API Response Mocker

The API Response Mocker is a free, browser-based tool that lets you design a JSON schema and instantly generate realistic mock API responses. Whether you are building a frontend prototype, writing integration tests, or documenting an API, having realistic sample data speeds up every stage of development.

Start by defining fields with names and types — choose from string, number, boolean, date, email, UUID, URL, enum, nested object, or array. Each field supports a configurable nullable probability, and array fields let you set minimum and maximum lengths. For complex payloads you can nest objects inside objects to any depth.

Once your schema is ready, pick a response wrapper. Bare returns a single object or a plain JSON array. Data array wraps items in { "data": [...] }. Paginated adds a meta block with total, page, perPage, and totalPages fields, which mirrors common REST pagination patterns. Adjust the item count slider from 1 to 100 to control how many records appear.

The HTTP status selector lets you generate error responses too. Selecting 400, 404, or 500 produces a structured error body with code, message, and details fields that follow widespread API conventions. This is useful for testing how your UI handles different failure modes.

Built-in presets for User API, Product API, and Blog Posts API give you a head start. Load a preset, tweak the fields, and generate data in seconds. The output panel displays syntax-highlighted JSON that you can copy or download.

All processing happens entirely in your browser. No data is sent to any server — the random values are generated client-side using JavaScript's Math.random(), so there are no network requests or third-party services involved.

If you need to validate the generated JSON against a schema, try the JSON Schema Generator. To format or inspect the output further, open it in the JSON Formatter. For generating large volumes of realistic test records, check out the Mock Data Generator.

1. Define your response schema by adding fields. Enter a field name and choose a type (string, number, boolean, date, email, uuid, url, enum, object, or array).
2. For enum fields, enter comma-separated values in the text box that appears.
3. For object or array fields, click the expand arrow and use Add sub-field to create nested structures.
4. Set the nullable probability (0–100%) for any field to control how often it returns null.
5. Adjust the Item Count slider (1–100) and choose a Wrapper format: bare, data array, or paginated.
6. Select an HTTP Status code. Choosing 400, 404, or 500 generates a structured error response instead.
7. Click Generate Response or load a preset (User API, Product API, Blog Posts API). Copy the output with the Copy button or press Ctrl+Shift+C. Use Download to save as a .json file.