a tool that generates mock HTTP responses so you can test frontend applications without a real backend. It lets you control the status code, headers, body content, and response delay to simulate different server scenarios. This is essential for frontend development, testing, and API documentation.
No. The API Response Simulator generates the raw HTTP response text so you can see exactly what your API response would look like. It does not send any requests to a server or set up a real mock server. All processing happens locally in your browser - no data is transmitted.
The tool includes 19 common HTTP status codes covering all major categories: 2xx Success (200, 201, 202, 204), 3xx Redirection (301, 302, 304), 4xx Client Errors (400, 401, 403, 404, 405, 409, 422, 429), and 5xx Server Errors (500, 502, 503, 504). You can also customise the status text for any code.
The tool supports JSON (application/json), XML (application/xml), HTML (text/html), plain text (text/plain), and JavaScript (application/javascript). JSON bodies are automatically formatted with proper indentation. The Content-Type header is set automatically based on your selection.
The latency slider lets you set a delay between 0ms and 5000ms (5 seconds). This simulates the time it would take for a real API to respond. Use this to test loading states, spinners, skeleton screens, and timeout handling in your frontend application. The delay is shown in the generated response for reference.
CORS (Cross-Origin Resource Sharing) headers tell browsers that it's safe to allow cross-origin requests. The Access-Control-Allow-Origin: * header allows any website to access the API. When building frontend applications, you often need to test with CORS headers to ensure your app can communicate with the API correctly.
The tool does not currently save simulations to a server. However, you can copy the generated raw HTTP response text and save it to a file, bookmark the page, or use the preset system to quickly rebuild common response patterns. All processing happens locally in your browser.
The Content-Length header value is calculated automatically based on the byte size of the response body using UTF-8 encoding. This is added to the output headers to provide a realistic HTTP response format. The byte count is also displayed in the response summary.
JSON supports structured data with objects, arrays, strings, numbers, booleans, and null values. XML allows hierarchical data with custom tags. HTML lets you simulate full web pages. Plain text is for any raw text content. JavaScript is for API endpoints that return JS files or JSONP-style responses.
Yes! The API Response Simulator is completely free with no signup, no account, and no API keys required. All response generation happens locally in your browser - no data is sent to any server, and no usage limits apply. Use it as much as you need.